Concentration

1. When a solute dissolves in a solvent such as water, it spreads evenly through the liquid to form a solution.
2. The concentration of a solution tells you how much solute is present in a fixed volume of that solution.
3. A concentrated solution has a large amount of solute in a given volume, while a dilute solution has a small amount in the same volume.
4. In terms of particles, a more concentrated solution has more solute particles in each unit of volume, so they are closer together.
5. Adding more water to a solution makes it more dilute, because the same amount of solute is spread through a larger volume.
6. Concentration is always measured per volume of the final solution, not per volume of solvent added.

The cubic decimetre

1. Chemists measure the volume of a solution in cubic decimetres, written ${dm}^3$, rather than in litres.
2. A cubic decimetre is the volume of a cube $10cm$ long on every side, so it holds $10\times 10\times 10=1000{cm}^3$.
3. One ${dm}^3$ is the same volume as one litre, which is about the size of a large carton of juice.
4. Smaller volumes, such as those read from a measuring cylinder or a burette, are measured in cubic centimetres, ${cm}^3$.

Concentration in grams per cubic decimetre

1. A concentration in ${g/dm}^3$ gives the mass of solute in grams dissolved in each $1{dm}^3$ of solution.
2. Read the unit as "grams per cubic decimetre", where the solidus, $/$, means "per".
3. A solution labelled $40{g/dm}^3$ contains $40g$ of solute in every $1{dm}^3$ of solution.
4. This unit is useful when the mass of solute has been measured on a balance.
5. The same unit is sometimes printed as ${g\; dm}^{-3}$, which means exactly the same thing.

Concentration in moles per cubic decimetre

1. A concentration in ${mol/dm}^3$ gives the amount of solute in moles dissolved in each $1{dm}^3$ of solution.
2. The mole, symbol $mol$, is the unit chemists use for an amount of substance, and it counts particles rather than measuring their mass.
3. Read the unit as "moles per cubic decimetre".
4. A bottle of dilute hydrochloric acid labelled $1.0{mol/dm}^3$ contains $1.0mol$ of hydrogen chloride in every $1{dm}^3$ of solution.
5. Most laboratory solutions, such as acids and alkalis, have their concentrations given in ${mol/dm}^3$.
6. The same unit is sometimes printed as ${mol\; dm}^{-3}$.

Comparing concentrations

1. Two concentrations can be compared directly only when they are given in the same unit.
2. Within one unit, the larger number always means the more concentrated solution.
   1. A $2.0{mol/dm}^3$ solution of sodium hydroxide is twice as concentrated as a $1.0{mol/dm}^3$ solution of sodium hydroxide.
3. Always write the unit with the number, because $0.5$ on its own could mean $0.5{g/dm}^3$ or $0.5{mol/dm}^3$, which are very different amounts.