By the end of the subtopic, learners should be able to:
  1. Describe the states of matter and explain their inter-conversion in terms of the kinetic particle theory.
  2. Describe and explain diffusion.
  3. Carry out experiments on diffusion.

  • All substances that surround us are made of matter.
  • The matter is composed of small, invisible pieces that are referred to as particles.

1.1 States of matter

All substances that surround us are made of matter. The matter is composed of small, invisible pieces that are referred to as particles.
  • There are three states that matter can assume.
  • These are solid, liquid and gas.
  • The particles in the three states are arranged differently hence they have different properties.
  • The most common examples of the states of matter which are encountered in everyday life are ice (solid), water (liquid) and water vapour/ steam (gas).
  • The particles in a solid are held together by very strong forces of attraction in a rigid structure.
  • Because of these strong forces, the volume and shape of solids are therefore fixed.
  • Solid particles have a regular arrangement.
  • The close packing of particles in a solid causes the density of solids to be higher than that of liquids and gases.
  •  The same molecular mass of particles of a solid occupies a smaller volume than that occupied by a liquid or a gas.
  • Due to the unavailability of free space between the particles, a solid cannot be compressed.
  • The particles in a liquid are held together by less strong forces of attraction as compared to solids.
  • The particles are able to move around and therefore they can change shape and be poured.
  • Liquid particles have an irregular arrangement
  • There are negligible (very little) forces of attraction which exist between particles in a gas and therefore they can move freely and fill up the container.
  • Particles in a gas have spaces between them and hence can be compressed.
  • Gas particles are randomly arranged.
  • Table 1.1.1 below gives a summary of the properties of the three states of matter.
Properties Solid Liquid Gas
Arrangement of particles Tightly packed in a regular arrangement Close together in an irregular arrangement Far apart in a random arrangement
Movement of particles Can only vibrate within fixed places Move around and slide over each other Rapidly move freely colliding with each other and bouncing back
Shape and volume Fixed shape and volume Fixed volume, assumes shape of the container Assumes volume and shape of container
Compressibility Cannot be compressed Cannot be compressed Can be easily compressed
Diagram 1.jpg (75 KB) 2.jpg (80 KB) 3.jpg (82 KB)

Conversion of states of matter

  • The states of matter can be converted from one state to another.
  • The kinetic particle theory explains the changes of states using the idea that all matter is composed of small invisible particles which are in constant motion.
  • The space between the particles and the movement of the particles vary according to the state of matter.
  • A solid can be converted to a liquid and a liquid to a gas by supplying heat.
  • Likewise, a gas can be changed to a liquid and a liquid to a solid by cooling.
  • It is important to note that only the arrangement of particles and their movement changes when one state is converted to another and there is no change on the particles themselves.
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5. Fig 1.1.1.jpg (103 KB)
  • The conversions of the states of matter are discussed below:
1. Melting
  • Melting refers to the process through which a substance is converted from the solid state to the liquid state.
  • Heat supplied is absorbed and converted to kinetic energy resulting in increased vibration of the particles within their fixed positions; this breaks the forces of attraction holding the particles together in the regular shape resulting in a liquid.
2. Vaporisation
  • Vaporisation is the process through which a liquid is converted into a gas.
  • The particles in the liquid absorb heat and begin to move more rapidly breaking the forces of attraction holding the particles together resulting in a gas.
  • There are two processes through which vaporisation takes place, these are evaporation and boiling.
a) Evaporation
  • Evaporation is the conversion of a liquid to a gas at any temperature below the boiling temperature.
  • The process is slow and occurs only at the surface of the liquid.
b) Boiling
  • Boiling is the conversion of a liquid to a gas at a constant boiling temperature.
  • The process is rapid and occurs throughout the whole liquid.
3. Condensation
  • Condensation refers to the process through which a gas changes to a liquid.
  • Heat energy is removed and therefore the gas particles lose their kinetic energy and movement becomes slow and move closer to one another forming a liquid.
4. Freezing
  • Freezing is the process through which a substance is converted from a liquid to a solid.
  • Cooling of the liquid is done and the particles lose their kinetic energy and move closer together forming a solid, particles move into fixed position and obtain a regular arrangement.
5. Sublimation
  • Sublimation refers to the change of state from either a solid to gas or a gas to a solid without the formation of a liquid.

Heating Curve of water

  • Fig 1.1.2 shows the heating curve water.
  • The curve shows the conversion of states from solid to liquid and then to gas as heat is supplied.
  • As melting takes place, the temperature remains constant as all the heat energy supplied is used to break the forces of attraction that hold the particles together.
  • When the boiling point is reached, the temperature remains constant as the heat energy absorbed is used to break the forces of attraction between the particles.
  • The melting point refers to the temperature at which the melting occurs.
  • The boiling point refers to the temperature at which boiling occurs.
  • A cooling curve will be a mirror image of the heating curve.
6. Fig 1.1.2.jpg (119 KB)

1.2 Diffusion

  • There are everyday encounters which give evidence of the fact that particles are present in matter and that they have the ability to move.
  • The smell of bread being baked in a bakery spreads to the surrounding areas as the gas particles from the bread collide with the air particles and bounce away, in this way they spread to the areas in the near the bakery.
  • A drop of ink also spreads in water colouring it as the ink particles move and collide with the water particles and bounce away again, in this way the ink particles spread throughout the liquid giving it its colour.
  • Diffusion is the mixing and spreading of particles in gases and liquids through collisions.
  • As the particles collide, they bounce and change direction and spread out.
  • In the process of diffusion, particles move from a higher concentration to lower concentration.
  • The process of diffusion in gases can be demonstrated experimentally using bromine gas.

Experiment 1.2.1: Diffusion of gases
Materials: Bromine, two gas jars, glass cover
  1. Place a few drops of bromine in one of the gas jars.
  2. Immediately cover the jar with the glass cover.
  3. Place the second jar on top of the glass cover.
  4. Remove the glass cover.
  5. Observe the two gas jars after about an hour.
Expected observations
  • Fig 1.2.1 A shows the gas jars before the cover is removed and fig 1.2.1 B shows the gas jars an hour after the cover is removed.
7. Fig 1.1.3_a.jpg (111 KB)
7. Fig 1.1.3_b.jpg (106 KB)
  • The drops of Bromine evaporate forming a vapour.
  • The bromine and air particles will be in constant motion in the separate jars, when the cover is removed, the particles collide and bounce in different directions therefore they spread and continue to mix until the bromine particles and the particles in air are completely mixed giving the two jars the colour of bromine gas.
  • The bromine vapour particles move from their region of high concentration to a region of low concentration.
  • The rate of diffusion is faster in gases than in liquids because the particles in a gas move more rapidly and have negligible forces of attraction between the particles therefore they can freely move to mix and spread.
  • The diffusion in liquids can be demonstrated by dissolving copper sulphate crystals in water

Experiment 1.2.2: Diffusion of a dissolving solid in water
Materials: beaker, glass tube with a small diameter, copper sulphate crystal, water
  1. Place water in the beaker.
  2. Use the glass tube to place the copper sulphate crystals at the bottom of the beaker.
  3. Observe the beaker after a few minutes.
  4. Observe what will happen in the beaker after a day.
Expected observations
  • Fig 1.2.2 shows how the process of diffusion occurs in a beaker.
  • After a few minutes the water that surrounds the area where the copper sulphate crystals were placed turns blue.
  • After about 24 hours the water in the beaker will have a uniform blue colour which is the colour of the copper sulphate crystals.
  • When the copper sulphate crystals are added, they first dissolve in the water giving a dark blue colour to the water around the area where the crystals were placed.
  • The particles then move randomly and collide with the water molecules therefore they spread and mix with the water and as time progresses the water becomes uniformly coloured with the blue colour of copper sulphate.
  • The particles of copper sulphate move from a region of higher concentration (dark blue colour) to a region of lower concentration until they become uniformly distributed in the water.
8. Fig 1.1.4.jpg (126 KB)
Factors that affect the rate of diffusion
  • The factors that affect the rate of diffusion include the molecular mass of the substance and the temperature at which the process takes place.
1. Effect of molecular mass
  • Molecular mass refers to the mass of a molecule.
  • The mass of a molecule is calculated by adding the masses of each atom that make up a molecule.
  • Gases with lower molecular mass diffuse more faster than those which are heavier.
  • The dependence of the rate of diffusion on the molecular mass of the gases can be demonstrated in an experiment described below.

Experiment 1.2.3: To show that the rate of diffusion depends on the molecular mass of the gas
Materials: aqueous ammonia, hydrogen chloride, cotton wool, glass tube, rubber stoppers
  1. Soak a piece of cotton wool in ammonia and another in hydrogen chloride.
  2. Insert the soaked cotton wools into each end of the glass tube. This can be done by two people at the same time. The glass tube should be immediately closed with the rubber stoppers.
  3. Observe the glass tube. Record your observations.
Expected observations.
9. Fig 1.1.5.jpg (134 KB)
  • Fig 1.2.3 shows the expected results.
  • A white ring is formed nearer to the cotton wool soaked in hydrogen chloride.
  • Aqueous ammonia vapour and hydrogen chloride gas diffuse through the tube and a white ring of ammonium chloride forms closer to the hydrogen chloride side.
  • This shows that the hydrogen chloride has diffused at a slower rate and therefore it is heavier than the aqueous ammonia vapour.
  • To calculate the molecular mass of a gas you have to know its molecular formular and the atomic masses of the atoms present in the gas.
  • The molecular formular of ammonia is NH3. N has an atomic mass of 14, H has an atomic mass of 1. There are 3 H in the ammonia molecule, this has to be considered when adding.
  • The molecular mass of ammonia (NH3) is 14 + (1 x 3) which gives a total of 17.
  • The molecular formular of hydrogen chloride is HCl. Cl has an atomic mass of 35.5.
  • The molecular mass of HCl is 1 + 35.5 which gives a total of 36.5.
  • The calculated values show that HCl has a higher molecular mass than NH3.
2.  Effect of temperature
  • The higher the temperature, the higher the amount of heat energy absorbed by the particles in the matter and they gain more kinetic energy and move faster resulting in a higher rate of diffusion.
  • A lower temperature results in the particles losing their kinetic energy and therefore they will move slower thereby reducing the rate of diffusion.