Chapter map — Chemical Changes

The one definition everything rests on

Chemical change A change in which one or more new substances are formed. The starting substances are the reactants; the new substances are the products. Chemical changes are usually not easily reversed.

In a physical change no new substance is formed, and the change can usually be reversed.

Atoms and mass

A chemical reaction is a rearrangement of atoms. No atoms are created and none are destroyed.
The number of atoms of each element is therefore the same before and after.
So mass is conserved: total mass of reactants = total mass of products. Proved by Lavoisier.
Reactions are written as word equations, with any condition (heat, light, electricity) written above the arrow.
The number of reactants and the number of products may not be the same.

The four types of chemical change

Type	What happens
Combustion	A substance is heated in the presence of oxygen; heat and light are given out.
Thermal decomposition	A substance is broken down into two or more simpler substances by heat. Not the same as melting.
Oxidation	A substance gains oxygen. Includes rusting and cellular respiration.
Neutralisation	An acid reacts with an alkali to form a salt and water.

Every word equation in the chapter

Reaction	Word equation
Combustion of carbon	`carbon + oxygen → carbon dioxide`
Combustion of methane	`methane + oxygen → carbon dioxide + water`
Combustion of magnesium	`magnesium + oxygen → magnesium oxide`
Thermal decomposition	`calcium carbonate --heat--> calcium oxide + carbon dioxide`
Rusting	`iron + water + oxygen → rust`
Cellular respiration	`glucose + oxygen → carbon dioxide + water`
Photosynthesis	`carbon dioxide + water --light--> glucose + oxygen`
Neutralisation	`acid + alkali → salt + water`
Acid + metal	`acid + metal → salt + hydrogen`
Acid + carbonate	`acid + carbonate → salt + carbon dioxide + water`
Electrolysis of water	`water --electricity--> hydrogen + oxygen`
Electrolysis of salt	`sodium chloride --electricity--> sodium + chlorine`

Naming the salt hydrochloric acid → a chloride · sulfuric acid → a sulfate · nitric acid → a nitrate. The first half of the name comes from the metal.

The four gas tests

Gas	Test	Positive result
Hydrogen	Burning splint	Extinguishes with a squeaky “pop”
Oxygen	Glowing splint	Rekindles into a flame
Carbon dioxide	Bubble through limewater	Limewater turns chalky — white precipitate
Ammonia	Damp red litmus paper	Paper turns blue

Acids, alkalis and indicators

Type of solution	Red litmus	Blue litmus	pH
Acidic	remains red	turns red	less than 7
Neutral	remains red	remains blue	exactly 7
Alkaline	turns blue	remains blue	more than 7

The trap Red litmus staying red does not prove a solution is acidic — it stays red in neutral solutions too. Only blue litmus turning red proves an acid.

Universal Indicator colours: red below pH 4 · orange ~5 · yellow ~6 · green 7 · blue ~8 · indigo ~9 · violet above 10.

The five triggers — how a change is caused

Mixing — acids with alkalis, metals or carbonates.
Heating — combustion and thermal decomposition.
Exposure to light — photosynthesis; fading of dyes in sunlight.
Interaction with oxygen — rusting, cellular respiration, copper turning green.
Using an electric current — electrolysis and electroplating.

Beneficial and harmful

Beneficial

Cooking · respiration · decay returning nutrients to the environment · electroplating to protect and decorate.

Harmful

Rusting weakening structures · decay spoiling food · combustion producing air pollutants, global warming and ocean acidification.

Preventing rust — painting, oiling, galvanising (zinc coating), drying. All work by keeping water and oxygen off the iron.

Air pollutants from combustion — carbon monoxide (toxic, reduces oxygen carried by red blood cells), sulfur dioxide and oxides of nitrogen (both dissolve in rainwater to form acid rain).

Ocean acidification — carbon dioxide dissolves in seawater to form carbonic acid, lowering the pH. Shells and corals made of calcium carbonate react with the acidic seawater and are broken down. 1875: 280 mg/l CO₂, pH 8.2, thicker shells. 2011: 400 mg/l CO₂, pH 8.1, thinner shells.