Changes of State

P1.2-1 Solid, Liquid, Gas

It is the bond between the particles that decides which state of the material we have: solid, liquid or gas. 

In a solid strong bond keeps particles fixed in their positions. Particles can only vibrate in their position.

If the solid receives heat energy, particles vibrate more and move further from each other (expansion). If this continues the bonds, holding the particles in place, break and solid turns into liquid (melting). 

If liquid receives heat energy same thing happens and particles escape into the atmosphere and material turns into gas (boiling). During all this process, mass remains constant, and molecules (or atoms) do not change (i.e. it is a physical change, not chemical), and they all are reversible.

Figure 6: Phase changes of matter, source

Difference between vaporisation and evaporation: 

Evaporation: a liquid turns into gas. This can happen at any temperature (liquid doesn’t need to be boiling)! Only molecules at the surface of the liquid escape to the atmosphere. Evaporation cools the material, because faster molecules evaporate and leave behind the slower ones in the liquid. 

Vaporisation: means both boiling and sublimation! If a liquid is boiling all of its molecules can escape to atmosphere! 

A little thinking:

Q1. Why does our body cool down when we sweat?

Q2. Why is a steam burn worse than being burned by boiling water?

Q3. Why wet clothes do not dry if left in a pile, and why we hang them?

P1.2-2 Internal Energy

The heat energy received by the material is spent on two things: 1. vibration of particles (temperature); 2. Breaking the bonds between particles (change of state).

The heat energy spent on temperature is kinetic energy, and the one spent on breaking the bonds is potential energy. The sum of the two is called internal energy of the material. 

When kinetic energy changes, potential does not; and vice versa! This means when material is changing state its temperature does not change!

Figure 7: Temperature vs. time; source (Attempted to contact for authorization, website was not accessible)

Question: How does adding ice cubes to a glass water, cools it down? 

Answer: The faster moving molecules of water, transfer heat energy to the ice. The KE of water decreases and cools down, and the KE of ice increases and it gets warmer. When water and ice reach the same temperature (0 ^oC) there is no more exchange of thermal energy between them.

P1.2-3 Specific Heat

Specific Heat Capacity: amount of heat energy needed to change the temperature of 1 kg of a material by 1 degree. 

Compare some specific heat capacities:

Little Practice:

Q1: 252 kJ of energy was transferred to 2 kg of water to raise its temperature from 20 to 50 ^oC. Find the specific heat capacity of water?

Specific latent heat (SLH): amount of energy needed to change the state of 1 kg of a material, without any change in temperature. There are two types if it:

1. Specific Latent Heat of fusion: amount of energy to melt 1 kg of a solid (also to freeze a liquid).

   2. Specific Latent Heat of Vaporisation: Amount of energy to vaporise 1 kg of a liquid (also to condense a gas).    SLH of vaporisation is larger than SLH of fusion, because in vaporisation all of the bonds between the particles    should be broken and separate them totally. 

Little Practice:

Q1. An electric heater has a power output of 580 W. It is used to heat ice at temperature of -20 ^oC to water of temperature 90 ^oC. If this process takes 13 minutes, calculate the mass of the ice which was initially put in the heater. 

Specific heat capacity of ice: 2.1 kJ/kg ^oC

Specific latent heat of fusion of ice: 334 kJ/kg

Specific heat capacity of water: 4.2 kJ/kg ^oC

Q2. Figure below shows temperature-time graph for melting paraffin wax. 

1. What is the melting point of paraffin?
2. What is happening between 7 and 15 minutes?

Answers: 

1. Melting point is about 50 ^oC;
2. Between 7 and 15 minutes, the temperature does not change and that means the paraffin is melting.

Q3. A cube of brass at 0 ^oC is put into a container of hot water of temperature 80 ^oC, until they reach thermal equilibrium at 70.7 ^oC. 

The brass cube has a mass of 150 gr and the water in the container has mass of 250 gr. Specific heat capacity of water is 4200 J/kg^oC. Calculate the specific heat capacity of the brass. Ignore any loss of thermal energy loss to the surroundings.

Answer: thermal equilibrium means both material will reach the same temperature eventually (55 ^oC), and the decrease in energy store of water is equal to increase in energy store of brass.

P1.2-4 Pressure

Particles in a gas move randomly and freely. The hotter the gas the faster the particles move and gain kinetic energy. 

Particles in a gas collide with each other and the walls of the container. This applies a force on the wall, which causes the pressure of the gas! 

Imagine a gas in a sealed container being warmed up, the particles collide the walls of the container more forceful and more regularly (because they have more KE!), this causes the gas pressure to go up! 

If the container is not sealed, the gas escapes (volume and mass are not constant) and pressure does not change! (Or at least would not change by the same amount, if the container was sealed).