Freezing

Freezing is a process when a substance changes from a liquid to a solid. The temperature at which a liquid becomes a solid is called its freezing point. Hence, heat energy is given out during the cooling process.

Condensation

The change of a vapour or a gas into a liquid is called condensation. This process is accompanied by the evolution of heat. When water vapour touches a cold surface, condensation occurs and liquid water is finally obtained.

Sublimation

There are a few substances that change directly from a solid to a gas when they are heated without ever becoming a liquid. This phenomenon is known as sublimation. Cooling causes a change from a gas directly back to a solid. Examples- carbon dioxide and iodine.

When dry ice (carbon dioxide) is exposed to a temperature higher than -78ºC it turns into carbon dioxide gas directly without melting. 

Dry ice sublimes in heating and can be used for creating a special effect on stage.

 

Diffusion

The process by which different substances mix as a result of the random motions of their particles. It depends on these two factors:

A) The mass of the particles

The particles in hydrogen chloride gas are twice as heavy as those in ammonia gas. So which gas do you think will diffuse faster? Let’s have a look,

1. Cotton wool soaked in ammonia solution is put into one end of a long tube (at A ). It will give off ammonia gas.

2.  At the same time, cotton wool soaked in hydrochloric acid is put into the other end of the tube (at B). It will give off hydrogen chloride gas.

3. The gases diffuse along the tube. White smoke forms where they meet. 

The white smoke forms closer to B. So, the ammonia particles have travelled further than the hydrogen chloride particles that means they have travelled faster.

So, we can also say, the lower its relative molecular mass, the faster a gas will diffuse.

B) Temperature

When a gas is heated, its particles move faster-taking heat energy. They collide with more energy and bounce further away. So the gas diffuses faster.

So we can say, the higher the temperature, the faster a gas will diffuse.

Effusion

Consider a balloon filled with Helium gas. 

The pressure of helium gas inside and outside the balloon is not the same. The pressure inside the balloon is much higher. The passage of gases from a region of high pressure to a region of low pressure through the fine pores is called effusion.