Kinetic Particle Theory

Matter refers to anything that has mass and takes up space. Light and sound are not matter as they do not have mass and they do not take up space.

The kinetic particle theory is used to explain the different properties of matter. The theory states the following points:

• All matter is made up of particles that are too small to be seen directly.
• There are forces of attraction between the particles in matter.
• The particles in matter have kinetic energy and are in constant random motion.

Brownian motion is the constant random motion of particles suspended in a fluid (liquid or gas) due to collisions with the fluid molecules.

For example, if smoke is observed under a microscope with intense illumination, the smoke particles appear as bright specks that are constantly moving about in a random manner. This is because the air particles, which are in constant random motion, are colliding with the smoke particles.

Since particles in matter are too small to be seen with the naked eye, scientists use a model known as the particulate nature of matter to show the arrangement of particles in solids, liquids and gases respectively.

At different physical states, the arrangement and movement of particles, as well as the energy of particles in a substance, will be different:

Expansion refers to the increase in volume of a substance when heated. For example, when a solid absorbs thermal energy from the surroundings (which is converted to kinetic energy of its particles), its particles vibrate and rotate more quickly about their fixed positions and move further apart from each other, hence the volume of the solid increases.

Contraction refers to the decrease in volume of a substance when cooled. For example, when a solid releases thermal energy to the surroundings (and the kinetic energy of its particles decreases), its particles vibrate and rotate more slowly about their fixed positions and move closer to each other, hence the volume of the solid decreases.

Matter can undergo a change in state when it absorbs thermal energy from the surroundings or releases thermal energy to the surroundings, as shown below.

• When a substance absorbs thermal energy from the surroundings, some of the thermal energy is converted to kinetic energy of its particles. Since the average kinetic energy of particles increases, the temperature of the substance increases.
• When a substance releases thermal energy to the surroundings, the average kinetic energy of particles decreases, hence the temperature of the substance decreases.

The change in temperature of Substance X as it is heated over time is shown in the heating curve below. Substance X changes from solid to liquid state.

The change in temperature of Substance X as it is heated over time is shown in the heating curve below. Substance X changes from liquid to gaseous state.

Evaporation is a process where a substance changes from liquid to gaseous state at a temperature between its melting point and boiling point.

• When a liquid absorbs thermal energy, some of it is converted to the kinetic energy of its particles — they slide past each other more quickly and move further apart.
• Evaporation occurs when particles at the surface of the liquid have sufficient (kinetic) energy to overcome the forces of attraction between neighbouring particles and escape as vapour.

• Sublimation: only certain substances can change directly from solid to gaseous state when heated gently. Common examples include iodine, dry ice and naphthalene (found in mothballs).
• Vapour deposition: these same substances can also change directly from gaseous to solid state when their vapours are allowed to cool.

The change in temperature of Substance Y as it is cooled over time is shown in the cooling curve below. Substance Y changes from gaseous to liquid state. For any pure substance, its condensation point is equal to its boiling point.

The change in temperature of Substance Y as it is cooled over time is shown in the cooling curve below. Substance Y changes from liquid to solid state. For any pure substance, its freezing point is equal to its melting point.

1. A few drops of liquid bromine are placed in a covered gas jar.
2. Liquid bromine evaporates, filling the jar with reddish-brown bromine vapour.
3. Another gas jar filled with air is placed above the jar filled with bromine vapour, and the lids between the two jars are removed.
4. After a few hours, the mixture appears uniformly pale brown as the bromine particles and air particles have spread out evenly in both jars.

1. Potassium manganate(VII) crystals are added to a beaker of water. The crystals dissolve to form a deep purple solution at the base of the beaker.
2. After a few hours, the solution appears uniformly pale purple as the potassium manganate(VII) particles and water particles have diffused throughout the solution.

The factors affecting the rate of diffusion are temperature and mass of particles.

The rate of diffusion increases as temperature increases. As temperature increases, the average kinetic energy of particles increases, and the particles move more quickly. Hence, the rate of diffusion increases.

The rate of diffusion decreases as the mass of particles increases.

In the setup below, cotton wool soaked in concentrated hydrochloric acid and concentrated aqueous ammonia are inserted into opposite ends of a sealed glass tube. Concentrated hydrochloric acid evaporates to form hydrogen chloride gas while concentrated aqueous ammonia evaporates to form ammonia gas. The gases then diffuse and react with each other.

After a period of time, a white solid of ammonium chloride is produced from the reaction between hydrogen chloride gas and ammonia gas. The white solid is formed closer to the end containing cotton wool soaked in concentrated hydrochloric acid.

The relative molecular mass (M_r) of a molecular substance is calculated by adding up the relative atomic mass (A_r) of all atoms shown in the chemical formula of the molecular substance.