CHEMISTRY · Stage 8 · Chapter 8: Pure Substances and Solutions

Chemistry 8.4 Solutions and concentration — Dilute, Concentrated and Particle Ratios

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8.4 Solutions and concentration — Dilute, Concentrated and Particle Ratios — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define concentration, dilute and concentrated, explain them with the particle model and calculate concentration in g/dm3.Solution = a mixtur…

Concentrated solution
Dilute solution
Ethanoic acid
Amount, not strength
Same volume
Same number of particles

Part 1 of 6Solutions, solute and solvent

Solution a mixture that forms when a solute dissolves in a liquid.
Solvent the liquid in which the solute dissolves.
Solute the substance that dissolves.

Part 2 of 6Dilute and concentrated

Concentrated solution — it has a large amount of solute dissolved in a given volume and is mixed with little water.
Dilute solution — it has a small amount of solute in a given volume and is mixed with a large amount of water.
Ethanoic acid — in vinegar it is dilute and safe to eat in pickles, but concentrated ethanoic acid in a lorry is corrosive and can burn eyes and skin.
- Concentrated acids are more corrosive than dilute ones.
Squash — it is made as a concentrated solution that you dilute by adding water.
Amount, not strength — concentrated and dilute describe the amount of solute, whereas a strong acid is one that ionises completely.

Part 3 of 6Concentration and the particle model

Concentration = a measure of the number of solute particles in a volume of solution.
- A concentrated solution has more solute particles in the same volume, so the ratio of solute to solvent particles is higher.
Same volume — in two cups of tea with the same volume, the cup with more tea particles has the higher concentration.
Same number of particles — in two beakers with the same number of tea particles, the one with the smaller volume has the higher concentration.
Strength of particle diagrams — they show that a concentrated solution has more solute particles in a certain volume.
Limitation of particle diagrams — they are not to scale, since a real cup contains millions and millions of water and solute particles.

Part 4 of 6Calculating concentration

Units — volume is measured in cubic decimetres, where 1 dm³ = 1000 cm³ = 1 litre.
Formula — concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).
- Rearranged, mass = concentration × volume.
Converting — divide cm³ by 1000 to get dm³ before calculating.
Volume used — use the volume of the whole solution, not the volume of the solvent.

Part 5 of 6Dilution

Adding water — the amount of solute stays the same but the volume increases, so the concentration falls.
Doubling the volume — the concentration halves.

Solution	Mass of solute (g)	Volume (dm³)	Concentration (g/dm³)
A	10	0.5	20
B	10	1.0	10

Part 6 of 6Acid spill and making a solution

1Acid spill2 points

— firefighters wear special clothes to protect their skin and eyes and add water to a leak of concentrated sulfuric acid, H₂SO₄.

The water dilutes the acid and makes the area safe.
Sulfuric acid contains hydrogen, sulfur and oxygen atoms.

2Making a known concentration1 point

— weigh the solute, dissolve it in less than the final volume of water, stir, and transfer to a volumetric flask.

Rinse the beaker into the flask and make up to the mark with water.

3Equipment

— a measuring cylinder gives rough volumes, and a volumetric flask or pipette gives accurate ones.

4Colour

— a more concentrated coloured solution is darker.

Last updated: 13 October 2026