Changes of State
Particles require heat energy to change state between a solid, liquid and a gas. As heat energy is added, the particles gain more heat energy until they undergo the process of changing state, at which point their temperature remains constant. The amount of energy required to change the temperature of a material is referred to as the specific heat capacity.
Solids, liquids and gases
The particles in a solid are organised in a regular arrangement. The atoms vibrate in a fixed position and are physically touching the other atoms. These particles are low in energy.
In a liquid, the atoms are still touching but are slightly further apart and can slide over one another. Each atom is free to move and has a moderate amount of energy.
The atoms within a gas are arranged randomly and are far apart from one another. They are moving quickly and have a high amount of energy.
Kinetic theory
Heating a substance causes its internal energy (thermal energy) to increase and causes the particles to move around more. The more thermal energy particles have, the larger their kinetic energy. Eventually, the particles will have enough energy to change state. When this happens, heat energy is now being used to break intermolecular bonds between particles rather than raising the temperature. This means that when a solid melts or a liquid boils, the temperature of the substance remains constant.
Evaporation
Evaporation occurs when particles escape from the surface of a liquid and become a gas. Particles near the surface of a liquid can escape if they are travelling fast enough and in the right direction. This means that the fastest molecules are most likely to evaporate from the liquid’s surface.
After the gas molecules escape, the remaining liquid particles have a lower average kinetic energy (and therefore a lower temperature), which means that evaporation causes the liquid to cool down (just like how evaporation of sweat helps our body to cool down).
The difference between boiling and evaporation is that boiling involves the whole liquid, whereas evaporation only happens to a small proportion of the liquid particles at its surface. Particles evaporate from a liquid at lower temperatures than the liquid’s boiling point.
Temperature-time graphs
The fact that temperature remains constant when solids, liquids and gases change state can be shown experimentally by carrying out the following steps:
Place a thermometer into a beaker containing ice and record its temperature.
Gradually heat the ice using a Bunsen burner.
Record the temperature every 20 seconds from the inside of the beaker as the ice melts into water. Carry on recording until the water starts to boil.
Plot a graph of temperature against time. You should get a graph that looks something like the one on the right.
Specific Heat Capacity
Specific heat capacity refers to the amount of energy needed to increase the temperature of one kilogram of an object by 1 oC. Different materials have different heat capacities, which means they require different amounts of energy to heat up.
For example, the specific heat capacity of water is 4200 J/kg which means it takes 4200 Joules of energy to increase the temperature of 1 kg of water by 1 oC. In contrast, the specific heat capacity of copper is 385 J/kg which means it takes 385 Joules of energy to increase the temperature of 1 kg of copper by 1 oC. Since its value for heat capacity is much lower, less energy is required to heat up copper compared to water.
To calculate the change in heat energy of an object, you need to use the following equation:
Did you know..
Water has an exceptionally high heat capacity compared to other liquids, which means it requires a lot of heat energy to cause it to change temperature. This is really handy for us, since our bodies are made up of mostly water, and it prevents us from fluctuating in temperature. It also keeps the temperatures of seas and oceans relatively stable to ensure the survival of marine life.
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