Physics
Cambridge IGCSE

TOPIC 2: THERMAL PHYSICS

2.1a States of Matter

This topic is all about heat, and how it affects substances. Before we can understand the effects of heat, we need to have a clear understanding of the particle nature of matter, and how scientists have come to understand the differences in arrangments in particles in the three states of matter.

There are three states of matter that you should be able to describe in detail: Solids, liquids and gases. We are all familiar with ice, water and steam, and how heat energy can cause ice to melt and water to boil. In this section, we will look at the differences between the states of matter, and also what happens when a change of state occurs. To start, we need to take a closer look at each state of matter.

Solids

The particles in a solid have several features as shown in this animation:

• They vibrate around a fixed position.
• They are close together (tightly packed).
• They are held in position by strong forces (bonds) between them.

It takes energy to break these bonds and pull the particles apart. The higher the temperature, the faster the particles vibrate, and eventually they will have enough kinetic energy to break the bonds.

Figure 1. Particles in a solid

Liquids

The particles in a liquid:

• Move past each other randomly.
• Have weak forces between them.
• Are still close together.
• Take the shape of the bottom of the container they are in.

Gravity will pull them to the bottom of the container. The weak forces tend to keep the particles together in clumps as they slide past each other.

Figure 2. Particles in a liquid

Gases

The particles in a gas:

• Have wide spaces between them.
• Have (effectively) no forces acting between them.
• Move freely and randomly.
• Fill the container they are in.

For most gases, these particles are gas molecules of 2 or more atoms joined together. As gases are widely spaced, they can be easily compressed, unlike solids and liquids which are hard to compress.

Figure 3. Particles in a gas
All particle animations by Julio Miguel A Enriquez and Monica MuĂ±oz -Wiki Learning Tec de Monterrey. CC BY-SA 4.0

Particles

What are the particles shown above? These could be atoms in a substance like aluminium, or molecules of water. You should know that the forces, motion and distances between these particles affects the properties of all substances. In addition, we could add in here the forces, motion and and distances between ions and electrons which will also affect the physical as well as chemical properties of all matter.

Changing state

When a solid is heated, the particles vibrate faster and faster, and the bonds between them get weaker. Eventually, the particles have enough energy to break free of the strong bonds, and move more freely. The solid has melted and become a liquid. if cooled, it will freeze and become solid again.

Similarly for liquids, heating will increase the energy again and cause them to move faster as they gain kinetic energy. Eventually even the liquid's weak bonds are broken and the substance becomes a gas.

Some particles move faster than others due to random collisions. this means that some particles will break free before others. This is how a liquid evaporates.
If it is heated further, all of the particles will have enough energy to break the bonds and the liquid will boil - all turning to gas.

You should be familiar with all of the terms shown in figure 4:

melting

LIQUID

boiling/ evaporating

GAS

freezing

condensing

Figure 4: Changing state

We will study changes in state in more detail in section 2.2c.

Questions:

1. Liquid oxygen can be stored in containers for transporting.

• a) Describe the arrangement of oxygen molecules in a liquid.
• b) Describe two changes to this arrangement as the oxygen is released from the container as a gas.

a) The molecules are close together, moving past each other, taking the shape of the (bottom of the) container, moving randomly, have weak forces between them holding them near to each other

b) The molecules move far apart, move faster, have no forces keeping them together, fill the container they go into or spread through the atmosphere.

2. Some ice cubes are left outside on a sunny day.

• a) Describe the changes that take place to the molecular arrangement in the ice as it melts.
• b) Explain what is meant by a change of state.

a) Ice is a solid, so to begin with the molecules are arranged in a fixed pattern and are held in place by strong forces between them. They vibrate around a fixed point, and are close together. As the ice melts, heat energy is used to break/weaken the bonds between the molecules, allowing them to move more freely with only weak forces between them. They stay close together, moving randomly.

b) A change of state occurs when a substance changes between a solid, liquid or gas state. For example, the ice cube changes state from solid to liquid as it melts.

Brownian Motion

In 1827, a Botanist called Robert Brown was looking at pollen grains under a microscope. The pollen grains were suspended in water. He noticed they seemed to be moving randomly. This random motion of small particles was later observed with smoke particles suspended in air, so this effect was nothing to do with the pollen grains being a living thing.

This motion remained a mystery for over 80 years, until a young Albert Einstein came up with an explanation for the seemly random motion of the suspended particles.

Fig 5 . The Brownian Motion of Polymer Particles
Yasrena, CC BY-SA 4.0, via Wikimedia Commons

Einstein's Explanation

He put forward that the pollen grains (or similar microscopic particles) were being hit by atoms/molecules. These were moving at high speed, and the random collisions would seem to make the pollen grains move small amounts in random directions. This explanation fitted the observations, but was also ground breaking at the time, as it meant that scientists now had observational proof that atoms exist. Before this point, the idea of all states of matter being made of particles was just a theory.

Fig 6 . Brownian Motion, showing the atoms causing the motion
Lookang, CC BY-SA 3.0, via Wikimedia Commons

In summary, Brownian motion is a really important development in science, as it first demonstrated the existence of atoms and molecules of air, and caused a major change in scientifc thinking at the turn of the 20th Century.

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