A mist appears on the inside of a window in a bus full of people. Steam is produced from the surface of the water in a pan when the water is heated before it boils. Ice cubes taken from a freezer thaw out. Water put into a freezer gradually turns to ice. Solid carbon dioxide turns to a gas. Carbon dioxide gas turns into a solid. Freezing/ Solidifying Melting Vaporisation Condensation Condensation Sublimation 29

Changes of State Gas Boiling Sublimation Condensing Solidifying Liquid Solid Boiling Sublimation Condensing Solidifying Melting Describe how materials change state. Explain latent heat Be able to calculate latent heat

Changes of State and Latent Heat You will be able to answer the following questions. How do materials change state? What is latent heat? How can I calculate specific latent heat? Describe how materials change state. Explain latent heat Be able to calculate latent heat

Heating/Cooling Curves To go from solid to liquid to gas, energy must be input. This energy is required to break the bonds that hold molecules together. Describe how materials change state. Explain latent heat Be able to calculate latent heat

Heating/Cooling Curves Describe how materials change state. Explain latent heat Be able to calculate latent heat

Heating a metal from a solid to a liquid. Latent Heat Note that when the material is changing state, even though the amount of energy is still changing, temperature remains constant. Example Heating a metal from a solid to a liquid. Describe how materials change state. Explain latent heat Be able to calculate latent heat

Latent Heat As you heat the metal the atoms gain kinetic energy (vibrate more about their fixed position in the lattice) and the temperature of the metal increases. When the metal starts to melt, energy is used to break the bonds. So while the metal melts temperature remains constant. The energy absorbed while melting is called the latent heat of fusion Describe how materials change state. Explain latent heat Be able to calculate latent heat

Energy transferred = mass x specific latent heat of fusion Equation Just as with specific heat capacity, we can measure the latent heat per unit mass. Energy transferred = mass x specific latent heat of fusion E = mLf J kg J/kg Describe how materials change state. Explain latent heat Be able to calculate latent heat

Latent heat of vaporisation. As boiling water is heated, the energy supplied is used to overcome the intermolecular forces holding the molecules together in a liquid state Where there is a change in state without a change in temperature the energy involved is called latent heat vaporisation. Describe how materials change state. Explain latent heat Be able to calculate latent heat

This is known as the specific latent heat of vaporisation Lv. The quantity of energy needed to vaporise a sample depends on the mass of the sample. As with heat capacity, comparisons can be made between different materials by measuring the energy needed to turn 1 kg of a material at its boiling point into vapour. This is known as the specific latent heat of vaporisation Lv. Describe how materials change state. Explain latent heat Be able to calculate latent heat

specific latent heat of vaporisation, L, can also be calculated using: E=mLv Units of L: J/kg. Describe how materials change state. Explain latent heat Be able to calculate latent heat

Why is a scald from steam worse than from boiling water at the same temperature? E=mLv E=mcΔθ Your hand is at 37oC initially, if 10g of water at 100oC is accidentally put on your hand assume all the heat energy is transferred from the water. How much energy is transferred? (Cwater = 4200 J/kgoC. How much energy is transferred from 10g steam (remember there is a change of state first!) Lvaporisation= 2.26 x 106J/kg Describe how materials change state. Explain latent heat Be able to calculate latent heat

The energy from the 10g water at 100OC is 2646J The energy from the 10g steam at 100oC is 25246J Now that’s going to hurt! Describe how materials change state. Explain latent heat Be able to calculate latent heat

How do we tackle long answer questions in this topic? The story of ice melting long answer preparation questions.pptx Describe how materials change state. Explain latent heat Be able to calculate latent heat

Experiment: Measuring the Latent heat of Ice Health and safety: The heaters will be hot! Describe how materials change state. Explain latent heat Be able to calculate latent heat

Diagram and concise method. Results. Analysis. Calculate L. To complete: Diagram and concise method. Results. Analysis. Calculate L. Evaluation. Compare your results to the ideal value. What has caused these differences? Lice = 334 000 J/kg Describe how materials change state. Explain latent heat Be able to calculate latent heat

Can you answer these Questions? How do materials change state? What is latent heat? How can I calculate specific latent heat? Describe how materials change state. Explain latent heat Be able to calculate latent heat