How does the arrangement of particles explain the three states of matter, and how do we separate mixtures?
Describe the three states of matter using the particle model, explain changes of state, and choose suitable methods to separate mixtures
A focused N(A)-Level answer on matter. The particle model of solids, liquids and gases, changes of state, and choosing filtration, evaporation, distillation or chromatography.
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What this dot point is asking
SEAB wants you to describe the three states of matter using the particle model, to explain what happens to the particles during changes of state, and to pick the right method to separate a mixture. The central idea is that the way particles are arranged and how much energy they have decides whether a substance is a solid, liquid or gas.
The answer
The particle model
All matter is made of tiny particles. The state depends on how they are arranged and how much they move:
- Solid: particles packed closely in a fixed, regular pattern, vibrating about fixed positions. A solid has a fixed shape and volume.
- Liquid: particles close together but with no fixed pattern, able to slide past each other. A liquid has a fixed volume but takes the shape of its container.
- Gas: particles far apart, moving quickly and randomly in all directions. A gas spreads to fill its container.
Changes of state
Heating gives particles more energy so they move more and break free; cooling does the reverse. The changes are:
- melting: solid to liquid,
- boiling or evaporating: liquid to gas,
- condensation: gas to liquid,
- freezing: liquid to solid.
During a change of state the temperature stays the same while the energy is used to change the arrangement of the particles.
Separating mixtures
A mixture contains substances that are not chemically joined, so they can be separated by physical methods. Choose the method to match the mixture:
- filtration: separates an insoluble solid from a liquid (for example sand from water). The solid stays as residue; the liquid passes through as filtrate.
- evaporation or crystallisation: gets a dissolved solid back from its solution (for example salt from salt water).
- distillation: gets the pure liquid (the solvent) back from a solution by boiling it off and cooling it.
- chromatography: separates and identifies coloured substances such as dyes in ink.
Examples in context
Example 1. Getting drinking water from sea water. Distillation boils the sea water to make steam, leaving the salt behind, then cools the steam back to pure liquid water. This is how some countries with little fresh water produce drinking water from the sea.
Example 2. Testing food colourings. Chromatography spots different food dyes on paper and lets a solvent carry them up at different rates. The pattern of spots shows which dyes are present, which is useful for checking what is really in a sweet or drink.
Try this
- Cue. Describe the particles in a liquid. Close together, no fixed pattern, able to slide past one another.
- Cue. Name the method to separate pure water from ink. Distillation (boil off and condense the water).
- Cue. State what happens to the temperature while ice melts. It stays constant until all the ice has melted.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marks(a) Describe the arrangement and movement of particles in a solid and in a gas. (b) Name the change of state when a gas turns into a liquid.Show worked answer →
(a) In a solid the particles are packed closely in a fixed, regular pattern and only vibrate about fixed positions. In a gas the particles are far apart, arranged randomly, and move quickly in all directions.
(b) A gas turning into a liquid is called condensation.
What markers reward: close, fixed, vibrating particles for a solid; far apart, random, fast-moving for a gas; and naming condensation.
Original3 marksA student has a mixture of salt dissolved in water with some sand at the bottom. (a) Name the method to remove the sand. (b) Name the method to get the salt back from the salt solution.Show worked answer →
(a) Filtration removes the insoluble sand, which stays as residue in the filter paper while the salt solution passes through.
(b) Evaporation (evaporating the water away) leaves the salt behind as crystals.
What markers reward: filtration to remove the insoluble solid, and evaporation (crystallisation) to recover the dissolved salt.
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