States of Matter
The Three States of Matter
The three states of matter are solid, liquid and gas. They are distinguished by the arrangement, movement and energy of their particles.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, closely packed | Random, close together | Random, widely spread apart |
| Movement | Vibrate only (fixed positions) | Flow and slide past each other | Move rapidly and freely in all directions |
| Energy | Least kinetic energy | More kinetic energy than a solid | Most kinetic energy |
| Shape | Fixed shape | Takes the shape of its container | Fills its container completely |
| Volume | Fixed volume | Fixed volume | No fixed volume |
Interconversions Between States
State changes are physical changes, not chemical ones. The particles themselves remain the same; only the forces between them change. Physical changes are reversible.
| Change | Name | Energy Change |
|---|---|---|
| Solid to liquid | Melting | Energy taken in (endothermic) |
| Liquid to solid | Freezing | Energy released (exothermic) |
| Liquid to gas | Boiling | Energy taken in (endothermic) |
| Gas to liquid | Condensing | Energy released (exothermic) |
| Liquid to gas (at surface) | Evaporation | Energy taken in |
| Solid directly to gas | Sublimation | Energy taken in |
Melting and freezing occur at the same temperature: the melting point. Boiling and condensing occur at the same temperature: the boiling point.
The stronger the forces of attraction between particles, the more energy is needed to overcome them, and therefore the higher the melting and boiling points.
What happens during state changes?
- Going from solid to liquid to gas: particles gain kinetic energy, move more freely, become more randomly arranged and further apart.
- Going from gas to liquid to solid: particles lose kinetic energy, move less, become more regularly arranged and closer together.
Evaporation vs Boiling
Evaporation occurs only at the surface of a liquid, at temperatures below the boiling point. High-energy particles at the surface can escape. It occurs over a range of temperatures; a larger surface area and higher temperature speed it up.
Boiling occurs throughout the liquid, with bubbles of gas forming below the surface, at a single specific temperature.
Sublimation
A few solids, such as iodine and solid carbon dioxide (dry ice), sublime directly from solid to gas without passing through the liquid state.
Predicting Physical State
Given melting point and boiling point data for a substance, you can predict its state at any temperature:
- Below the melting point: solid
- Between melting point and boiling point: liquid
- Above the boiling point: gas
Diffusion
Diffusion is the movement of particles from an area of high concentration to an area of low concentration. It occurs because particles are in constant, random motion.
- Diffusion does not occur in solids because particles cannot move from place to place (they can only vibrate).
- Diffusion occurs in liquids and gases.
- The results of experiments involving the dilution of coloured solutions (e.g. adding a drop of dye to water) and the diffusion of gases (e.g. the smell of a gas spreading across a room) are explained by the movement of particles from high to low concentration.
Solubility
Solvent: the liquid in which a solute dissolves (e.g. water).
Solute: the substance that dissolves in the liquid to form a solution (e.g. salt).
Solution: the mixture formed when a solute dissolves in a solvent.
Saturated solution: a solution in which no more solute can dissolve at that temperature.
Higher Tier
Solubility is measured in grams of solute per 100 g of solvent. For example, if the solubility of potassium nitrate at 20 °C is 31 g/100 g water, then 31 g will dissolve in 100 g of water at that temperature.
Solubility curves show how solubility changes with temperature. In general:
- The solubility of solids increases as temperature increases.
- The solubility of gases increases as pressure increases.
- Any mass below the solubility curve for a given temperature means the solution is unsaturated.
- Any mass above the curve means the solution would be supersaturated (unstable).
Exam Questions
Q: Describe the arrangement and movement of particles in a liquid.
Particles in a liquid are randomly arranged and close together. They can flow and move past each other but are not in fixed positions.
Q: Explain why diffusion does not occur in solids.
In solids, particles can only vibrate in fixed positions; they cannot move from place to place, so diffusion cannot occur.
Q: What name is given to the process in which a gas turns into a liquid?
Condensing (condensation).
Q: Explain why melting is a physical change.
Melting is a physical change because no new substance is formed. The particles are the same before and after; only the forces between them change. The change is reversible.
Q: A substance has a melting point of 50 °C and a boiling point of 120 °C. What is its physical state at 80 °C?
At 80 °C the substance is above its melting point but below its boiling point, so it is a liquid.
Exam Tip
When describing a state change, always refer to the energy of the particles and the forces between them. Saying “the particles move faster” alone is not enough; explain why (they gain kinetic energy) and what this means for the forces between them.