What Is Concentration?
Concentration measures how much of a solute (the dissolved substance) is packed into a given volume of solution. A concentrated solution has a lot of solute; a dilute solution has very little.
Units and Formula
Concentration is often expressed in grams per decimetre cubed (g/dm³) or moles per decimetre cubed (mol/dm³):
Note: 1 dm³ = 1 litre = 1000 cm³
Example: 4 g of salt dissolved in 0.5 dm³ of water.
Concentrated vs Dilute
- Concentrated: high ratio of solute to solvent — strong flavour, strong reaction, more colour
- Dilute: low ratio — weaker properties
Adding more solvent (water) dilutes a solution. The amount of solute stays the same, but the concentration falls.
Dilution Calculations
When a solution is diluted:
where C = concentration and V = volume.
Example: 50 cm³ of 2 mol/dm³ acid is diluted to 200 cm³. What is the new concentration?
When diluting concentrated acids in the lab, always add acid to water — never water to acid. Adding water to concentrated acid produces intense heat and can cause dangerous spattering.
Concentration and Reaction Rate
A higher concentration means more particles per unit volume, so collisions between reactant particles are more frequent → reactions happen faster. This is why a 2 mol/dm³ acid reacts faster than 0.5 mol/dm³ acid with the same metal.
Real-World Examples
- Medicine: drugs are given in doses calculated per kg body weight; concentration in the bloodstream matters
- Cooking: strength of flavour in sauces depends on concentration of dissolved compounds
- Water quality: safe levels of nitrates, chlorine, and fluoride in drinking water are specified in mg/dm³
A serial dilution is used to make a series of solutions of decreasing concentration. You take a fixed volume, add solvent, mix, then take from that to dilute again. This is used in hospitals and labs to make a wide range of concentrations precisely.
Real world: Drug dosage — concentration in your bloodstream
When a doctor prescribes a medicine, the dose is calculated based on concentration in the bloodstream. Too low a concentration and the drug has no effect; too high and it becomes toxic. Drugs are given in milligrams per kilogram of body weight precisely because larger people have more blood volume (more solvent), diluting the drug more. Chemotherapy drugs have extremely narrow therapeutic windows — a small increase in concentration can kill healthy cells. Getting concentration right is literally a matter of life and death.
Real world: Nitrate concentration in drinking water is legally regulated
Fertilisers contain nitrates, which dissolve in rain and leach into groundwater. In high concentrations (above 50 mg/dm³), nitrates in drinking water are dangerous, especially for infants — they can cause "blue baby syndrome" (methaemoglobinaemia) where nitrites block oxygen transport in blood. The EU and WHO set maximum nitrate concentrations in tap water. Water companies continuously measure and manage nitrate concentration, applying treatment to remove excess. Concentration monitoring protects public health.
Real world: Squash drinks — concentrated to dilute
Fruit squash (cordial) is sold as a concentrated solution — typically dilute 1:4 to 1:10 with water. The concentrated form is cheaper to transport (less water weight) and has a longer shelf life (higher sugar and acid concentrations inhibit microbial growth). Using C₁V₁ = C₂V₂: if you dilute 50 cm³ of 10× squash concentrate to 500 cm³, the final concentration is (10 × 50) ÷ 500 = 1× — just right. This dilution calculation is the same chemistry as diluting acids in a school laboratory.
Where do you see this in real life? Drug dosing based on body weight (concentration in bloodstream), nitrate limits in drinking water (regulated in mg/dm³), squash drinks sold concentrated then diluted, espresso vs filter coffee (different concentrations of the same dissolved compounds), bleach solutions diluted for cleaning — concentration is the most important variable in medicine, food science, and water quality.