How Strong Is a Solution?
- Chemistry
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- A solution is a solute dissolved in a solvent, the same all the way through.
- Concentration is the amount of solute per volume of solution, such as grams per litre.
- Dilution adds solvent, lowering the concentration without removing any solute.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Solute, solvent, solution
When sugar dissolves in water, chemists name the parts. The sugar, which dissolves, is the solute. The water it dissolves in is the solvent. The mixture is a solution, and it is the same all the way through: a spoonful from the top tastes as sweet as one from the bottom. Water dissolves so many things that it is called the universal solvent, though oil and sand will not dissolve in it. The same words work for other solutions: air is oxygen and other gases in nitrogen, and brass is zinc dissolved in copper. What matters most is often how much is dissolved.
Grams per litre
Concentration is the amount of solute in a given amount of solution. One common unit is grams per litre. Dissolve 10 grams of salt in enough water to make 1 litre and the concentration is 10 grams per litre. Dissolve the same 10 grams in only half a litre and it is 20 grams per litre, twice as strong, though the salt is the same. The rule is always mass of solute divided by volume of solution. Seawater has about 35 grams of salts per litre. Drink labels give sugar in grams per 100 millilitres, and medicines are measured in milligrams per millilitre, where a mistake matters. Concentration lets a nurse or a cook give the same strength every time.
Diluting
Adding more solvent lowers the concentration without removing any solute; this is called dilution. Squash is sold concentrated because moving less water is cheaper, and the label says to add, say, four parts water to one part squash. The sugar and flavour are all still there, spread through five times the volume, so the drink is one fifth as concentrated. Chemists dilute in steps. Add 90 millilitres of water to 10 millilitres of a solution and it is one tenth as concentrated. Do it again and it is one hundredth. After a few steps a dye fades beyond what the eye can see, though it is still there. Not seeing a substance never proves it is absent.
Curriculum links: NGSS MS-PS1-2 · CCSS 6.RP.A.3