1 STATES OF MATTER
CIE IGCSE Chemistry
Cambridge Chapter 1
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Chapter 1 Explanation presentation
Kinetic Theory
Solids, Liquids & Gases
Solids
Strong forces of attraction between particles, particles are packed very closely together in a fixed and regular pattern.
Atoms vibrate in position but can’t change position or move.
Solids have a fixed volume, shape and high density.
Liquids
Weaker attractive forces in liquids than in solids, particles are close together in an irregular, unfixed.
Particles can move and slide past each other which is why liquids adopt the shape of the container they’re in and also why they are able to flow.
Liquids have a fixed volume but not a fixed shape and have a moderate to high density.
Gases
No intermolecular forces, particles are in random movement and so there is no defined pattern.
Particles are far apart and move 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).
No fixed volume, since there is a lot of space between the particles, gases can be compressed into a much smaller volume. Gases have low density.
Note
Solids, liquids and gases have different physical properties. The difference in these properties comes from differences in how the particles are arranged in each state.
States of Matter
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.) which is unique to each pure solid.
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.) which is unique to each pure liquid.
Freezing
Freezing is when a liquid changes into a solid.
This is the reverse of melting and occurs at exactly 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°C.
Requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature which is unique for each pure substance.
Evaporation
When a liquid changes into a gas. 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.
No heat is required, and evaporation occurs over a range of temperatures.
Condensation
When a gas changes into a liquid, usually on cooling. 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.
No energy is required for condensation to occur and it takes place over a range of temperatures.
Sublimation
When a solid changes directly into a gas.
This happens to only a few solids such as iodine or solid carbon dioxide.
The reverse reaction also happens and is also called sublimation (sometimes called deposition or desublimation).
Sublimation occurs at a specific temperature which is unique for a pure substance.
Note
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.
Read the question carefully and pick the direction of the change in state that the question refers to.
Gaseous Particles
Gaseous particles are in constant and random motion.
An increase in temperature increases the kinetic energy of each particle, as the thermal energy is transformed to kinetic energy, so they move faster.
Decreasing the temperature has the opposite effect.
The pressure that a gas creates inside a closed container is produced by the gaseous particles hitting the inside walls of the container. As the temperature increases, the particles in the gas move faster, impacting the container’s walls more frequently.
Therefore an increase in temperature causes an increase in pressure.
Moving particles of gas colliding with each other and the container walls:
Heating Curve
Brownian Motion & Diffusion
Brownian Motion
Brownian motion is defined as the random movement of particles in a liquid or a gas produced by large numbers of collisions with smaller, often invisible particles.
The observation of Brownian motion proves the correctness of the kinetic particle theory.
Large particles show jerky and erratic movement caused by many collisions with smaller particles:
Diffusion
This is the process by which different gases or different liquids mix and is due to the random motion of their particles.
Diffusing particles move from an area of high concentration to an area of low concentration.
Eventually the concentration of particles is even as they spread out to occupy all of the available space.
Diffusion happens on its own and no energy input is required although it occurs faster at higher temperatures.
Diffusion of potassium manganate (VI) in water. After a few hours the concentration of KMnO4 is the same everywhere in the solution:
Changes in State & Kinetic Theory
When substances are heated, the particles absorb thermal energy which is converted into kinetic energy. This is the basis of the kinetic theory of matter.
Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solid expands until the structure breaks and the solid melts.
On further heating, the now liquid substance expands more and some particles at the surface gain sufficient energy to overcome the intermolecular forces and evaporate.
When the b.p. temperature is reached, all the particles gain enough energy to escape and the liquids boils.
These changes in state can be shown on a graph which is called a heating curve.
Cooling down a gas has the reverse effect, and this would be called a cooling curve.
These curves are used to show how changes in temperature affect changes of state.
Note
While changing state, the temperature of the substance remains the same as the heat energy is rapidly converted into kinetic energy. This is called latent heat and corresponds to the horizontal sections of a heating / cooling curve.
Diffusion
Brownian Motion
Explanation and evidence for Brownian motion
An example of Brownian motion is the observed jerky and erratic motion of smoke particles as they are hit by the unseen molecules in the air which can be seen under a microscope.
In 1905, physicist Albert Einstein explained that pollen grains in water were being moved by individual water molecules.
In all cases, larger and visible particles are caused to move by the random bombardment of smaller, invisible particles.
Diffusion & Molecular Mass
Diffusion occurs much faster in gases than in liquids as gaseous particles move much quicker than liquid particles.
At the same temperature, different gases do not diffuse at the same rate.
This is due to the difference in their relative molecular masses.
Lighter gas particles can travel faster and hence further, therefore the lower its relative mass the faster a gas will diffuse.
NH3 molecules have less mass than the HCl molecule, so diffuse faster, hence the product (a white cloud of NH4Cl) forms closer to the end where the HCl is.