A chemical reaction is a process where reactants change into new products. Recognising the types of reactions and balancing equations are standard exam questions.
Signs of a Chemical Reaction
- Change in colour.
- Change in temperature (heat released or absorbed).
- Formation of a gas or a precipitate.
- Change in state.
Types of Chemical Reactions
- Combination: two or more substances combine into one.
- Decomposition: one substance breaks into two or more.
- Displacement: a reactive element replaces a less reactive one.
- Double displacement: exchange of ions between two compounds.
Oxidation and Reduction
Oxidation is the gain of oxygen or loss of electrons. Reduction is the loss of oxygen or gain of electrons. A reaction where both happen together is a redox reaction.
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Balancing Equations
A chemical equation must be balanced to obey the law of conservation of mass. The number of atoms of each element must be equal on both sides. Mass can neither be created nor destroyed in a reaction.
Everyday Examples
- Rusting of iron is slow oxidation (corrosion).
- Burning of fuel is a combustion reaction.
- Rancidity is oxidation of fats and oils in food.
- Respiration is an exothermic reaction.
Quick Revision Points
- Equations are balanced to obey the law of conservation of mass.
- Oxidation = gain of oxygen / loss of electrons.
- Reduction = loss of oxygen / gain of electrons.
- Rusting of iron is an example of corrosion.
- Combustion releases heat and light.
- Photosynthesis is a decomposition-type endothermic reaction.
- Respiration is an exothermic reaction.