A water report says 12 mg/L. A soil test says 12 ppm. A bottle in the lab says 1 mol/L. A cleaning product says 5%. All of these answer the same question, "how much is in here?", but they are not the same kind of answer, and mixing them up is one of the easiest ways to get a lab result wrong.
Every concentration is a fraction
A concentration is always an amount of the thing you care about divided by an amount of something else. The trick is knowing what it is divided by.
- Moles per litre divides by the volume of the whole solution.
- Milligrams per kilogram divides by the mass of the whole sample.
- Percent can divide by mass or by volume, and labels do not always say which. 5% by mass and 5% by volume are different amounts.
Before you trust or compare any concentration, ask what it divides by.
The mistake almost a third of scientists may make
To make one litre of a solution at a set strength, you dissolve the solid in a bit less than a litre of water, pour it into a special flask marked at exactly one litre, and top it up to the mark. What you should not do is add the solid to a full litre of water. The solid takes up room, so you end up with more than a litre and a weaker solution than the label says.
That sounds like a beginner's mistake. But a 2020 survey of people who make solutions in their work concluded that almost a third of scientists may be doing exactly this. The fix costs nothing: dissolve first, then fill to the mark.
"ppm" needs a denominator too
For soil, ppm usually means milligrams per kilogram. For water, it usually means milligrams per litre. Those only match when a litre of the liquid weighs a kilogram, which is roughly true for plain water and not true for much else. 12 ppm of lead in soil and 12 ppm of lead in water are not the same kind of number.
Diluting: the amount stays the same
When you dilute something, you add water and nothing else, so the amount of the stuff you care about does not change. Only the volume does. That is why labs can calculate dilutions with one simple rule.
One habit saves a lot of trouble: say a dilution in a way nobody can misread. "One millilitre made up to ten" is clear. "1:10" is not, because some labs read it as 1 part in 10 total and others as 1 part plus 10.
Why this matters to you
The nutrition label on your food, the chlorine in a pool, the strength of a medicine, the lead in your tap water: all of them are concentrations. When you read one, the most useful question is the simplest: per what?
Want the full lesson, with the conversions worked through, dilution practice and the questions?
Read the full lesson → Free: sign in with your email, no password.
References
- D. Harvey, Analytical Chemistry 2.1, 2.2 Concentration (LibreTexts).
- R. D. Calvert, C. R. Crawford, C. A. Running, "The prevalence of improper solution-making technique places molar solutions in crisis", Journal of Food Science Education 19 (2020) 183–191.
- A. Thompson, B. N. Taylor, Guide for the Use of the International System of Units (SI), NIST Special Publication 811 (2008).