Chapter 11: Chemical Changes (Summary Sheet)

Part A · What makes a change chemical

1 Physical or Chemical?

	Physical change	Chemical change
New substances?	No new substances are produced	New substances are produced
Properties of the products	The same as the reactants	Different from the reactants
Reversibility	Usually reversible	Usually irreversible
Examples	Dissolving sugar, melting ice, a metal bar expanding when heated, stretching a rubber band	Rusting iron, burning a candle, baking a cake, souring milk, photosynthesis

Only one thing decides it: whether new substances have formed. A change in colour, size, state or shape on its own is not enough.

2 What Happens Inside a Reaction, and How We Write It

Chemical changes are usually called chemical reactions. The substances that react are the reactants; the substances produced are the products.
The atoms are rearranged — they are never created or destroyed.
So the total mass of the reactants equals the total mass of the products: mass is conserved.
The total number of each type of atom stays the same before and after the reaction: in sulfur + oxygen → sulfur dioxide, there are 1 sulfur and 2 oxygen atoms on each side.
The general form of an equation is reactants → products, with the conditions of the reaction written above the arrow — read the arrow as “reacts to form”.
Example: copper + oxygen → copper(II) oxide. Copper and oxygen are the reactants; copper(II) oxide is the product. Word equations, not formulas, are what this chapter asks for.

Part B · The five triggers — what starts a chemical change

4 The Five Interactions at a Glance

Interaction	Type of chemical change
Heat	Thermal decomposition — one compound breaks down when heated
Oxygen	Oxidation — combustion, cellular respiration and rusting
Electricity	Electrolysis and electroplating
Light	Photochemical reactions, for example photosynthesis
Other substances (mixing)	Neutralisation, reactions of acids, synthesis

5 Heat: Thermal Decomposition

Type	Examples (heat above the arrow)
Metal carbonate	zinc carbonate → zinc oxide + carbon dioxide
Metal hydroxide	copper(II) hydroxide → copper(II) oxide + water
Metal nitrate	lead(II) nitrate → lead(II) oxide + nitrogen dioxide + oxygen
Hydrated salt	hydrated copper(II) sulfate → anhydrous copper(II) sulfate + water

General form: AB → A + B — one compound breaks down, giving two or more simpler substances.

Exam tip: thermal decomposition takes one compound in and gives two or more simpler substances out. The opposite of synthesis, which is A + B → AB.

6 Oxygen: Oxidation, Combustion and Rusting

Oxidation is a reaction in which a substance reacts with oxygen. Examples: combustion, cellular respiration and rusting.

Reaction	Word equation and what to notice
Complete combustion	propane + oxygen → carbon dioxide + water. Plenty of oxygen; a clean flame.
Incomplete combustion	propane + oxygen → carbon monoxide + water. Limited oxygen; a yellow flame and soot, and the carbon monoxide is poisonous.
Cellular respiration	glucose + oxygen → carbon dioxide + water. The same reaction that releases energy in living cells.
Rusting	iron + water + oxygen → iron(III) oxide (hydrated iron(III) oxide). Rusting needs both water and oxygen.

Preventing rust	How it works
Painting, oiling, greasing	Coats the metal so that air and water cannot reach it.
Galvanising	Coats the object with zinc. Zinc is preferentially oxidised (due to its higher reactivity) instead of the iron, so the iron is protected even where the coating is scratched.
Drying	Removes water, so one of the two things rusting needs is missing.

7 Electricity: Electrolysis and Electroplating

Change	Word equation and what to notice
Electrolysis of water	water → hydrogen + oxygen (electricity above the arrow). Bubbles of gas are seen at both electrodes, because both products are gases.
Electrolysis of a molten compound	lead(II) bromide (molten) → lead + bromine. Silvery beads of lead form at one electrode, red-brown bromine vapour at the other.
Electroplating	A metal is deposited onto an object by electrolysis: a shiny, hard, corrosion-resistant surface at a fraction of the cost of solid metal.

8 Light: Photochemical Reactions

Reaction	Word equation
Photosynthesis	carbon dioxide + water → glucose + oxygen (light above the arrow, and chlorophyll as the green pigment that absorbs the light)
Photography	silver bromide → silver + bromine (light above the arrow)

9 Mixing: Reactions of Acids, and Synthesis

Reaction	Word equation
Neutralisation (acid + alkali)	acid + alkali → salt + water
Acid + metal	acid + metal → salt + hydrogen
Acid + carbonate	acid + carbonate → salt + carbon dioxide + water
Alkali + ammonium salt	alkali + ammonium salt → salt + ammonia + water
Synthesis element + element	iron + sulfur → iron sulfide zinc + oxygen → zinc oxide
Synthesis compound + compound	sodium oxide + water → sodium hydroxide lead(II) oxide + sulfur trioxide → lead(II) sulfate
Synthesis element + compound	carbon + carbon dioxide → carbon monoxide oxygen + sulfur dioxide → sulfur trioxide

General form of synthesis: A + B → AB — two or more substances combine to form one single compound, so nothing else comes out.

Gas	Test and observation
Hydrogen	Bring a lighted splint to the gas. The lighted splint extinguishes with a “pop” sound.
Carbon dioxide	Bubble the gas through limewater. A white precipitate forms — the limewater turns milky.
Ammonia	Hold a piece of damp red litmus paper in the gas. The paper turns blue — the alkaline properties show only once the gas has dissolved in water.

Part C · Observations and effects

10 What You See During a Chemical Change

Observation	What to write
Change in colour	Say which colour to which colour, and which substance changed.
Formation of an insoluble solid	A solid formed in a solution is called a precipitate.
Formation of gas	Describe the bubbles, then name and test the gas.
Change in temperature, or emission of light	The mixture gets hotter or colder, or a flame or glow appears.

Beneficial effects	Harmful effects
Cooking food; cellular respiration; combustion of fuels; producing medicines; manufacturing daily products	Rusting; air pollution; decay of food; global warming; ocean acidification

11 Air Pollution and Acid Rain

Pollutant	Source	Effect
Carbon monoxide	Forest fires; incomplete combustion in vehicle engines	Respiratory problems; can cause death
Sulfur dioxide	Volcanic eruptions; burning sulfur impurities in coal and petroleum	Irritates eyes, nose and lungs; causes acid rain
Oxides of nitrogen	Lightning; fuel burning at high temperatures in engines and power stations	Irritates eyes, nose and lungs; causes acid rain

Acid rain forms when sulfur dioxide and oxides of nitrogen dissolve in rainwater and react with oxygen and water:
sulfur dioxide + oxygen + water → sulfuric acid
nitrogen dioxide + oxygen + water → nitric acid
Wind carries the gases across borders, so the country that suffers the acid rain is often not the one that released the gas.

12 Global Warming and Ocean Acidification

Greenhouse gas	Where it comes from
Carbon dioxide	Respiration and aerobic decomposition; complete combustion of coal, petroleum and natural gas; deforestation
Methane	Decaying plant matter in landfills and paddy fields; cattle and other livestock

Greenhouse gases trap infrared radiation, so the Earth's average temperature rises: global warming. Melting ice and expanding seawater raise sea level and speed up coastal erosion.
Ocean acidification: the oceans absorb the extra carbon dioxide, which dissolves and reacts with water to form carbonic acid — carbon dioxide + water → carbonic acid.
That acid attacks the carbonates in the shells and skeletons of oysters, clams and corals. Corals with damaged skeletons die, the reef crumbles, and the food chain, fishing and tourism suffer.

Exam tip: same cause, different mechanism — global warming is heat trapped in the atmosphere; ocean acidification is carbonic acid formed in the sea.

13 Say It Like This

If the question asks…	Say this
Is this a chemical change?	“New substances are formed: the products have different properties from the reactants.”
Why does the mass stay the same?	“The atoms are rearranged, not created or destroyed, so the total mass of the reactants equals the total mass of the products.”
Complete or incomplete combustion?	“Plenty of oxygen gives carbon dioxide and water; limited oxygen also gives carbon monoxide and soot.”
Test for hydrogen?	“A lighted splint extinguishes with a pop sound.”
Test for carbon dioxide?	“Bubble it through limewater; the limewater turns milky.”
Test for ammonia?	“Damp red litmus paper turns blue.”
Synthesis or thermal decomposition?	“Synthesis is A + B → AB: two or more substances to one single compound. Thermal decomposition is AB → A + B: one compound broken into two or more simpler substances.”