Wait, what? Two laboratory reports appear to disagree. Report A says a material contains 20 mg/kg of a substance. Report B says 8 mg/kg. At first glance, A looks much higher. Then you notice two small phrases: A says dry weight; B says wet weight.
Those words are not decoration. They tell you what mass was used as the denominator. If water has been removed before the result is expressed, one kilogram of “dry material” is not the same physical amount of original wet material as one kilogram of “wet material.” A concentration can therefore look numerically larger on a dry-weight basis without any extra substance being created.
This is a real-world evidence-reading problem. A Primary 5 or Primary 6 learner does not need specialist chemistry to solve it. The important habit is to ask: 20 milligrams per kilogram of what exact kilogram?
Quick Answer
No. “20 mg/kg dry weight” means 20 milligrams of the measured substance for each kilogram of the material after its water contribution has been excluded according to the reporting method. A wet-weight result uses the wet sample mass as the basis. Because water adds mass to the wet sample, the same underlying material can have different numerical concentrations on wet- and dry-weight bases.
The Exact Learner Job This Reality Lab Owns
This Reality Lab teaches one narrow job: when a scientific report gives a concentration on a dry-weight, wet-weight or as-received basis, identify the denominator before comparing the number with another result.
It does not become a chemistry lesson about every laboratory preparation method, nor a general percentages lesson. Existing PSLE Science owners retain measurement, ratio, fair-comparison and quantity-identity skills. Here we apply those skills to a common reporting label that can quietly change the meaning of a number.
Rebuild the Claim From the Denominator Up
Imagine a composite soil-like sample that has a wet mass of 1.0 kg. Of that mass, 0.6 kg is dry solid material and 0.4 kg is water. Suppose the sample contains 12 mg of Substance X.
| Reporting basis | Numerator | Denominator | Reported concentration |
|---|---|---|---|
| Wet weight | 12 mg Substance X | 1.0 kg wet sample | 12 mg/kg wet weight |
| Dry weight | 12 mg Substance X | 0.6 kg dry material | 20 mg/kg dry weight |
The amount of Substance X did not increase from 12 mg to 20 mg. The denominator changed. The dry-weight number asks, “How much Substance X is present per kilogram of dry material?” The wet-weight number asks, “How much is present per kilogram of the original wet sample?”
Observed, Claimed and Inferred
| Layer | Example | Careful interpretation |
|---|---|---|
| Observed | Masses and an analytical result were obtained from the sample. | These measurements support a concentration on a stated basis. |
| Claimed | “20 mg/kg dry weight.” | The denominator is dry sample mass, not wet sample mass. |
| Inferred | “The wet material also contains 20 mg/kg.” | Unsupported unless moisture is zero or a proper conversion shows that value. |
Why Scientists Use Different Bases
Wet samples can contain very different amounts of water. A fruit, sediment, soil, plant tissue or other material may be wetter one day than another. Reporting on a dry-weight basis can help compare the substance relative to the solid material rather than letting changing water content dominate the denominator. In other situations, wet-weight results are the useful basis because the question concerns the material as it is actually present or used.
The learner should not decide that one basis is “more scientific.” The correct question is: Which basis matches the claim, and are the numbers being compared on the same basis?
Worked Case 1: The Same Sample, Two Correct Numbers
A wet sample weighs 500 g. After drying, 300 g of dry material remains. Laboratory analysis shows the original sample contained 6 mg of Substance Y.
On a wet-weight basis, 6 mg is spread across 0.5 kg of wet material, giving 12 mg/kg wet weight. On a dry-weight basis, the same 6 mg is related to 0.3 kg of dry material, giving 20 mg/kg dry weight. Both numbers can be correct because they answer different denominator questions.
This is an important evidence habit: two different-looking numbers do not automatically mean one laboratory is wrong. First check whether they describe the same scientific quantity on the same basis.
Worked Case 2: A False Trend Created by Moisture
Two samples contain the same amount of Substance Z per kilogram of dry solid. Sample P is much wetter than Sample Q. If both are reported on a wet-weight basis, P can show a smaller mg/kg number simply because extra water contributes to the denominator. A headline that says “P contains less Z” may therefore be describing a difference in reporting basis and moisture, not necessarily a difference in the dry material itself.
A careful learner asks whether the comparison is about the wet material as collected, the dry solid, or something else. The claim must match the basis.
The Representation Check: Small Labels Can Carry Large Meaning
Scientific tables often abbreviate the basis: “dw,” “ww,” “dry wt,” “wet wt,” “as received,” or a footnote below the table. A graph may show only “mg/kg” on the vertical axis while the caption explains the basis. If the reader ignores the caption, two unlike quantities can look directly comparable.
So read in layers: title, axis, unit, footnote, caption, method. The number is only fully meaningful when its basis travels with it.
The Comparison Check
- Are both results dry weight?
- Are both results wet weight?
- If one is dry and one is wet, is moisture information available for a justified conversion?
- Were the same material fraction and preparation rules used?
- Are the units otherwise the same?
- Does the claim concern the wet object, the dry solids, or the total amount?
If the bases differ, do not compare the raw numbers first. Align the quantity meaning first.
Method and Measurement Check
Dry-weight reporting depends on how moisture is determined and what material remains after drying. Temperature, preparation, subsampling and method rules can matter. For a Primary learner, the important point is not to memorise specialist procedures. It is to notice that “dry weight” is the outcome of a defined measurement process, not an imaginary sample with all science problems removed.
If a report compares dry-weight results across time, ask whether the same basis and compatible method were used. If not, an apparent rise or fall may partly reflect a change in how the denominator was defined or measured.
Alternative Explanations for a Higher Number
A higher mg/kg value could mean more of the measured substance relative to the relevant mass. But it could also reflect a different moisture content, a different reporting basis, a different sampled fraction, or a different preparation method. Before explaining the number with a scientific mechanism, confirm that the quantities are comparable.
What Evidence Strengthens the Comparison?
- The reporting basis is stated clearly beside the result.
- Moisture content is measured when conversion between wet and dry bases is needed.
- The same sample type and material fraction are compared.
- Preparation and analytical methods are compatible.
- Units and denominators are made explicit.
- The conclusion says whether it concerns wet material, dry solids or total amount.
- Uncertainty and sample variation are considered when differences are small.
Tempting Reasoning That Fails
| Tempting thought | Why it fails | Better move |
|---|---|---|
| “20 mg/kg is always twice 10 mg/kg.” | Only if the kilograms refer to the same basis and material. | Read the denominator label first. |
| “Drying created a higher amount.” | Removing water changes the denominator; it does not create the measured substance. | Track numerator and denominator separately. |
| “Dry weight is automatically the real value.” | Different bases answer different questions. | Choose the basis that matches the claim. |
| “If two reports use mg/kg, they are directly comparable.” | The same written unit can hide different wet/dry bases or sample fractions. | Check footnotes, captions and methods. |
How Far Can the Conclusion Travel?
A dry-weight concentration supports a statement about the measured substance relative to the dry mass of the tested material. Converting that into a wet-weight concentration needs moisture information and a justified calculation. Generalising to every item in a batch needs sampling evidence. Turning a concentration into a health or safety conclusion needs the appropriate authoritative framework and is outside this Reality Lab.
The scientific habit is simple: move one step at a time and make the denominator travel with the number.
PSLE-Style Transfer Case
Original case: Sample A contains 15 mg/kg dry weight of Substance M. Sample B contains 12 mg/kg wet weight of Substance M. A poster says, “Sample A definitely contains more M than Sample B.”
Question 1: What is the first problem with the comparison?
Explained answer: The results use different mass bases. The 15 mg/kg is relative to dry sample mass while the 12 mg/kg is relative to wet sample mass.
Question 2: What extra information would help?
Explained answer: Moisture information and a valid way to express both measurements on the same basis would allow a fairer numerical comparison.
Question 3: Why should the learner not simply declare B lower?
Explained answer: The raw numbers are attached to different denominators, so their numerical order does not yet answer the intended comparison.
Delayed Independent Return
Later, answer these without looking back: Why can the same sample have a larger numerical concentration on a dry-weight basis? What must you know before converting between dry and wet weight? What is the first question to ask when two reports both say mg/kg but disagree?
Explained Practice
- A sample contains the same 10 mg of a substance before and after water is removed. Does drying create more of the substance? No. The amount remains 10 mg; the sample mass used as denominator changes.
- Two fish-tissue reports say 0.2 mg/kg and 0.3 mg/kg, but one is wet weight and one dry weight. Can you rank them immediately? No. Align the basis first.
- Two sediment results are both reported dry weight using compatible methods. Is the comparison now better formed? Yes. You still need sampling and uncertainty checks, but the denominator basis is aligned.
Routes to the Canonical PSLE Science Owners
For the general skill of checking that two numbers measure the same scientific quantity before comparison, use How to Check That Two PSLE Science Numbers Measure the Same Scientific Quantity Before Comparing Them. If the issue becomes how much of a system or sample was actually measured, continue with How to Decide Whether a PSLE Science Investigation Should Measure the Whole System or a Sample. If a method difference is discovered, use How to Decide What a PSLE Science Investigation Can Still Tell You After You Find a Method Flaw.
Parent and Tutor Teaching Guide
Use a sponge to make the denominator visible. Weigh a wet sponge, then imagine it contains a fixed amount of coloured salt that does not disappear when water is removed. Ask the learner what happens to “amount per kilogram” when the water mass is removed but the salt amount stays. The goal is not a formula drill. It is to see how a denominator can change the numerical concentration.
Next give two mock report cards with identical units but different footnotes: one “wet weight,” one “dry weight.” Ask the learner to circle the words that prevent a direct comparison. Strong learners can then explain why a conversion needs moisture data rather than guessing a factor.
Authoritative Sources and Further Reading
- Singapore Examinations and Assessment Board — 2026 PSLE Science syllabus and assessment objectives.
- Ministry of Education, Singapore — Primary Science Syllabus 2023.
- U.S. Geological Survey — Professional Paper 1655, which explains dry-weight and wet-weight concentration reporting and conversion using sample moisture.
- U.S. Geological Survey — Spatial and temporal trends of mercury in fish, 2007–24, a current example showing dry-weight analysis, moisture measurement and wet-weight conversion.
The Quiet Habit to Keep
When a scientific number says “per kilogram,” do not stop at the unit. Ask the question hidden inside the denominator: per kilogram of what exact material, on what basis?
