An alkene has an unusually useful feature: a carbon-carbon double bond that can undergo addition reactions. That single structural fact explains why ethene may become ethane with hydrogen, ethanol with steam, or a dibromo compound with bromine. The first skill is noticing the double bond. The next is tracing every atom into the correct product. The advanced skill is knowing how far a test observation can support a conclusion.
The official 2027 SEC G3 Chemistry code is K324. The syllabus includes alkanes and alkenes, aqueous bromine as a comparison for saturation, alkene addition with bromine, steam and hydrogen, and addition polymerisation. It specifies Paper 1 multiple choice (1 hour, 40 marks, 30%), Paper 2 structured/free response (1 hour 45 minutes, 80 marks, 50%) and Paper 3 practical (1 hour 50 minutes, 40 marks, 20%). See the 2027 SEAB G3 subject directory and the official K324 syllabus. These original practice scenarios explain concepts, not directions for carrying out chemical demonstrations.
What the double bond changes
Alkanes are saturated hydrocarbons with single carbon-carbon bonds. Acyclic alkenes with one carbon-carbon double bond follow the general formula CnH2n. They are called unsaturated because their carbon-carbon multiple bond can participate in addition. In a simple ethene addition, the double bond becomes a single bond and new atoms attach to the two carbon atoms. The carbon skeleton is often preserved.
For an ethene example, write CH2=CH2 first. If H2 adds, one new hydrogen attaches to each carbon and the product becomes CH3–CH3. That product is ethane. The process is chemical, not a mere change in how a drawing is displayed. The molecular equation C2H4 + H2 → C2H6 accounts for every atom.
Three central additions
Bromine addition: C2H4 + Br2 → C2H4Br2. Hydrogen addition: C2H4 + H2 → C2H6. Steam addition: C2H4 + H2O ⇌ C2H5OH in the industrial hydration process under appropriate conditions. The added species determines the chemical family of the product. Do not use these three equations interchangeably just because the starting material is ethene.
Aqueous bromine provides a syllabus-level test for distinguishing saturated and unsaturated hydrocarbons. When an appropriate alkene reacts, bromine’s colour disappears. The classroom-level explanation is addition at the unsaturated bond. This is a chemistry reasoning example, not an invitation to handle hazardous chemicals. Practical work belongs in a properly supervised, authorised laboratory.
A five-stage reasoning sequence
First identify the homologous series and functional bond. Second identify the other reactant or condition. Third draw or imagine the bond change. Fourth balance the atoms and write the product. Fifth distinguish what an observation actually proves from what it merely suggests. This sequence makes written explanations, data interpretation and reaction comparisons much easier to control.
Twenty-four worked chemical reasoning cases
1. Spot the double bond
Question. A structural formula is CH2=CH2. Decide whether the compound belongs to the alkene family and explain why. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. It is ethene, an alkene with a carbon-carbon double bond. Its molecular formula is C2H4, consistent with the general alkene formula CnH2n. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Start with structure, not a remembered colour chart. A carbon-carbon double bond is the defining feature for the simple alkene reactions studied here. The reaction changes that bond and adds atoms from a second reactant. If you cannot locate the double bond, drawing the structural formula first is safer than guessing from the product name. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Naming it ethane ignores both the double bond and the difference in hydrogen number. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Sketch the corresponding two-carbon alkane and compare hydrogen counts. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
2. The saturated comparison
Question. Two compounds are C2H4 and C2H6. Identify the ordinary alkene and alkane without relying on a memorised name alone. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. C2H4 is ethene, with a double bond. C2H6 is ethane, a saturated alkane in which the carbon-carbon bond is single. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Apply conservation of atoms to every equation. Carbon atoms are not silently created in hydrogenation, and bromine addition cannot make one bromine atom disappear. Write the molecular formulas, count each element on both sides and then explain the structural change. Conservation is both a science concept and a powerful examination check. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. The formula test alone may be misleading for molecules outside the stated straight-chain acyclic context; also inspect the structure. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Explain why ethane contains two more hydrogen atoms. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
3. Bromine and ethene
Question. The question supplies ethene and bromine as reactants. Predict the addition product’s molecular formula. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. One Br2 molecule adds across one C=C bond: C2H4 + Br2 → C2H4Br2. Each carbon gains one bromine bond and the carbon-carbon connection becomes single. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. A chemical observation supports only the claim that the circumstances permit. Decolourisation in a controlled textbook comparison supports a reaction consistent with unsaturation. It does not necessarily identify the complete composition of an unlabelled mixture. Keep observation, interpretation and chemical identity in three separate sentences when the question asks for evaluation. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Do not remove hydrogen atoms; the addition reaction adds atoms rather than substituting an existing atom. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Draw the product as BrCH2–CH2Br and count its atoms. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
4. Name the dibromo product
Question. A displayed product is BrCH2–CH2Br. Give its identity and explain where the bromine atoms came from. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. This is 1,2-dibromoethane. It can be formed by addition of bromine to ethene, one bromine atom bonding to each former alkene carbon. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Different reactions can begin with the same compound. The appropriate product is controlled by the second reactant, the bond being changed and the specified conditions. Hydrogenation, bromine addition, hydration, polymerisation, combustion and cracking have different atom bookkeeping. An exam answer should identify the process before writing the equation. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Calling it bromoethane loses one bromine atom and changes the molecular formula. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Write the balanced molecular addition equation. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
5. Bromine-water evidence
Question. A known unsaturated hydrocarbon decolourises aqueous bromine in a theoretical classroom question. Explain the observation. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. The unsaturated hydrocarbon reacts with bromine by addition across a carbon-carbon multiple bond, so bromine’s characteristic colour is lost. In the syllabus comparison, this supports unsaturation. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. The name of the product follows from its structure. Count the carbon atoms, locate the multiple bond or functional group and retain the numbering that distinguishes positional isomers. A molecular formula reports atom totals but cannot always identify a unique structure. When the question gives a displayed formula, make use of the information in the drawing. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. The colour alone does not establish the exact identity of an unknown substance or exclude other reactions in an uncontrolled sample. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Name one additional structural fact needed before identifying a specific alkene. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
6. A negative result with an alkane
Question. Ethane and ethene are compared under the usual conceptual bromine-water test conditions. Which is expected to react in the stated comparison? Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. Ethene is expected to decolourise bromine water by addition. Ethane lacks the C=C bond needed for that alkene addition; it is not expected to give the same result under the stated conditions. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. The most useful examination explanation has three parts: what is present before the reaction, what changes and what evidence supports the proposed product. Avoid extra claims about industrial yield, purity, safety or suitability unless evidence is supplied. Specific answers score more reliably than impressive but unsupported generalisations. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Do not infer that alkanes are chemically inert in every situation; combustion and substitution are separate reactions. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Explain why the exam question must specify relevant conditions and substance class. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
7. Propene plus bromine
Question. Write the formula equation when propene reacts by addition with bromine. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. Propene, C3H6, combines with Br2 to form C3H6Br2. A suitable displayed product is CH2Br–CHBr–CH3, 1,2-dibromopropane. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Start with structure, not a remembered colour chart. A carbon-carbon double bond is the defining feature for the simple alkene reactions studied here. The reaction changes that bond and adds atoms from a second reactant. If you cannot locate the double bond, drawing the structural formula first is safer than guessing from the product name. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Do not use C3H6Br as the final formula; a molecule of Br2 contributes two atoms. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Count carbon, hydrogen and bromine atoms on both sides. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
8. Hydrogenation of ethene
Question. Predict the product when hydrogen adds to ethene across its carbon-carbon double bond. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. C2H4 + H2 → C2H6. The product is ethane. Hydrogenation removes the carbon-carbon double-bond character by adding a hydrogen atom to each carbon. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Apply conservation of atoms to every equation. Carbon atoms are not silently created in hydrogenation, and bromine addition cannot make one bromine atom disappear. Write the molecular formulas, count each element on both sides and then explain the structural change. Conservation is both a science concept and a powerful examination check. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Do not describe the product as ethyne or add two H2 molecules to one ordinary ethene C=C bond. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Compare the displayed structural formula before and after addition. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
9. Hydrogenation of propene
Question. An alkene has formula C3H6. It receives one H2 in a hydrogenation equation. Identify the product. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. C3H6 + H2 → C3H8. Propene becomes propane, a saturated alkane with the same three-carbon skeleton. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. A chemical observation supports only the claim that the circumstances permit. Decolourisation in a controlled textbook comparison supports a reaction consistent with unsaturation. It does not necessarily identify the complete composition of an unlabelled mixture. Keep observation, interpretation and chemical identity in three separate sentences when the question asks for evaluation. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. The carbon count does not change during this addition; adding carbon atoms would require a different reaction. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Explain what bond change accompanies the two new C–H bonds. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
10. Hydration of ethene
Question. A textbook asks for the alcohol obtained by adding steam to ethene in the industrial process. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. Ethene reacts with steam to form ethanol: C2H4 + H2O ⇌ C2H5OH. Industrial hydration uses appropriate catalytic conditions; the organic product is an alcohol. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Different reactions can begin with the same compound. The appropriate product is controlled by the second reactant, the bond being changed and the specified conditions. Hydrogenation, bromine addition, hydration, polymerisation, combustion and cracking have different atom bookkeeping. An exam answer should identify the process before writing the equation. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Do not confuse this with fermentation, which also yields ethanol but begins with sugars and follows a different process. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Name one reason why knowing the route matters when describing the process. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
11. Account for oxygen in hydration
Question. A student balances ethene plus water as forming C2H6 without an oxygen-containing product. Evaluate the claim. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. The proposed product omits the oxygen atom from water. A valid hydration product is ethanol, C2H5OH, which contains two carbons, six hydrogens and one oxygen. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. The name of the product follows from its structure. Count the carbon atoms, locate the multiple bond or functional group and retain the numbering that distinguishes positional isomers. A molecular formula reports atom totals but cannot always identify a unique structure. When the question gives a displayed formula, make use of the information in the drawing. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Correct equations conserve every element, not only carbon and hydrogen. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Count C, H and O atoms before and after writing the corrected equation. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
12. Addition versus substitution
Question. Compare bromine addition to ethene with chlorine substitution of methane in an appropriate textbook setting. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. In ethene addition, atoms attach across a double bond without replacing hydrogen. In methane substitution, a chlorine atom replaces hydrogen, producing chloromethane and hydrogen chloride under the required conditions. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. The most useful examination explanation has three parts: what is present before the reaction, what changes and what evidence supports the proposed product. Avoid extra claims about industrial yield, purity, safety or suitability unless evidence is supplied. Specific answers score more reliably than impressive but unsupported generalisations. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Calling both mechanisms ‘addition’ hides the different atom bookkeeping. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Write the conceptual product sets for each reaction without suggesting a practical procedure. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
13. Recognise the homologous series
Question. Give the formulas of ethene, propene and butene, identifying what changes as the series advances. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. Ethene is C2H4, propene C3H6 and butene C4H8. Successive members differ by CH2, while the acyclic monoalkenes share the general formula CnH2n. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Start with structure, not a remembered colour chart. A carbon-carbon double bond is the defining feature for the simple alkene reactions studied here. The reaction changes that bond and adds atoms from a second reactant. If you cannot locate the double bond, drawing the structural formula first is safer than guessing from the product name. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. The formula alone does not specify the exact position of the double bond in a larger molecule. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Show how C5H10 fits the same formula pattern. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
14. But-1-ene structure
Question. An examiner supplies CH2=CH–CH2–CH3. Name the four-carbon alkene and identify its double-bond location. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. This is but-1-ene. The double bond begins at carbon 1 when numbering the chain from the nearer end. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Apply conservation of atoms to every equation. Carbon atoms are not silently created in hydrogenation, and bromine addition cannot make one bromine atom disappear. Write the molecular formulas, count each element on both sides and then explain the structural change. Conservation is both a science concept and a powerful examination check. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Calling it but-2-ene places the double bond between different carbon atoms. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Draw but-2-ene and compare the connectivity. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
15. But-2-ene structure
Question. Another four-carbon alkene is CH3–CH=CH–CH3. State how it differs from but-1-ene. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. This is but-2-ene. Both forms have molecular formula C4H8, but the carbon-carbon double bond lies in a different position. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. A chemical observation supports only the claim that the circumstances permit. Decolourisation in a controlled textbook comparison supports a reaction consistent with unsaturation. It does not necessarily identify the complete composition of an unlabelled mixture. Keep observation, interpretation and chemical identity in three separate sentences when the question asks for evaluation. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. The compounds should not be treated as having identical displayed structures merely because their molecular formulas match. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Explain structural isomerism in one sentence without using the word ‘different’ alone. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
16. What a colour test cannot identify
Question. A fictional unlabelled sample causes bromine water to lose colour. A student announces ‘it must be ethene’. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. Within a controlled hydrocarbon question the observation suggests unsaturation, but it does not identify ethene uniquely. Other alkenes can also react by addition. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Different reactions can begin with the same compound. The appropriate product is controlled by the second reactant, the bond being changed and the specified conditions. Hydrogenation, bromine addition, hydration, polymerisation, combustion and cracking have different atom bookkeeping. An exam answer should identify the process before writing the equation. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. The common error is treating a diagnostic observation for a family as a unique fingerprint for one member. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. State what extra evidence would be needed before claiming a specific identity. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
17. No bubbles are required
Question. A student expects bubbles when bromine reacts by addition with a liquid alkene in a written theory question. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. The familiar test’s stated observation is decolourisation of aqueous bromine, not necessarily gas production. No gaseous product appears in the simple bromine-addition equation. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. The name of the product follows from its structure. Count the carbon atoms, locate the multiple bond or functional group and retain the numbering that distinguishes positional isomers. A molecular formula reports atom totals but cannot always identify a unique structure. When the question gives a displayed formula, make use of the information in the drawing. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Inventing effervescence without an appropriate gas-forming reaction loses evidential discipline. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Describe the correct observation and match it to the balanced formula equation. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
18. Hydrogen demand for one double bond
Question. A theoretical equation asks how many H2 molecules add across one ordinary carbon-carbon double bond. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. One H2 molecule supplies two hydrogen atoms and converts one C=C bond into a C–C bond with two additional C–H bonds. For ethene, the 1:1 mole ratio appears directly in C2H4 + H2 → C2H6. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. The most useful examination explanation has three parts: what is present before the reaction, what changes and what evidence supports the proposed product. Avoid extra claims about industrial yield, purity, safety or suitability unless evidence is supplied. Specific answers score more reliably than impressive but unsupported generalisations. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Doubling the H2 coefficient would produce an atom imbalance and an implausible product. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Use atom conservation to check a proposed 2:1 H2-to-ethene ratio. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
19. From unsaturated oil to a more saturated product
Question. The syllabus connects hydrogenation with treating unsaturated vegetable oil in the manufacture of margarine. Explain the chemical direction. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. Hydrogen adds across some carbon-carbon double bonds in unsaturated hydrocarbon chains, reducing unsaturation and altering the material’s physical properties. The precise product depends on the process and extent of hydrogenation. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Start with structure, not a remembered colour chart. A carbon-carbon double bond is the defining feature for the simple alkene reactions studied here. The reaction changes that bond and adds atoms from a second reactant. If you cannot locate the double bond, drawing the structural formula first is safer than guessing from the product name. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. It is inaccurate to claim that every molecule becomes completely saturated or that all fats have identical properties. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Explain why the phrase ‘more saturated’ can be safer than ‘fully saturated’. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
20. Polyunsaturated is not a percentage
Question. A food description calls an oil polyunsaturated. What does the word communicate chemically? Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. Polyunsaturated means that fatty-acid chains contain more than one carbon-carbon double bond. It does not specify a mass percentage, an energy content or an automatic health verdict. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Apply conservation of atoms to every equation. Carbon atoms are not silently created in hydrogenation, and bromine addition cannot make one bromine atom disappear. Write the molecular formulas, count each element on both sides and then explain the structural change. Conservation is both a science concept and a powerful examination check. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Interpreting ‘poly’ as ‘high concentration by mass’ confuses bond structure with composition. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Compare the meanings of saturated, monounsaturated and polyunsaturated. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
21. Addition polymerisation
Question. Many ethene molecules react to make poly(ethene). Explain what happens to the double bonds. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. In addition polymerisation, ethene monomers join in a long chain through the conversion of carbon-carbon double bonds into single bonds within the growing polymer backbone. The repeat unit is –CH2–CH2–. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. A chemical observation supports only the claim that the circumstances permit. Decolourisation in a controlled textbook comparison supports a reaction consistent with unsaturation. It does not necessarily identify the complete composition of an unlabelled mixture. Keep observation, interpretation and chemical identity in three separate sentences when the question asks for evaluation. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. A polymer is not simply one ethene molecule with extra bromine or hydrogen; the transformation joins many monomers. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Explain the difference between a repeat unit and a whole polymer chain. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
22. Cracking compared with addition
Question. A question contrasts cracking of decane with subsequent addition to an alkene product. Give an atom-balanced example of the first stage. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. A valid illustrative cracking equation is C10H22 → C8H18 + C2H4. Cracking breaks a larger hydrocarbon into smaller molecules, whereas addition uses an alkene’s double bond to combine reactants. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. Different reactions can begin with the same compound. The appropriate product is controlled by the second reactant, the bond being changed and the specified conditions. Hydrogenation, bromine addition, hydration, polymerisation, combustion and cracking have different atom bookkeeping. An exam answer should identify the process before writing the equation. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Do not reverse the arrow or call cracking a hydrogenation step. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Identify which cracking product could undergo bromine addition and write its product formula. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
23. Combustion is another branch
Question. A theory question asks for complete combustion of ethene rather than bromine addition. Write the balanced equation. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. C2H4 + 3O2 → 2CO2 + 2H2O. Complete combustion uses oxygen and produces carbon dioxide and water; it does not preserve the carbon skeleton as an addition product does. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. The name of the product follows from its structure. Count the carbon atoms, locate the multiple bond or functional group and retain the numbering that distinguishes positional isomers. A molecular formula reports atom totals but cannot always identify a unique structure. When the question gives a displayed formula, make use of the information in the drawing. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. Adding bromine or naming ethanol would answer a different reaction question. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Verify carbon, hydrogen and oxygen counts before settling on the coefficients. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
24. Choose the right evidence and claim
Question. Three cards read: ethene loses bromine colour; ethene plus hydrogen yields ethane; ethene plus steam yields ethanol. Write one precise conclusion. Begin by identifying which feature of the reactant matters rather than trying to remember a complete answer.
Reasoned solution. The cards illustrate distinct addition reactions at ethene’s carbon-carbon double bond. The reactant added determines the product: bromine yields a dibromo compound, hydrogen an alkane, and steam an alcohol. For a formula question, make the atom count visible. For an observation question, separate what is seen from what is inferred.
Mechanism and transfer. The most useful examination explanation has three parts: what is present before the reaction, what changes and what evidence supports the proposed product. Avoid extra claims about industrial yield, purity, safety or suitability unless evidence is supplied. Specific answers score more reliably than impressive but unsupported generalisations. In this example, the distinction matters because the examination wording asks for a particular identity, product, process or level of certainty. Do not replace that job with a general paragraph about all organic compounds.
Common wrong turn. The presence of a shared starting alkene does not make all additions chemically identical or give a unique diagnostic observation. Locate the earliest incorrect assumption and repair the structure or evidence claim at that point. A corrected explanation should not need invented observations or unspecified apparatus.
Independent retest. Construct a comparison table in words listing added species, bond change and product family. Write one sentence linking the starting structure to the final answer. Then read the original question again to check that your explanation, formula and terminology all address it.
A complete reasoning chain: from cracking to a verified product
Suppose a written question describes cracking a larger hydrocarbon into octane and ethene. C10H22 → C8H18 + C2H4 is a balanced example. The first reaction supplies an alkane and an alkene. If the ethene subsequently undergoes hydrogen addition, it becomes ethane by C2H4 + H2 → C2H6. If a different portion reacts by bromine addition, it forms C2H4Br2. These are different branches from the same original alkene, not three simultaneous descriptions of one product.
The structure explains the choice. In the cracking stage, the carbon skeleton divides. In the hydrogenation and bromination stages, the two-carbon skeleton remains but the double bond no longer remains in the final simple addition product. For a mark-worthy explanation, state the relevant chemical change, write a balanced equation, and avoid claiming that a bromine-water result establishes an exact formula without other evidence.
Imagine the task then asks which observation would support an alkene in a controlled alkane-versus-alkene comparison. State decolourisation of aqueous bromine for the alkene under the expected conditions. Do not invent gas bubbles. Do not claim that the same observation reveals a precise chain length. Examiners can test this boundary because it distinguishes a memorised test from an understanding of what the test measures.
Mistake repair: chemistry and scientific evidence
Keep four columns after practice: bond or functional group, reactant, balanced products, and justified observation. Mark errors according to the first column that breaks. If the bond was mistaken, revise the structural formula. If the reactant was mistaken, identify the different chemical process. If the equation was unbalanced, count each element. If the conclusion was too strong, weaken it to match the observation. Retest with a different alkene rather than copying the corrected ethene example.
Chemistry also requires a careful boundary between theory and practical work. The official syllabus includes practical assessment, but dangerous chemical handling should be performed only in appropriate supervised settings. For independent study, use structural formulae, equations, supplied experimental observations, tables and teacher-approved past-paper questions. The learner’s job here is chemical reasoning, not reproducing a laboratory demonstration.
Route back to foundations and forward to materials
Begin with the PSLE Learning Guide and Secondary and Post-Secondary Science owner when basic ideas about materials and fair observations need repair. Volume 0348: crude oil fractions explains the refinery starting point; Volume 0350: cracking hydrocarbons explains how smaller alkenes arise. This article takes the next chemical step: what the alkene double bond permits. For the polymer direction, revisit Volume 0303: polymers and repeat units.
Final exam check
Does the reactant have a C=C double bond? Which species is being added? Did you conserve every carbon, hydrogen, halogen and oxygen atom? Did the new structure receive the correct name? Are the observation and the inference stated separately? Have you confused addition with cracking, combustion or substitution? A strong answer is usually short because it identifies the single chemical change that controls the whole question.
