Lesson 6

Lesson 6: Changes of State.

What happens when matter changes state?

The three most familiar states of matter are solid, liquid, and gas. A change of state is the change of a substance from one physical form of matter to another. When a substance undergoes a physical change, it does not change its identity, just its appearance.

To change a substance from one state to another, energy must be added or removed. When a substance gains or loses energy, its temperature changes or its state changes. These 2 changes do not happen at the same time; the temperature remains constant until the change of state is complete.

All matter is made of tiny particles that are in constant motion. During a change of state, the motion of the particles changes. Particles can break away from each other and gain more freedom to move. Particles can also attract each other more strongly and have less freedom to move. During the change of the substance energy must be lost or gained from the environment, but the total amount of energy is conserved. Energy is never created or destroyed. 

How do solids and liquids change states?

Particles in a liquid can slide past each other, but particles in a solid can only move enough to vibrate. Removing energy from a liquid can cause it to change to a solid as the particles stop sliding past each other.

• By freezing: it is the change of state in which a liquid becomes a solid. When a liquid is cooled, its particles have less energy than they did before. The particles slow down and the attractions between the particles increase. The temperature at which a liquid substance changes into a solid is the liquid's freezing point. 
• By melting: Particles in a solid have an ordered arrangement. When a solid is warmed, its particles have more energy than they did before. The particles speed up and the attraction between them decreases. Eventually the particles are able to slide past one another. This change of state is called melting. 
  

How do liquids and gases change state?

Particles in a gas have a great deal of energy. Removing enough energy from a gas cause it to change into a liquid or a solid. The process in which a liquid or a solid changes to a gas is vaporization. 

• By evaporation or boiling: As a liquid is warmed, its particles gain energy. Some particles gain enough energy that they escape from the surface of the liquid and become a gas. This process is called evaporation. It occurs slowly at a range of temperatures but it happens more quickly at higher temperatures. A fast change from a liquid to a gas or vapor is called boiling. This change takes place throughout a liquid not just at the surface. The specific temperature at which bubbles form because of heat is called the boiling point. The greater the air pressure, the higher the boiling point of a liquid.
• By condensation: Particles in a gas have very little attraction to one another. As a gas is cooled its particles lose energy. The attraction between particles overcomes the speed of their motion, and a liquid forms. This change of state from a gas to a liquid is called condensation. It is the opposite of evaporation. 

How do solids and gases change state?

Under the right conditions some solids and gases can change state without ever becoming a liquid. The substance must gain or lose a great deal of energy for this to happen.

• By sublimation: Dry ice is frozen carbon dioxide. It changes from its solid state directly into a gas. As the particles of solid dry ice gain energy their motion completely overcomes the attraction between the particles. Snow and ice are formed at below freezing temperatures. 
• By depostion: It is the change of state in which a gas transforms directly into a solid. It is the process by which ice crystals form in clouds. When conditions are right, deposition occurs when the particles of a gas lose energy. 
  

What happens to matter when a change of state occurs?

When matter changes from one state to another, it remains the same kind of matter. Its physical state changes, but its chemical identity does not. 

The mass of a substance does not change when its state changes, Each state contains the same amount of matter.