Change of State MATTER Chapter 5

Change of State A CHANGE OF STATE occurs if a material changes from one state to another because ENERGY is either GIVEN TO IT or TAKEN AWAY FROM IT. If we think of ice, water and steam…

The graph below shows how the temperature of a material being heated would change against time. To begin with the material is in its SOLID STATE. Using Particle Theory to explain change of state

A CHANGE OF STATE is an example of a PHYSICAL CHANGE taking place, because it is possible to return to the MATERIAL WE STARTED WITH. In other words the change is REVERSIBLE if we were to start off with a GAS and then COOL IT DOWN to a LIQUID and then to a SOLID we could get a graph which is a mirror image of the one above.

TWO IMPORTANT POINTS 1.When a CHANGE OF STATE takes place the MASS BEFORE THE CHANGE is EQUAL to the MASS AFTER THE CHANGE. 2.DIFFERENT SUBSTANCES would have graphs SIMILAR TO ABOVE although they would CHANGE STATE at DIFFERENT TEMPERATURES.

There are 5 changes of state

Remember that heat is energy, and remember that there are two types of energy - kinetic and potential. The heat exchanges, or energy exchanges, present during a change in phase are changes in potential energy. These energy exchanges are not changes in kinetic energy. If heat is coming into a substance during a phase change, then this energy is used to break the bonds between the molecules of the substance. The example we will use here is ice melting into water. After the molecular bonds in the ice are broken the molecules are at a higher potential energy state, however they are not on the average moving any faster, so their average kinetic energy remains the same, and thus, their Kelvin temperature remains the same. Remember that heat is energy, and remember that there are two types of energy - kinetic and potential. The heat exchanges, or energy exchanges, present during a change in phase are changes in potential energy. These energy exchanges are not changes in kinetic energy. If heat is coming into a substance during a phase change, then this energy is used to break the bonds between the molecules of the substance. The example we will use here is ice melting into water. After the molecular bonds in the ice are broken the molecules are at a higher potential energy state, however they are not on the average moving any faster, so their average kinetic energy remains the same, and thus, their Kelvin temperature remains the same.

Look at the following diagram and continue to read the text below it.

In the ice the molecules are strongly bonded to one another, thus forming a rigid solid. When heat is added to the ice, these bonds are broken and the molecules, now at a higher potential energy state, bond to one another with less strength. Water is formed. Now, before the melting, molecules were actually moving when in the solid state. They were vibrating back and forth. So, they had an average speed, and thus, an average kinetic energy. So, they had a Kelvin temperature proportional to this average kinetic energy.

After the melting the water molecules are moving, also. And they have the same average speed and the same average kinetic energy as they had before the melting. So, the water is at the same temperature the moment after the melting that the ice was at the moment before the melting. Heat came into the situation, but it was not used to change the speed of the molecules. It was used to change the bonding between the molecules. To break the bonds between the molecules of the ice required energy, and this is was the use for the heat.

In a similar way heat enters a liquid to change the molecular bonding when the liquid boils or evaporates into a gas, and heat enters a solid to change the molecular bonding when it sublimates into a gas. In an inverse way heat leaves a gas to change the molecular bonding when the gas condenses into a liquid, and heat leaves a liquid to change the molecular bonding when it freezes into a solid. In none of these changes of state is heat input used to speed up the molecules, nor is heat output ever occurring as a result of the molecules slowing down. The average speed of the molecules is the same before and after a phase change, so, the average kinetic energy is the same. And thus, again, we state that the temperature does not change during a change in phase.