Topic 1: States of Matter

(The content below are notes for students to use to revise the topic. The notes will include a condensed overview of the topic, including all relevant IGCSE questions and answers given in point form) 

1.1 Solids, Liquids and Gases  

Q. Describe the structure of solids, liquids and gases in terms of particle separation, arrangement and movement

• Solids: regular arrangement, movement limited to simple vibrations. Particles are always touching each other and cannot be further compressed
• Liquids: random arrangement. Molecules take up the space of the container, and slide over each other. Particles are somewhat touching each other 
• Gases: random arrangement. Particles are the furthest apart. Movement is random and in all directions 

This  is a heating curve 

The substance above is a pure  substance. How do we know? Because the curve shows sharply defined gradients with no smooth curves 

2. Explain, in terms of kinetic particle theory, what happens at melting (2 marks) 

• Heat energy is given to the particle when it is in the state of a solid. The heat energy is converted to kinetic energy'
• The kinetic energy is enough to overcome the intermolecular forces of attraction, and causes the intermolecular forces in the solid to weaken or break. This allows the particles to move further apart
• Therefore, the movement of the molecules go from simple vibration, where they are constantly touching each other, to sliding over each other. 

The same points as mentioned above applies to evaporation as well, except in evaporation, the molecules go from sliding past each other to moving more randomly

3. Is melting/ evaporation an exothermic or endothermic reaction? 

• It is endothermic. Heat is being given/ absorbed by the substance. The substance itself does not emit any heat. 

This is a cooling curve. 

• In a cooling curve, at the plateu points, heat energy is being released, which makes it an exothermic reaction. 
• New bonds are being formed between the molecules, which strengthens their intermolecular forces of attraction. 
• When this happens, the movement of the molecules is restricted. Once there are enough bonds formed to cause a change of state (for example, gas to liquid; condensation), the temperature begins to decrease again 

How to determine the state of a substance given its temperature

1. Use the heating or cooling curve given to you 
2. Determine where the number falls. 

For example, water melts at 0°C, and boils at 100 °C

At 39°C, the water will be a liquid, because 39 is higher than 0 and lower than 100

At -3°C, the water will be a solid, because -3 is below 0 (freezing point of water). 

At 103°C, the water will be a gas, because 103 is above 100 (boiling point of water).