State which of the following involve a chemical change and which involve a physical change:
making toast, melting chocolate, freezing cordial, water evaporating , putting a solute tablet in water, burning gas and cutting up cheese.
A physical change occurs when either the phase of the substance changes, i.e. it changes between being a liquid, solid or gas. Or if in either of the states it changes shape.
A chemical change on the other hand involves the formation of a new chemical compound.
In the cases you have given:
- making toast(without burning the bread): this is a physical change as some of the water in the bread evaporates
- melting chocolate: here the chocolate changes phase from solid to liquid
- freezing cordial: here again there is a phase change from liquid to solid
- water evaporating: again a change in phase from liquid to gas
- putting a solute tablet in water: the substance dissolves in the water which is a physical change (the assumption made here is that the chemicals that making up the tablet do not react with water)
- burning gas: a chemical change, the gas reacts with oxygen to form other chemical compounds
- cutting up cheese: a physical change with the shape of the solid changing