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What is a chemical reaction?

A chemical reaction is a change that takes place when one or more substances (called reactants) form one or more new substances (called products).

reactants products

For example:

carbon carbon dioxideoxygen

+

+

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How can you spot a chemical reaction?

Chemical reactions can appear very different.

As you observe a chemical reaction, you may detect:

a colour change

precipitate (solid) forming

energy being produced (fizzing, burning)

an odour being produced.

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Does mass change during a reaction?

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What is a word equation?

For example, when a piece of sulfur is burned in oxygen gas it produces a white solid called sulfur dioxide.

A word equation uses the names of the reactants and products to show what happens in a chemical reaction.

+ sulfur oxygensulfur

dioxide

The word equation for this reaction is:

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What is a symbol equation?

A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction.

This equation shows that one atom of sulfur (S) reacts with one molecule of oxygen (O2) to make one molecule of sulfur dioxide (SO2).

A symbol equation must be balanced to give the correct ratio of reactants and products.

+ S O2 SO2

+

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What do state symbols show?

State symbols are added to a symbol equation to show whether the reactants and products are:

With state symbols in place, this symbol equation now shows that the sulfur is a solid, the oxygen is a gas and the sulfur dioxide is also a gas.

solid – symbol is (s)

liquid – symbol is (l)

gas – symbol is (g)

dissolved in water – symbol is (aq).

+ S (s) O2 (g) SO2 (g)

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Chemical reactions – true or false?

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