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Chemical Reactions

In a chemical reaction, atoms trade partners, and none are lost.

When a cake bakes, a nail rusts, or a match burns, new substances form that weren't there before. In this unit you'll learn how to tell a chemical change from a physical one, read and balance chemical equations, see why mass is conserved, and find out what makes reactions give off heat or go faster.

Balance an equation Take the self-guided quiz
The balanced equation 2H₂ + O₂ → 2H₂O drawn as molecules: two hydrogen molecules and one oxygen molecule on the left, two water molecules on the right, with 4 hydrogen and 2 oxygen atoms on each sideHHHH+OO→HHOHHO2H₂ + O₂ → 2H₂O

Physical and chemical changes

In a physical change, a substance changes form but stays the same substance. Melting ice is still water, and cut paper is still paper.

In a chemical change, or chemical reaction, the atoms rearrange to make new substances. Burning, cooking, rusting, and food rotting are all chemical changes. You met atoms in the Atoms and the Periodic Table unit. In a reaction, the bonds holding atoms together break and new ones form.

Signs of a reaction

You can't see atoms rearranging, but you can look for clues:

  • bubbles of a new gas (not just boiling)
  • a color change, like a sliced apple turning brown
  • a change in temperature, or light given off
  • a solid forming when two liquids are mixed
  • a new smell

These are clues, not proof. Boiling water makes bubbles, but it's a physical change.

Reading chemical equations

A chemical equation is a short way to write a reaction. The starting substances, the reactants, go on the left. The new substances, the products, go on the right. The arrow means "react to make."

2H₂ + O₂ → 2H₂O means "two hydrogen molecules and one oxygen molecule react to make two water molecules."

  • A small number after a symbol, the subscript, tells how many of that atom are in one molecule. H₂O has 2 hydrogen atoms and 1 oxygen atom.
  • A big number in front, the coefficient, tells how many molecules there are. When there is no number, it means 1.

Conservation of mass

In a chemical reaction, atoms are not created or destroyed. They just rearrange. This is the law of conservation of mass (also called conservation of matter): the total mass of the products equals the total mass of the reactants.

That's why equations must be balanced, with the same number of each kind of atom on both sides. In 2H₂ + O₂ → 2H₂O there are 4 hydrogen and 2 oxygen atoms on the left, and 4 hydrogen and 2 oxygen atoms on the right. To balance an equation you change the coefficients, never the subscripts, because changing a subscript would make a different substance.

Try the interactive equation balancer at jugami.com/physical-science/chemical-reactions/

Try it: balance an equation

Pick a reaction, then use the − and + buttons to change how many molecules of each substance there are. Balance it so every atom on the left also shows up on the right.

H₂ + O₂ → H₂O.

Key: grey H = hydrogen, red O = oxygen, dark C = carbon. Simplified drawings, not to scale. The drawings don't move.

Energy in reactions

  • An exothermic reaction releases heat. Burning wood and the air-activated hand warmers you shake in winter are exothermic.
  • An endothermic reaction absorbs heat, so its surroundings get colder. Baking soda and vinegar fizzing together feel slightly cooler, and some instant cold packs work by absorbing heat.

See the Energy unit for more on how energy changes form.

Speeding reactions up

Particles have to bump into each other to react. Anything that makes them collide more often, or harder, usually speeds a reaction up:

  • Higher temperature: particles move faster. That's one reason food lasts longer in the fridge.
  • More surface area: a powder reacts faster than a lump, because more particles are exposed.
  • Higher concentration: more reacting particles packed into the same space.
  • A catalyst: a substance that speeds up a reaction without being used up. Enzymes in your body are catalysts.

Safety

  • Never mix household cleaners. Some combinations, like bleach and ammonia, make dangerous gases.
  • Wear eye protection for fizzing experiments and do them over a sink or tray.
  • Never taste anything in an experiment, and wash your hands afterward.
  • Do any experiment with heat or flame only with an adult.

Experiments to try at home

Ask an adult before you start. Wear safety glasses or goggles, work over a sink or tray, and never taste anything.

Self-guided quiz

1. Which of these is a chemical change?

2. In 2H₂ + O₂ → 2H₂O, how many hydrogen atoms are on each side?

3. Baking soda and vinegar react in a sealed bag. What happens to the total mass?

4. A reaction that releases heat is called what?

5. Which change usually makes a reaction go faster?

6. When you balance an equation, what does changing a coefficient change?

Talk about it

  • Which chemical reactions did you depend on today, from breakfast to the bus ride?
  • Why might a scientist want to slow a reaction down instead of speeding it up?
  • If atoms are never destroyed, where does a burning log's mass go?

Sources

Credits: All illustrations on this page are original, hand-coded SVG by Galileo. The share image was drawn in code.