Chemical Reactions and Energy: An Introduction
Chemical reactions are happening all around us — and inside us — every single second. When you digest food, when a car burns petrol, when a bonfire crackles and glows, chemical reactions are at work. One of the most important things to understand about chemical reactions is that they almost always involve an energy change.
A process in which substances (called reactants) are transformed into new substances (called products), often involving a change in energy.
Energy released in chemical reactions can take many different forms:
- Heat — warming up the surroundings (like a bonfire)
- Light — produced by burning or even by living organisms (like glow-worms)
- Kinetic energy — causing muscles to contract and move
- Electrical energy — generated in power stations using fuel combustion
Think of chemical energy like money stored in a bank account. Reactants "deposit" energy into chemical bonds. When a reaction happens, that energy is either "withdrawn" (released) or more energy is "deposited" (absorbed) to form new bonds in the products.
An important principle underpins all of this: energy is never created or destroyed — it is only transferred from one store to another, or transformed from one form to another. This is called the conservation of energy. In a chemical reaction, the chemical energy stored in the reactants doesn't disappear — it is converted into heat, light, or other forms.
The principle that energy cannot be created or destroyed, only transferred between stores or transformed from one form to another. The total amount of energy in a closed system remains constant.
The chemical reactions involved in respiration allow living organisms to release the energy that keeps them alive. We also use fuels to generate energy, enabling us to heat our homes, power machines, and provide electricity.
Evidence That a Chemical Reaction Is Happening
How do we know a chemical reaction is taking place? Scientists look for observable evidence — signs that new substances are being formed.
Key indicators of a chemical reaction include:
- Temperature change — the mixture gets noticeably hotter or colder
- Gas production — bubbles appear, often a colourless or coloured gas is released
- Colour change — the mixture changes colour (e.g., copper carbonate is green; copper oxide is black)
- Precipitate formation — a solid appears in a liquid
- Light or sound — flashes, flames, or bangs may occur
Citric acid and baking soda reaction:
When you mix citric acid and sodium bicarbonate (baking soda) in water, you can observe:
- Bubbles forming — carbon dioxide gas is being produced
- Temperature drop — the mixture feels cold; the reaction is absorbing heat from its surroundings
This tells us a chemical reaction is definitely occurring — new substances are being formed and energy is being transferred.
A temperature change is one of the most reliable indicators of a chemical reaction. If the temperature rises, energy is being released. If it falls, energy is being absorbed from the surroundings.
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