States of Matter

Kinetic Theory

Solids, liquids & gases

Solids
 Solids have a fixed volume and shape, and they have a high density.
 The atoms vibrate in position but cannot change .
 The particles are packed very closely together in a fixed and regular pattern.

Liquids
 Liquids also have a fixed volume but adopt the shape of the container.
 They are less dense than solids (an exception is water), but much denser than gases.
 The particles move and slide past each other which is why liquids adopt the shape of the
container and why they can flow freely.

Gases
 Gases do not have a fixed volume, and, like liquids, take up the shape of the container.
 Gases have an incredibly low density.
 Since there is a lot of space between the particles, gases can be compressed into a much
smaller volume.
 The particles are far apart and move randomly and quickly (around 500 m/s) in all directions.
 They collide with each other and with the sides of the container (this is how pressure is
created inside a can of gas)

Summary of the properties of solids, liquids, and gases

Solid Liquid Gas

Diagram

Arrangement
Regular arrangement Randomly arranged Randomly arranged
of particles
Movement of Vibrate about a fixed Move around each
Move quickly in all directions
particles position other
Closeness of
Very close Close Far apart
particles

Density High Medium Low

Energy of Low energy Greater energy Highest energy

particles
Exam Tip
You can explain the differences in the physical properties of solids, liquids, and gases by referring to
the arrangement and motion of particles. This is called the kinetic theory of matter.

States of Matter

State changes.

 State changes occur when:

o Solids become liquids.
o Liquids become gases.
o Gases become liquids.
o Liquids become solids.
 Each state change requires a change in the energy, arrangement, and movement of the
particles.

The state changes.

The inter-conversions / state changes are shown in relation to energy.

Melting

 Melting is when a solid changes into a liquid.

 Requires heat energy which transforms into kinetic energy, allowing the particles to move.
  Occurs at a specific temperature known as the melting point (m.p.)

Freezing

 Freezing is when a liquid changes into a solid

 This is the reverse of melting and occurs at the same temperature as melting, hence the
melting point and freezing point of a pure substance are the same. Water, for example,
freezes and melts at 0 ºC.
 Requires a significant decrease in temperature (or loss of thermal energy) and occurs at a
specific temperature.

Boiling

 Boiling is when a liquid changes into a gas.

 Requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for
liquid particles to escape from the surface and within the liquid.
 Occurs at a specific temperature known as the boiling point (b.p.)

Evaporation

 Evaporation occurs when a liquid changes into a gas and occurs over a range of
temperatures.
 Evaporation occurs only at the surface of liquids where high energy particles can escape
from the liquid's surface at low temperatures, below the b.p. of the liquid.
 The larger the surface area and the warmer the liquid surface, the more quickly a liquid can
evaporate.

Condensation

 Condensation occurs when a gas changes into a liquid on cooling and it takes place over
a range of temperatures.
 When a gas is cooled its particles lose energy and when they bump into each other they lack
the energy to bounce away again, instead they group together to form a liquid.

Exam Tip
Questions on the particle theory of matter show interconversion of states with a reversible arrow:
⇌, which means that the process can go forwards and backwards.

State changes & kinetic theory.

 When substances are heated, the particles absorb thermal energy which is converted into
kinetic energy.
o This is the basis of the kinetic theory of matter.
 Heating a solid cause its particles to vibrate more.
o As the temperature increases, the particles vibrate so much that the
solid expands until the structure breaks.
o This is when the solid melts into a liquid.
 Heating a liquid causes its particles to move more and spread out.
o Some particles at the surface gain sufficient energy to overcome the intermolecular
forces.
o This is when a liquid starts to evaporate.
 o When the boiling point is reached, all the particles gain enough energy to escape,
and the liquids boil into a gas.

 These changes in state can be shown on a graph called a heating curve:

A heating curve showing the states, state changes and temperature changes as time
progresses.

 Cooling down a gas has the reverse effect, and this would be called a cooling curve:

A cooling curve is like a heating curve but is the mirror image.

 Heating and cooling curves are used to show how changes in temperature affect changes of
state.
 The horizontal sections occur when there is a change of state but there is no change in
temperature.

Pressure & Temperature in Gases

 Pressure & temperature in gases.

 A change in temperature or pressure affects the volume of gases.

 As the air inside a hot air-balloon is heated up, it expands, and the balloon gets bigger.
 This is because the volume of gas increases as temperature increases.

As the temperature increases gas volume increases. The density decreases as the volume
increases so the balloon rises.

 If you have a gas stored inside a container that is squeezed, the pressure increases as you
decrease the volume.
 This is what happens on a bicycle pump.

Pressure increases as volume decreases in a bicycle pump.

 As you compress the bicycle pump, the high pressure allows you to inflate a tyre.
 You can feel the force of the high pressure if you put your finger on the end of the pump.
 Gases & kinetic theory
Extended tier only

 Gaseous particles are in constant and random motion.

 The pressure that a gas creates inside a closed container is produced by the gaseous
particles hitting the inside walls of the container:

Moving particles of gas colliding with each other and the container walls

How does temperature affect the volume of a gas?

 Increasing the temperature increases the kinetic energy of each particle.

o Remember: The thermal energy from increasing the temperature is converted to
kinetic energy in the particles
 As the temperature increases, the particles in the gas move faster and spread out more.
 If the gas particles are inside a container, they will collide with the container walls
more frequently.
o If the container walls are flexible and stretchy then the container will get bigger and
bigger, just like the hot air-balloon!

Exam Tip
If you are talking about the particles, then make sure that you talk about them spreading out.

If you are talking about the material, then you can use the word expand.

You will lose a mark in an exam if you talk about particles expanding!
How does pressure affect the volume of a gas?

 Pressure is about the number of particles in each volume.

 Increasing the pressure means that there are the same number of particles but in a smaller
volume.
o Conversely, decreasing the pressure means that there are the same number of
particles but in a larger volume.
When the pressure increases, the volume decreases. This means that the molecules collide
with the container walls more frequently.

 Since the volume is decreased, the gas particles hit the container walls more frequently.
o If the pressure is too high, this can result in the container leaking gas or exploding.

Diffusion
Diffusion

 Diffusion occurs in gases and liquids, due to the random motion of their particles.
 It is where particles move from an area of high concentration to an area of low
concentration.
 Eventually the concentration of particles is even as the particles are evenly spread
throughout the available space.
 Diffusion happens on its own and no energy input is required.
o Although, it occurs faster at higher temperatures because the particles have more
kinetic energy.

Diffusion in liquids

 Potassium manganate (VII), KMnO4, in water is a typical demonstration of diffusion in liquids:

Diffusion of potassium manganate (VII), KMnO4, in water. After a few hours, the
concentration of KMnO4 is the same throughout the solution.
Diffusion in gases

 Diffusion is faster in gases than in liquids.

o This is because gaseous particles have more energy and move quicker than liquid
particles.
 For example, the diffusion of bromine gas and air:

Bromine diffuses until it is evenly spread throughout the container.

 At the start, the orange-brown bromine gas is an area of high concentration.

 It diffuses from a high to low concentration.
 After 5 minutes, the bromine gas will have diffused from the bottom jar until it is evenly
spread throughout both jars.
o The same can be said for the air, although it is less obvious as it is colourless.

Exam Tip
When you are talking about diffusion, you should (where appropriate) include:

 Ideas about areas of high and low concentration

 The energy / movement of the particles

Diffusion & molecular mass

How molecular mass affects diffusion

 At the same temperature, different gases do not diffuse at the same rate.
 This is due to the difference in their relative molecular masses.
 Gases with a lower relative molecular mass are "lighter" which means that they:
o Travel faster.
o Travel further in the same amount of time.
 The reverse argument is true for gases with a high relative molecular mass, they:
o Travel slower.
o Do not travel as far in the same amount of time.
 For example, the reaction between ammonia and hydrogen chloride

Diffusion of ammonia and hydrogen chloride

 Ammonia gas and hydrogen chloride gas react together to form solid ammonium chloride.
 NH3 (g) + HCl (g) → NH4Cl (s)

 The Mr of ammonia is 17, while the Mr of hydrogen chloride is 36.5.

o This means that ammonia will travel faster and further than hydrogen chloride.
o Therefore, the ammonium chloride will form nearer to the hydrogen chloride.
 This is seen as a white "smoke" ring inside the gas tube.

Ammonia molecules have less mass than HCl molecules, so they diffuse faster and the product
forms closer to the HCl end.