All physical changes occur because of what happens to the particles making up a substance. These particles are invisible to the eye and even the most powerful optics microscopes. Therefore, to understand physical change, scientist use models to help explain and understand their observations.

Understanding physical change.

Most physical changes can be understood by using the Particle model. The particle model is a simplified representation of solids, liquids and gases. Int can explain some but not all of their properties.

The individual particles in solids, liquids and gases are invisible, and so scientists have developed the particle model through indirect observation. This means that scientists use their observations about the behavior of solids, liquids and gases ro deduce information about the particles that make up each state.

The particle model assumes that all forms of matter (solids, liquids and gases) are made up of invisible, ball-like particles that are:

• Hard, in compressible (not able to be squashed) and indivisible (unable to be split)
• attracted to each other
• constantly moving

In solids, the attraction between particles binds them tightly together. For this reason, solids are incomprehensible and hold their own shape. Although the particle sin solids are fixed in position, they vibrate on the spot. These vibrations increase with temperature.

In liquids, the particles are packed tightly together, which makes liquids in compressible. However the particles in a liquid are not stuck to each other as much as they are in solids. This gives liquids the ability to flow and take on the shape of the bottom of their container. Particles in a liquid vibrate but can also move freely though the liquid.

In gases, the particles are not stuck to each other at all and there are large spaces between gas particles. For this reason gases are highly compressible (able to be squashed). The particles are free to move anywhere within their container. The particles move in straight lines until they collide with another gas particle or the side of the container. This is why gases always fill their container.

Expansion and contraction

The physical changes of expansion and contraction can be understood by looking at how the motion of particles changes with temperature. In solids, the particles vibrate in fixed positions. As the temperature increases, so too do the vibrations- pushing the particles further apart and causing the solid to expand (get larger) . When the solid is cooled, the reverse happens. The particles vibrate less, allowing them to be packed more tightly and the solid contracts (shrinks).

The particles in liquids also vibrate more when they are hated. This causes liquids to expand when heated and contract when cooled. However , in liquids the particles are free to move, so liquids tend to expand and contract more than solids.

Gases will always expand or contract to fill their container. However, as the temperature of a gas is increased, the particles travel faster. This means they hit the side so container more frequently ads with more force. If the container is flexible, then increasing the temperature of the gas will cause the balloon to expand.

The reverse is also true. If a gas is cooled, then the particles travel slower and hit the sides of the container with less force and less frequently. If the gas is cooled in a flexible container, then the container will contract.

Extension topic

Gas pressure

A gas cylinder has rigid walls and so doesn’t expand like a balloon when the gas inside warms up. Instead, the pressure of the gas inside the cylinder increase . Pressure measures the force of a container’s walls. When a gas is heated, its particles travel faster and so they slam into the sides of the walls with greater force and greater frequency. This increases the force on the walls and the pressure within the container. In contrast, cooling a gas slows its particles. Fewer collisions occur and the force of each collisions is less.

Changes of state and the particle model

The particle model explains changes of state by looking at the relationship between how the particles move and the attraction between them.

Melting

In a solid, the particles vibrate but are held in position by the forces of attraction between them. As the temperature increases, the vibrations increases and the solid expands. As the temperate increases further, the solid melts. this is because the vibrations become so energetic that attraction between the particles can no longer hold the particles in fixed positions. At this point, the particles become unstuck and start moving freely. However, there is still a small amount of attraction between the particles that holds them together as a liquid.

Freezing

Freezing is the opposite of melting. As the free-moving particles in the liquid are cooled, they slow and become less energetic. Eventually the attraction between the particles is able to fix them in position,forming a solid.

Evaporation

Evaporation occurs when particles in a liquid escape from the surface of the liquid to form a gas. The particle sin a liquid are stuck together but only weakly. As the liquid particles are heated, they move faster until some are able to break free from the other particles at the surface of the liquid. The escaped particles have formed a gas. They have evaporated. A the boiling point, the particles within the liquid are moving so fast that they fly apart and form bubbles within the liquid.

Condensation

Condensation is the reverse. As a gas is cooled, the particles move slower until the forces of attraction between the particles can hold and stick them together to form liquid droplets

Mixing

Mixtures are formed when two or more pure substances are mixed together. A mixture of sand and iron filings is an example of a mixture that you can see. However, the substances can also mix on the partlce level to form solutions and alloys. This process of mixing can also be understood through the particle model.

Solutions and the particle model

Solutions are an example of a mixture. When a solute dissolves, the particles carry them away. As a result, the solute seems to disappear. However, the solute particles are not destroyed. They are spread so thickly throughout the solvent that they seem invisible. the individual solute particles are so small that they cannot be seen with even the most powerful optical microscope.

Diffusion and the particle model

The particles of two gases or two liquids will mix evenly (or diffuse without stirring. This process is known as diffusion. For example, When a bottle of perfume is opened on one side of a room, the perfume particles will diffuse through the air particles. That is why you soon smell the perfume on the other side of the room.

During diffusion, gas particles travel in a zing-sag fashion. Each particle moves in a straight line until it collides with another particle and then changes direction. However, eventually the particles will be distributed evenly throughout the space. Increasing the temperature speeds up diffusion because the particles will travel faster.

Diffusion is even faster if there is a gust of wind or convection currents caused by different temperature. Even when you move or exhale you can send air particles flying across the room, making them diffuse faster through it.

Diffusion in liquids occurs less rapidly because the liquid particles are packed closer together. However, the liquids, will ultimately spread evenly through one another.