What is a Chemical Reaction? Equations, balancing and types

Strike a match and, in a second, wood and oxygen become carbon dioxide, water vapour, heat and light. The atoms are all still there — they have only changed partners. That is a chemical reaction.

This article explains what a chemical reaction is, how to read and balance a chemical equation, the main types of reaction, why some release heat and others absorb it, and what controls how fast they go.

Physical change or chemical change?

Melting ice is still water; dissolving sugar is still sugar. Those are physical changes — the substance keeps its identity. In a chemical change, new substances with new properties appear: iron + oxygen + water becomes rust, a crumbly red-brown solid that is not iron at all.

Signs that usually point to a reaction:

• a colour change (a cut apple browning);
• gas bubbles (baking soda in vinegar);
• a solid forming from two clear solutions (a precipitate);
• light or heat given off, or the mixture getting cold;
• a new smell (food cooking, milk going sour).

Reading a chemical equation

An equation is a sentence written in formulas. Reactants go on the left, products on the right, and the arrow means “react to form”:

The big numbers in front are coefficients: how many units of each substance take part. The small subscripts belong to the formula itself and never change — H₂O is water, H₂O₂ is hydrogen peroxide.

Balancing: atoms are conserved

In 1789 Antoine Lavoisier showed that in a closed container the total mass before and after a reaction is the same. Atoms rearrange; they do not appear or vanish. So a correct equation must have the same number of each kind of atom on both sides.

Balance by adjusting coefficients only, one element at a time. For hydrogen burning in oxygen:

• Unbalanced: H₂ + O₂ → H₂O — two O on the left, one on the right.
• Fix oxygen: H₂ + O₂ → 2 H₂O — now four H on the right, two on the left.
• Fix hydrogen: 2 H₂ + O₂ → 2 H₂O — 4 H and 2 O on each side. Balanced.

The Lab’s Reaction Lab balances equations automatically and works out quantities, the limiting reagent and theoretical yield.

Main types of reaction

The categories overlap — combustion is also redox — but they help predict products. Acid–base reactions trade protons; redox reactions trade electrons.

Energy: exothermic and endothermic

Breaking bonds costs energy; forming bonds releases it. The balance decides the sign:

• Exothermic reactions release energy, usually as heat or light — combustion, respiration in your cells, hand warmers.
• Endothermic reactions absorb energy — photosynthesis, baking soda reacting with citric acid, instant cold packs.

Even exothermic reactions often need a push to start, the activation energy: a match needs friction, a gas stove needs a spark.

What makes reactions faster

Particles have to collide with enough energy and the right orientation. Anything that increases useful collisions speeds a reaction up:

• Temperature: faster particles collide harder and more often — food spoils faster outside the fridge.
• Concentration or pressure: more particles in the same space, more collisions.
• Surface area: powdered sugar burns explosively; a sugar cube does not.
• Catalysts: provide an easier pathway with lower activation energy without being used up — enzymes, car catalytic converters.

Common misconceptions

“Burning destroys matter.”

The mass goes into gases you cannot see. Burn a candle in a sealed jar and the total mass does not change.

“To balance an equation you can change the subscripts.”

Changing a subscript changes the substance. Only coefficients may be adjusted.