Heat Energy and Changes of State

Learning Objective

In this lesson we will learn about the six types of changes in state of matter, and the energy changes that are associated with them.

Learning Outcomes

By the end of this lesson you will be able to:

  • Name and describe the six changes of state.

  • Use the particle model to explain the different changes of state in terms of heat energy.

  • Define melting point and boiling point.

  • Distinguish between boiling and evaporation.

 

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Introduction

  • We have previously learned that almost all pure substances can exist in the three different states – solid, liquid and gas.
  • We have also seen how substances, such as water, can change from a solid to a liquid, and from a liquid to a gas.
  • The reverse is also possible – water can change from a gas to a liquid, and from a liquid to a solid.
  • There six possible state changes, which result from changes in heat energy experienced by a substance.

 
volcano change of state molten lava gas

When the molten lava from this volcano cools, it will form solid rock.

(Image: skeeze, Pixabay)

 

Solid-Liquid Changes

  • Changing from solid to liquid is called melting.
  • Changing from liquid to solid is called freezing.
  • Melting and freezing are opposite processes.

 

melting solid liquid state change    freezing liquid solid state change

Melting and freezing are opposite, reversible processes.

(Images: Fotocitizen, Pixabay; Paul_Henri, Pixabay)

 

Liquid-Gas Changes

  • Changing from liquid to gas is called vaporisation.
  • There are two types of vaporisation – boiling and evaporation. We will compare these later.
  • Changing from gas to liquid is called condensation.
  • Vaporisation and condensation are opposite processes.

 

boiling vaporisation steam    condensation water

When hot steam comes into contact with a cooler surface, it condenses back to liquid water.

(Images: Republica, Pixabay; rkit, Pixabay)

 

Solid-Gas Changes

  • There are some substances that don’t exist in all three states.
    For example, dry ice does not exist as a liquid. It changes from a solid straight to a gas, and from a gas straight to a solid. The gas form of dry ice is called carbon dioxide.
  • Changing from solid to gas is called sublimation.
  • Changing from gas to solid is called deposition.
  • Sublimation and deposition are opposite processes.

 

dry ice deposition    dry ice sublimation

Dry ice readily sublimates at room temperature.

(Images: Richard Wheeler, Wikimedia Commons; Sarathtly, Wikimedia Commons)

 

Heat Energy

  • What causes substances to change state? What is involved when ice melts, or when water freezes?
    Well, just as the particle model explains the properties of solids, liquids and gases, it also explains the changes between them.
    Let’s continue with our water example.
  • When we take ice out of the fridge and leave it on the bench, it eventually melts and forms liquid water.
    So what is different about the freezer and the bench? That’s right – it is warmer on the bench than in the freezer. Therefore a change in state must have something to do with the change in temperature.
    However, let’s use the term heat energy instead of temperature. They are similar terms, but using the term heat energy is more correct when discussing the particle model. (Heat energy refers to the total energy of particles whereas temperature refers to the average energy of particles, but you don’t need to worry about that distinction for now).
    So, in conclusion, we can say that changes in state involve changes in heat energy.
    Let’s look closer at the different types of state changes, in terms of energy changes and the particle model.

 
steam locomotive heat energy boiling

Steam locomotives use heat energy to convert water into steam.

(Image: 1899441, Pixabay)

 

Transfer of Heat Energy

  • When a substance experiences an increase in heat energy, energy is transferred to the particles of that substance; in other words, the particles gain energy and the substance heats up.
    For example, when a bowl is placed in an oven, the heat energy moves from the air in the oven to the particles in the bowl. This causes the particles in the bowl to move faster, which heats up the bowl.

absorb heat energy heat up

When substances absorb heat energy, they heat up due to increased particle movement.

 

  • When a substance experiences a decrease in heat energy, energy is transferred away from the particles of that substance; in other words, the particles lose energy and the substance cools down.
    For example, when a bowl is placed in a fridge, the heat energy moves from the particles in the bowl to the air in the fridge. This causes the particles in the bowl to move slower, which cools down the bowl.

release heat energy cool down

When substances release heat energy, they cool down due to decreased particle movement.

 

Explaining Changes of State

Melting

  • When a solid gains heat energy, the particles begin to vibrate faster and a little further back and forth.
  • If enough heat energy is transferred to the solid, the movement of the particles will increase enough to partially overcome the bonds holding them together.
  • The particles can now slide past each other – the substance has melted and become a liquid.

 
melting metal heat energy

Melting metals requires a lot of heat energy.

(Image: Public Domain Pictures)

 

Vaporisation

  • When a liquid gains heat energy, the particles begin to move faster.
  • If enough heat energy is transferred to the liquid, the movement of the particles will overcome the bonds holding them together.
  • The particles can now move away from each other in all directions – the substance has vaporised and become a gas.

 
boiling water steam heat energy

Heat from the kettle gives water particles enough energy to overcome the bonds between them.

(Image: kboyd, Pixabay)

 

Condensation

  • When a gas loses heat energy, the particles begin to move slower.
  • If enough heat energy is lost from the gas, the particles will slow down enough for bonds to form between them.
  • The particles are now held together (although they can still slide past each other) – the substance has condensed and become a liquid.

 
condensation water window heat energy

Condensation forms when warm water vapour in the air comes into contact with a cold surface.

(Image: mh300dt88, Pixabay)

 

Freezing

  • When a liquid loses heat energy, the particles begin to move slower.
  • If enough heat energy is lost from the liquid, the bonds between the particles become stronger and they can no longer slide past each other.
  • The particles are now held in a fixed arrangement – the substance has frozen and become a solid.

 
frozen water ice heat energy

If enough heat energy is lost from a lake, it can freeze.

(Image: EvaEmaden, Pixabay)

 

Sublimation and Deposition

  • For substances that do not exist in the liquid state, the gain and loss of heat energy causes the particles to go directly from the solid state to the gaseous state, and vice versa.
  • In other words, during sublimation, as soon as the particles gain enough energy to overcome the bonds keeping them in a tightly-packed, fixed arrangement, they are able to move freely in all directions. The substance has changed from a solid directly to a gas.
  • Similarly, during deposition, as soon as the particles lose enough energy that bonds form between them, they become held in a tightly-packed, fixed arrangement. The substance has changed from a gas directly to a solid.

 
dry ice sublimation deposition heat energy

Very low temperatures are required to change carbon dioxide gas to dry ice.

(Image: KarolinaHalatek, Wikimedia Commons)

 

Comparing Boiling and Evaporation

  • Earlier we saw that vaporisation is when a liquid turns into a gas. We also saw that there are two types of vaporisation: boiling and evaporation.
    To explain the difference, let’s look at a couple of examples.

 

Boiling

  • If you place a pot of water on the stove and heat it, eventually, when the water gets hot enough, it will start to bubble vigorously.
    This is known as boiling, and the temperature that this happens at is called the boiling point.
    For water, the boiling point is 100 °C.
  • As the water boils, it vaporises to produce steam.
    Steam is hotter than boiling water; that is, it is hotter than 100 °C.
  • The conversion of water to steam by boiling will result in the level of water in the pot decreasing.

 
boiling water vaporisation steam

Water boils at 100 °C.

(Image: Three-shots, Pixabay)

 

Evaporation

  • Now imagine a swimming pool. If it was left for several weeks, the water level will start to go down. Where did the water go?
    The water has vaporised to produce water vapour, even though the temperature of the water didn’t get anywhere near the boiling point.
    This is known as evaporation.
    Water vapour is at the same temperature as the air.
  • How does evaporation happen?
    Particles don’t all have the same energy. Some of the water particles near the surface have enough energy to ‘escape’ into the air and form water vapour.
    This occurs at a very slow rate. That’s why the water level in the pool goes down very slowly compared to the water level in the pot.

 
evaporation water vapour swimming pool

Swimming pools need to be topped up occasionally, even though they never boil.

(Image: Hans, Pixabay)

 

  • The rate of evaporation is affected by temperature; the level of a swimming pool will drop faster in warm weather than cold weather.
    This can also be explained by the particle theory: the greater the temperature, the greater the average energy of the water particles; therefore, more water particles are able to become water vapour.

 
evaporation temperature water vapour puddle

Puddles ‘disappear’ (evaporate) faster in summer than in winter.

(Image: 3dman_eu, Pixabay)

 

Melting Point and Freezing Point

  • The temperature at which a solid melts is called its melting point.
  • The temperature at which a liquid freezes is called its freezing point.
  • Since melting and freezing are opposite processes, the melting point and freezing point of a substance are the same.

 
melting boiling point ice water

Ice melts if the temperature goes above 0°C; water freezes if the temperature goes below 0 °C.

(Image: bones64, Pixabay)

 

Boiling Point and Condensation Point

  • The temperature at which a liquid boils is called its boiling point.
  • The temperature at which a gas condenses is called its condensation point.
  • Since boiling and condensation are opposite processes, the boiling point and condensation point of a substance are the same.

 
boiling condensation point water steam

Water boils if the temperature goes above 100°C; steam condenses if the temperature goes below 100 °C.

(Image: José Luís Ávila Silveira / Pedro Noronha e Costa, Wikimedia Commons)

 

Summary

  • There are six different types of state changes:
    Melting – when a substance changes from a solid to a liquid.
    Freezing – when a substance changes from a liquid to a solid.
    Vaporisation – when a substance changes from a liquid to a gas.
    Condensation – when a substance changes from a gas to a liquid.
    Sublimation – when a substance changes from a solid directly to a gas.
    Deposition – when a substance changes from a gas directly to a solid.
  • Changes in state can be explained by the particle model in terms of heat energy.
    They involve the transfer of heat energy between particles and the surrounding environment.
  • Melting, vaporisation and sublimation involve a gain of heat energy by particles.
    Heat energy is transferred from the surrounding environment to the particles.
  • Condensation, freezing and deposition involve a loss of heat energy from particles.
    Heat energy is transferred from the particles to the surrounding environment.
  • The melting/freezing point of a substance is the temperature at which it melts/freezes.
  • The boiling/condensation point of a substance is the temperature at which it boils/condenses.

 
heat energy and changes of state of matter

Heat energy and changes of state of matter

 

(Header image: skeeze, Pixabay)

 

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