States of matter and separation
How particles are arranged and move in solids, liquids and gases, and how mixtures are separated and purity is checked.
📌 Key points
- In a solid particles are closely packed in a regular arrangement and only vibrate; in a liquid they are close together, randomly arranged and move past each other; in a gas they are far apart, randomly arranged and move quickly in all directions.
- Melting, boiling and evaporation need energy to overcome the forces of attraction between particles; freezing and condensation release energy. The temperature stays constant during a change of state.
- For a gas, raising the temperature at constant volume increases the pressure because particles move faster and hit the walls more often and with more force; raising the pressure at constant temperature decreases the volume.
- Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random movement. Gases with a lower relative molecular mass diffuse faster.
- Filtration separates an insoluble solid from a liquid; crystallisation obtains a soluble solid from its solution; simple distillation separates a solvent from a solution.
- Fractional distillation separates miscible liquids with different boiling points, such as ethanol and water, or the fractions in petroleum.
- Paper chromatography separates soluble substances. Rf = distance moved by substance ÷ distance moved by solvent front. A pure substance gives a single spot.
- A locating agent is sprayed on a chromatogram to make colourless spots visible.
- A pure substance melts and boils at a sharp, fixed temperature. Impurities lower the melting point and raise the boiling point, and make the substance melt over a range of temperatures.
📖 Key terms
- Diffusion
- Net movement of particles from higher to lower concentration due to random movement.
- Solvent
- The liquid in which a solute dissolves.
- Filtrate
- The liquid that passes through the filter paper.
- Residue
- The solid left behind in the filter paper.
- Rf value
- Distance moved by a substance ÷ distance moved by the solvent front.
- Pure substance
- A substance that contains only one element or compound, with nothing mixed with it.
✏️ Worked example
In a chromatogram the solvent front moved 8.0 cm from the baseline and a dye spot moved 3.2 cm. Calculate the Rf value of the dye.
- Rf = distance moved by substance ÷ distance moved by solvent front.
- Both distances are measured from the baseline, in the same units (cm).
- Rf = 3.2 ÷ 8.0
- Rf = 0.40 (no units, and always less than 1).
Draw the chromatography baseline in pencil (it does not dissolve) and above the solvent level, so the spots do not just dissolve into the solvent.
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