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7.2 Understanding physical change (The particle model (Liquids (Particles…
7.2 Understanding physical change
The particle model
Solids are compressible and hold their shape, liquid are incompressible and flow to take the shape of their container, gases are compressible and fill their entire container
Having a model of the structure of matter helps scientist to predict and explain the properties of matter
Most physical changes can be understood by using the particle model which is a simplified representation of solids, liquids and gases
The way matter behaves differently to what the model predicts, it gives clues to improve the model
The particles of solids, liquids and gases are too small for the naked eye which is why the model was developeed
The particle model assumes that all matter is made up of invisible, ball-like particles that are: hard, incompressible (not able to be squashed) and invisible (unable to be split), attracted to each other, constantly moving
Solids
Solids particles may be fixed in position but they vibrate on the spot, the vibrations increase as the temperature increases
In solids, the attraction between particles binds them tightly together
Liquids
Particles in a liquid are not stuck to each other as much as they are in solids which gives liquids the ability to flow and take on the shape of its container
Particles in a liquid vibrate but can also move freely through the liquid
In liquids, the particles are packed tightly together, which makes liquids incompressible
Gases
The particles move in straight lines until they collide with another gas particle or the side of the container
In gases, the particles are not stuck to each other at all and there are large spaces between gas particles this is why gases are highly compressible
Changes of state and the particle model
The particle model explains changes of the state by looking at the relationship between how the particles move and the attraction between the particles
Melting
As temperatures increase vibrations increase and the solid expands, as the temperature increases further, the solid melts
The solid melts because the vibrations become so energetic that the attraction between the particles that the particles can no longer hold themselves in a fixed position
In a solid, the particles vibrate but are held in position by the forces of attraction between them
Freezing
At one point the particles will move so slowly that the attraction between them in position and a solid is formed
Freezing is the opposite of melting, as the free moving particles in the liquid cool they slow and become less energetic
Evaporation
As 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
At the boiling point, the particles within the liquid are moving so fast that they fly apart and form bubbles within the liquid
Evaporation occurs when particles in a liquid escape from the surface of the liquid to form a gas
Condensation
As the temperature falls, the particles of gas move more slowly, until the forces of attraction between the particles stick them together
Condensation converts gases into liquids and is therefore the reverse of evaporation
Condensation occurs when a gas is cooled
Expansion and contraction
When a substance contracts, its volume decreases, the atoms become packed more closely together and the substance becomes more dense
The physical changes of expansion and contraction can be understood by looking at how the motion of particles changes with temperature
Expansion and contraction change the size of a substance, when a substance expands its volume increases and the atoms are spread furtherapart
In solids as the temperature increases, the particles vibrate more, the vibrations push the particles further apart and cause the solid to expand
When a solid is cooled, the reverse happens, the particles vibrate less, allowing them to be packed more tightly and the solid contracts
In liquids the particles are free to move so they 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 is what causes balloons to expand
If a gas is cooled, then the particles travel more slowly and hit the sides of their container less frequently and with less force
Mixing
Substances can also mix on the particle level to form solutions and alloys
Mixing can be understood through particle models
A mixture is formed when two or more substances are mixed together, a mixture of sand and iron filings is an example of a mixture that you can see
Solutions and the particle model
Solute particles are not destroyed, they are simply spread so thinly throughout the solvent that they seem invisible
Individual solute particles are so small that they cannot be seen
Solutions are an example of a mixture, when a solute dissolves, the solvent particles surround the solute particles and carry them away
Diffusion and the particle model
When a bottle of perfume is opened on one side of a room, the perfume particles will diffuse through the air particles which is why you soon smell the perfume on the other side of the room
During diffusion, gas particles travel in a zig-zag fashion, each particle moves, each particle moves in a straight line until it collides with another particle and then changes direction
When two liquids are mixed togethe, their particles will diffuse and mix evenly without stirring, this happens when two gases are mixed together this is known as diffusion
Eventually particle will be distributed evenly throughout the space as diffusion is the movement of particles will be distributed evenly throughout the space
Increasing the temperatures speeds up diffusion because the particles will travel faster