Solubility and concentration — Chemistry, 11–13
A substance can spread through a liquid, but only up to a limit. Concentration tells how much solute is present in a given amount of solution.
Dissolving and measuring it
When a solute dissolves, its particles spread among the particles of a solvent, such as water. Solubility is the most that can dissolve under particular conditions. Concentration compares the amount of solute with the amount of solution, so two clear liquids can still be very different.
Why amounts matter
A spoonful of sugar in a small cup tastes much sweeter than the same spoonful in a jug. People needed a way to describe this difference clearly when making drinks, medicines and laboratory solutions. Solubility also explains why adding more solid eventually leaves grains at the bottom instead of making the liquid endlessly sweeter.
Finding a concentration
Dissolve 10 grams of salt in enough water to make 200 millilitres of solution. Concentration in grams per 100 millilitres is found by dividing 10 by 200 and multiplying by 100. The result is 5 g per 100 mL. A 100 mL sample would therefore contain about 5 grams of salt if the solution is evenly mixed.
More liquid is not more solute
A common mistake is to say that a larger glass is more concentrated because it contains more liquid. Concentration depends on the comparison between solute and solution, not on the total amount alone. A large glass can contain more dissolved sugar but be less concentrated than a small glass with a stronger mixture.
Solutions in daily life
Cooks use dissolving when they make syrup or salt water, and doctors use carefully measured solutions in medicines and rehydration drinks. Water treatment also depends on substances dissolving and being present in safe amounts. The idea helps people follow instructions instead of guessing how much powder or liquid to add.
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