Introduction
Many of the activities in this topic focus on counter-intuitive effects, or at least effects that have an element of surprise and charm. Sometimes they are common phenomena, but students are challenged to look at them in new ways. Melting, evaporation and condensation are examples of physical change, or change of state, and are distinct from changes that cause new materials to form through a chemical reaction. Distinguishing between physical and chemical changes is not always easy, particularly if you do not have access to the concept of atoms and molecules. Some curricula talk of physical changes as being reversible, and chemical change being irreversible. Examples of irreversible chemical changes would be the burning of wood, or a candle, or the reaction of vinegar and baking soda to form carbon dioxide, water and a salt. However, while the notion of reversibility is important when discussing changes of state (solid to liquid, liquid to gas, and vice versa), some physical changes are not reversible (such as the separating of butter into its different constituents on heating, or the physical change to a car in an accident), and some chemical changes are reversible (although usually these would require a professional chemist to perform).
Key concepts of physical changes to matter
The activities in this topic are designed to explore the following key concepts:
Early years
- Substances can be solid, liquid or gas, or mixtures of these.
- Solids hold their shape, liquids flow and settle in containers, gases take up space in containers.
- Melting involves a change from a solid to a liquid, caused by heating.
- For a pure substance such as wax or ice, melting is reversible.
- Some substances, when heated, burn rather than melt, or melt but cannot be changed back (e.g. dairy products).
- Melting is distinct from mixing or dissolving, which involve adding a liquid.
- Water, when it boils and evaporates, changes to steam.
- Water will evaporate at ordinary temperatures, going into the air as a vapour (gas).
Middle years
- Liquids differ in a range of properties, including viscosity (runniness), density and transparency.
- Substances tend to melt at particular temperatures. For pure substances, the temperature is well defined. For substances that are mixtures, the transition may take place over a range of temperatures.
- Evaporation involves a change from the liquid to gas state, and condensation is the reverse of this.
- Fog or water appearing on cold surfaces is due to condensation of water vapour from the air.
- When substances dissolve in a liquid, their molecules intersperse amongst those of the liquid.
- Different substances will dissolve in different liquids.
- When substances form a mixture in a liquid rather than dissolving, they may remain as small solid particles (as with dust).
- The forms that substances can take are complex and varied, including suspensions and colloids, mixtures, etcetera.. As such they can be hard to classify.
Students’ alternative conceptions of physical changes to matter
Ideas about changes to matter are influenced by quite fundamental views about the nature of matter. Students’ concept of ‘matter’ can be very different to adult views. Young children will not, for instance, be very committed to the idea of matter having a continuing existence during changes, and will quite happily talk about clothes ‘drying up’ without being very interested in what has happened to the water in that case. They are more attuned to effects than to charting the changes to matter that accompany these effects. Similarly, they will put entities such as heat, or properties such as coldness, in the same category as dampness or fog. They can say things like ‘the coldness turned to fog’, which avoids the need to consider where the material that makes up the fog may have come from.
The task of the adult, therefore, in engaging students in these activities, is to learn to listen for these ambiguities in language, and to ask questions that challenge the children to focus on what is happening in terms of changes to matter, and to model through the discussion the language of these changes.
Research into students’ ideas about this topic has identified the following non-scientific conceptions:
- Only objects that are ‘hard’ and cannot be changed in shape are solid.
- Substances can disappear or be interchangeable with entities like heat.
- Melting involves substances turning to water.
- Melting and mixing with water are confused.
- Water can simply ‘dry up’ in the sun, or can be ‘overcome by the strength of the heat’.
- Water in puddles evaporates into the sun.
- Water appearing on cold bottles is simply coldness turning to fog, or it may be water leaching through.
- The smell of liquids such as perfume is separate from any physical material.
- Dissolving is confused with melting.