What do we mean by 'reaction'?

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In the conversation above, what misconceptions does student A have about chemical reactions?

Student A thinks of reactions in terms of one chemical being the active substance that brings about a change in another substance, or changes being caused by heating or stirring. In chemistry we talk about reactions or chemical reactions. These words can have unhelpful associations for some younger students because they think that reaction implies a response to something. This is the kind of meaning they may have used in physics when talking about forces.

What kind of language might you use when introducing the idea of chemical reactions to avoid these types of misconceptions?

It's a good idea to talk about substances 'reacting with...' rather than 'reacting to...'.

Thermal decomposition reactions

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In the conversation above, what misconceptions does student A have about the thermal decomposition?

Student A doesn't feel that changes due to heating are 'real' reactions because only one rather than two substances is involved. He also confuses this type of reaction with combustion reactions because both require that the substance is heated.

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Why is it better to illustrate thermal decomposition wiith copper(II) carbonate before using calcium carbonate?

The thermal decomposition of copper(II) carbonate brings about a colour change from green to black. It's therefore easy to see that a new substance has been formed and that the reactant is no longer present. There is no colour change when calcium carbonate is decomposed so it's much more difficult to convince students that a reaction has taken place.