Concentration is crowding
Drop one sugar cube into a teaspoon of water and it's intensely sweet; drop the same cube into a bathtub and you'd never taste it. The amount of sugar didn't change — the crowding did. Concentration is simply a number that captures this crowding: how much of the dissolved substance is packed into a given amount of liquid. More substance in less liquid means higher concentration; the same substance spread through more liquid means lower.
The dissolved substance is the solute; the liquid doing the dissolving is the solvent; together they make the solution. Concentration always answers the same question — *how much solute per how much solution* — but there are several honest ways to fill in those two blanks, and choosing the right one is most of the skill.
Molarity: moles per litre
The lab's everyday unit is molarity: the number of moles of solute in one litre of solution. We write it with a capital M, so "0.1 M sodium chloride" means one-tenth of a mole of salt is dissolved in each litre. The formula is short: molarity = moles of solute ÷ litres of solution.
Why moles and not grams? Because reactions count particles, and molarity tells you directly how crowded the *particles* are. Two solutions of equal molarity have equal particle crowding even if one weighs much more per litre. That is exactly what you want when you're about to make those particles react — it is the heartbeat of quantitative analysis.
Preparing a solution from scratch
Say you want 250 millilitres (a quarter litre) of 0.20 M sodium chloride, whose molar mass is about 58.5 grams per mole. How much salt do you weigh?
- Find moles needed: molarity × litres = 0.20 mole/L × 0.250 L = 0.050 mole.
- Convert moles to grams: 0.050 mole × 58.5 grams/mole ≈ 2.93 grams. Weigh that on the balance.
- Dissolve the salt in a little water in a 250 mL volumetric flask, swirl until clear, then add water exactly up to the mark.
- Stopper and invert several times to mix. You now have 250 mL of 0.20 M solution.
Stock solutions: make strong, keep handy
Analysts rarely weigh fresh solid every time they need a solution. Instead they prepare a concentrated stock solution once, store it, and weaken it on demand. A stock is just a deliberately strong solution kept as a starting point — like a concentrated cordial you'll dilute later. It saves weighing, reduces error from tiny masses, and keeps your concentrations consistent across a whole project.
We'll see the arithmetic of weakening a stock in a later guide. For now, hold onto the picture: weigh once to make something crowded, then thin it out as needed.