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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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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)