What Is a Chemical Reaction?

A chemical reaction is a process in which one or more substances (the reactants) are changed into one or more new substances (the products). In a chemical reaction, the atoms themselves do not change, but the way they are joined together does. Bonds between atoms are broken and new ones form. Chemical reactions are happening all around you, all the time: in the candle on your cake, the iron rusting on a fence, the food being digested in your stomach and the trees outside making oxygen from sunlight.

  • What it doesChanges substancesReactants -> products
  • What changesBonds between atomsAtoms themselves are preserved
  • Signs of a reactionHeat, light, gas, colourOr a new solid forming
  • Rule of massTotal mass stays the sameNo atoms are created or destroyed
  • Energy inEndothermic reactionCooking, ice packs, photosynthesis
  • Energy outExothermic reactionBurning, rusting, fireworks

The big idea

In a chemical reaction, the atoms in the starting substances are rearranged. The bonds holding them together are broken and new bonds form, producing different substances with different properties.

For example, when you burn methane (the gas you cook with):

  • Reactants: methane (CH4) and oxygen (O2)
  • Products: carbon dioxide (CO2) and water (H2O)
  • Equation: CH4 + 2 O2 -> CO2 + 2 H2O + heat

The carbon and hydrogen atoms from the methane combine with the oxygen atoms. The total number of each kind of atom is the same before and after, but they are now arranged into completely different molecules with completely different properties.

How can you tell a reaction has happened?

Five common signs that a chemical reaction is taking place:

  • Heat or cold: the temperature changes (a reaction is giving out or absorbing heat).
  • Light: glowing, sparking or flame.
  • Gas given off: bubbles or fizzing.
  • Colour change: a new colour appears.
  • Solid forms: a precipitate appears from a clear solution.

If you see one or more of these, chances are atoms are being rearranged.

Fact The law of conservation of mass says that the total mass of the reactants always equals the total mass of the products. Atoms are never created or destroyed in a chemical reaction. If you weigh a sealed jar containing all the products of a reaction, it will weigh exactly the same as the sealed jar of reactants before.

Examples of chemical reactions

  • Burning: a fuel reacts with oxygen, releasing heat and light. (See combustion.)
  • Rusting: iron reacts slowly with oxygen and water to form iron oxide. (See oxidation and rusting.)
  • Photosynthesis: plants use sunlight to turn carbon dioxide and water into sugar and oxygen. (See photosynthesis.)
  • Digestion: enzymes in your stomach and intestines break down food molecules into smaller ones your body can absorb.
  • Baking soda + vinegar: produces salt, water and bubbles of carbon dioxide.
  • Battery: chemicals inside react to push electrons through wires, powering your devices.
  • Fireworks: rapid combustion plus metal salts that glow in different colours when heated.

Physical changes are different

Not every change is a chemical reaction. A physical change alters how a substance looks or feels but does not change its chemical identity. Examples:

  • Ice melting into water (still H2O, just rearranged)
  • Water boiling into steam (still H2O)
  • Sugar dissolving in tea (still sugar)
  • Crushing a piece of chalk into powder (still chalk)

In all these cases, the molecules are still the same. Only their arrangement or state has changed. Physical changes can usually be reversed easily. Chemical reactions are much harder (or impossible) to reverse.

Did you know? Your body runs thousands of chemical reactions every second. Each cell contains millions of tiny machines called enzymes that speed up specific reactions: digesting food, building muscle, fighting germs, repairing DNA, producing energy. If even a few of these reactions stopped, you would die within minutes.

What makes a reaction go faster?

Five factors affect how quickly a chemical reaction happens:

  • Temperature: higher temperature, faster reaction. That is why food rots faster in summer than winter.
  • Concentration: stronger solutions react faster. Dilute acid reacts with metal slowly; concentrated acid reacts quickly.
  • Surface area: powders react faster than chunks because more atoms are exposed. Wood shavings catch fire much more easily than a log.
  • Catalysts: special chemicals that speed up reactions without being used up themselves. Enzymes in your body are biological catalysts. (See catalysts.)
  • Pressure: for gas reactions, higher pressure means molecules collide more often, speeding things up.

Energy in reactions

All chemical reactions involve energy. Some reactions release energy (usually as heat or light). These are called exothermic. Examples: burning, rusting, exploding fireworks.

Other reactions absorb energy from their surroundings. These are called endothermic. Examples: cooking, photosynthesis, ice packs. (See endothermic and exothermic.)

Try this Mix half a teaspoon of baking soda with a tablespoon of vinegar in a clear glass. Watch the fizzing: those bubbles are carbon dioxide gas, made by the reaction. Touch the side of the glass: it should feel slightly cooler than before (this reaction absorbs a little heat). What is left in the glass is sodium acetate dissolved in water. You started with two substances, ended with three different ones, plus heat absorbed. Five reaction signs in one experiment.
Deeper dive: writing balanced chemical equations

Chemists describe reactions with chemical equations: short symbolic recipes that show what goes in and what comes out. For example, the burning of methane is written as:

CH4 + 2 O2 -> CO2 + 2 H2O

The arrow (->) means "becomes". The numbers in front (called coefficients) tell you how many molecules of each substance are involved. The small numbers below the line (like the 4 in CH4) tell you how many atoms of that type are in each molecule.

A correctly written equation has to be balanced: the same number of each type of atom must appear on both sides. Looking at the example above:

  • Left side: 1 carbon, 4 hydrogen, 4 oxygen (from 2 O2 molecules).
  • Right side: 1 carbon, 4 hydrogen (from 2 H2O), 4 oxygen (2 in CO2, 2 in 2 H2O). Balanced.

Balancing reflects the law of conservation of mass: atoms are not created or destroyed, just rearranged. Chemistry students spend a lot of time learning to balance equations because the recipe must always add up. Adding the wrong number of molecules of a reactant or product will make a real reaction go wrong (or refuse to work at all).

Once balanced, the equation lets you predict exactly how much of each chemical you need. If you want to burn 16 grams of methane (which is 1 mole), you know from the equation you will need 64 grams of oxygen (2 moles), and you will produce 44 grams of carbon dioxide and 36 grams of water. The whole of industrial chemistry depends on this kind of careful counting.

For more, see combustion and endothermic and exothermic.