have a fixed volume, but they can flow and change their shape. Liquids occupy just slightly more space than when solid (water and ice are exceptions). Gases have no fixed shape or volume. They can be compressed easily.

To explain the properties of solids, liquids, and gases you use the kinetic theory of matter. It is based on the fact that all matter is made up of tiny particles and describes:

the movement of the particles, and
the average distance between particles within each state of matter.

Look at the diagrams to the left that represent the three states of matter.

Each particle in a solid is touching its nearest neighbours and they remain in this fixed arrangement. They cannot move around, but they do vibrate constantly.

The particles in a liquid are also very close together but they can move past each other. This results in a constantly changing, random arrangement of particles.

The particles in a gas have much more space, on average, between them.

They can move around at high speeds and in any direction. This means the particles have a random arrangement. The hotter the gas is, the faster the particles move. The pressure of a gas is caused by the particles colliding with the sides of the container. The more frequent and energetic the collisions are, the higher the pressure of the gas. So, in a sealed container, the pressure of the gas increases with temperature.

Changing state

If a solid is heated and changes directly to a gas without melting, that is, it does not pass through the liquid phase, the change of state is called sublimation.

Look at the changes of state that occur when water is heated and cooled:

Increasing energy

Solid

Melts

Boils

Gas

Liquid

Freezes

Condenses

(Solidifies)

Decreasing energy

Figure 2 The changes of state in water

The hotter a solid is, the faster its particles vibrate. Eventually, the vibrations will be so strong that the particles begin to break free from their neighbours. At this point the solid starts to melt and become a liquid.

The hotter a liquid is, the faster its particles move around. As the temperature rises, more and more particles gain enough energy to escape from the surface of the liquid. Its rate of evaporation increases. Eventually, the liquid boils and bubbles of gas rise and escape from within the liquid.

Each change of state is reversible. They are examples of physical changes. No new substances are formed in changes of state. For example, water molecules

H,0) are the same in ice as they are in liquid water or in water vapour.

Energy changes during changes of state

When you monitor the temperature of a solid as you heat it to beyond its melting point, the results are surprising. The temperature stops rising at the solid's melting point. It remains constant until all the solid has melted, and only then starts to rise again.

At its melting point, the energy provided in heating the solid is being absorbed to break the

quin

forces between the particles in

Melting (or freezing)

point

the solid. Once all the solid has melted, the energy from the heat source raises the temperature of the liquid as expected.

temperature / °C

Solid melts

Solio

Energy is being used here to separate particles from their neighbours

Changes of state which involve particles becoming closer together, that is, condensing and freezing (solidifying), transfer energy to the surroundings as stronger

time / min

Figure 3 The heating curve of a solid

forces form between particles.

Summary questions

Draw a table to summarise the general properties of solids, liquids, and gases, as well as the average distance, arrangement, and movement of their particles.
Describe the changes that occur to the particles as a gas is cooled down to a temperature below its freezing point.
Name the following changes:
a liquid → solid

b gas → liquid

c solid → liquid

d liquid → gas

e solid → gas (in a single step)

Using the kinetic theory of matter, predict how temperature and pressure affect the density of a fixed mass of gas.
Explain why substances have different melting points in terms of their particles.
Evaporation is the change of state that occurs when a liquid changes to a gas below its boiling point. You can investigate the factors that affect the rate of evaporation using a wet paper towel on a high resolution electronic balance.

Plan an investigation into one factor that might affect the rate of evaporation of water from the paper towel, writing a brief method.

States of matter

Practical

Cooling curve

Heat a test tube of stearic acid clamped in a water bath until its temperature reaches about

75°C. Then remove the test tube from the hot water and monitor the temperature as it falls. Plot or print off a graph of the results.

Interface

Temperature sensor

Clamp stand

Stearic acid cooling down
What is the melting point of stearic acid?
Explain the shape of the line on your graph.

Safety: Wear eye protection.

Key points

The three states of matter are solids, liquids, and gases.
The particles in a solid are packed closely together, fixed in their positions and vibrate.
The particles in a liquid are also close together but can slip and slide over each other in random motion.
The particles in a gas have, on average, lots of space between them and zoom around randomly.
Melting and boiling take in energy from the surroundings as they take place, whereas freezing and condensing transfer energy to the surroundings when they occur.

Flashcards Guide

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