P1.2 Changes of state
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P1.2 Changes of state
Mathematical learning outcomes:
PM1.2i apply: change in thermal energy = mass x specific heat capacity x change in temperature
PM1.2ii apply: thermal energy for a change in state = m x specific latent heat
Assessable content statements:
P1.2a describe how mass is conserved when substances melt, freeze, evaporate, condense or sublimate
P1.2b describe that these physical changes differ from chemical changes because the material recovers its original properties if the change is reversed
P1.2c describe how heating a system will change the energy stored within the system and raise its temperature or produce changes of state
P1.2d define the term specific heat capacity and distinguish between it and the term specific latent heat
P1.2e apply the relationship between change in internal energy of a material and its mass, specific heat capacity and temperature change to calculate the energy change involved (M1a, M3c, M3d)
P1.2f apply the relationship between specific latent heat and mass to calculate the energy change involved in a change of state (M1a, M3c, M3d)
P1.2g explain how the motion of the molecules in a gas is related both to its temperature and its pressure
P1.2h explain the relationship between the temperature of a gas and its pressure at constant volume (qualitative only)
Approaches to teaching the content
Learners have learnt about changes of state in key stages 2 and 3, some may have still have misconceptions about what happens. This can then hinder learners progressing into understanding the more difficult topics of specific heat capacity and specific latent heat. A good way of overcoming these basic challenges is to allow learners to heat a certain mass of ice until it boils. They should observe all of the changes taking place and discuss what is happening at each point.
There are a lot of important key concepts in this sub-topic surrounding heat, energy and changes of state. Learners should have an understanding from Key Stage 3 about the difference between heat and temperature, and about energy transfer between two adjacent objects. For clearer understanding learners need to know that the absorption of energy can change the state of matter, not just increase the temperature.
Constructing a heating curve for water allows learners to become familiar with not only the states, but also that there is no significant temperature rise during melting, freezing and boiling. For the most able learners a full explanation is needed about why temperature does not increase when a change of state occurs. They should be able to explain that energy supplied during a change of state is used to break inter-molecular bonds and this explains why temperature does not change. This is a good time to introduce the concept of specific latent heat.
Specific heat capacity can be another challenging area for learners to grasp as the term sounds similar to specific latent heat and the equation can seem daunting. It would be advisable to deliver specific heat and specific latent heat separately and using different concepts, e.g. energy changes in different fuels are a common experiment when investigating specific heat capacity. Learners can then collect results and use the equation to calculate the thermal energy change.
Common misconceptions or difficulties learners may have
To overcome some of the misconceptions learners may have about conservation of mass when changing states learners can complete simple experiments which can challenge their thinking, such as ice cubes in a bag, and discussing of what actually happens to the mass when it melts. Another is to consider a wet item of clothing on a washing line and thinking about where exactly the water goes when the item dries. Short observation based experiments followed by small group discussions can elicit deeper responses from learners. A link for these is provided in the resource section.
Wherever possible learners should develop the mathematical learning outcomes for this sub-topic based on practical work. This can make it seem less complex for learners who may find applying formulae difficult.
Learners may confuse the definitions for specific heat capacity and specific latent heat. The following definitions should be known:
Specific heat capacity of a material is the energy needed to raise the temperature of 1 kg by 1K and that different materials have different specific heat capacities.
Specific latent heat of a material is the energy needed to melt or boil 1 kg of it and that different materials have different latent heats.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
The knowledge of heat, temperature, changes of state and energy changes gained in this sub-topic is helpful when learners begin P1.3, exploring pressure and how molecules move with changing temperature.
This link provides a printable set of 15 cards, some that contain questions and others that contain general information.
Learners should work in small groups with one complete set of cards. The aim is to answer the questions using the information given.
This is a good introduction to specific heat capacity, and can take learners approximately 30 minutes to gather the information they require.
Approaches to teaching the content
The mathematical outcomes for this sub-topic are best delivered practically and using an industrial context that is topical and relevant such as the best fuel for a car, or could be based on fuels for a new housing development and learners act as energy consultants.
When considering changes of state, contexts such as the atmosphere and the effect of global warming on sea levels, may make it more relevant to some learners.
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