Chapter 11: Chemical Changes
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Part A · Physical and chemical changes
1 Two Types of Changes
- Matter undergoes two main types of changes: physical changes and chemical changes.
- In a physical change, no new substances are produced.
- In a chemical change, new substances are produced.
- The test is what happens to the substances: are they still the same substances at the end?
| Physical changes | Chemical changes | |
|---|---|---|
| New substances | No new substances are produced | New substances are produced |
| Properties of products | Same as those of the reactants | Different from those of the reactants |
| Reversibility | Usually reversible | Usually irreversible |
| Examples | Dissolving sugar in water, melting ice, expanding a metal bar when heated, stretching a rubber band | Rusting of iron, burning of a candle, baking a cake, souring of milk, photosynthesis |
Exam tip: the mark is for new substances are produced, named where the question lets you. A change in colour, size or state on its own is not evidence of a chemical change.
2 Chemical Changes Are Chemical Reactions
- Chemical changes are usually called chemical reactions.
- In a chemical reaction the atoms in the substances are rearranged, producing new substances.
- The substances that react together are the reactants; the substances produced are the products.
- No atoms are created or destroyed during a chemical reaction.
- The total mass of the reactants is therefore equal to the total mass of the products — mass is conserved.
Chemical reactions
- In the reaction above, sulfur burns in oxygen to form sulfur dioxide.
- Sulfur and oxygen are the reactants; sulfur dioxide is the product.
- Each oxygen molecule splits into two oxygen atoms, and each atom combines with one sulfur atom.
- The total number of each type of atom — 1 sulfur and 2 oxygen atoms — stays the same before and after the reaction.
Exam tip: use the word rearranged — atoms are rearranged, none created or destroyed, so the mass before and after is the same. If the masses do not match, look for a gas that escaped or a gas from the air that joined in.
3 Word Equations
- The general form is reactants → products, with the conditions of the reaction written above the arrow.
Word equations
- In the example above, copper and oxygen are the reactants and copper(II) oxide is the product.
Part B · Types of chemical changes
4 How Substances Change
- Substances undergo different types of chemical changes when they interact with the following:
| Interaction | Type of chemical change |
|---|---|
| Heat | Thermal decomposition — a 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 Interaction with Heat: Thermal Decomposition
- Substances may undergo chemical changes when heated to certain temperatures.
Thermal decomposition
Thermal decomposition is a chemical reaction in which a single compound breaks down into two or more simpler substances when heated.
- Metal carbonates → metal oxide + carbon dioxide
- Metal hydroxides → metal oxide + water
- Metal nitrates → metal oxide + nitrogen dioxide + oxygen
| 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 |
Exam tip: nitrogen dioxide is the brown gas given off by a metal nitrate — it turns damp blue litmus red. Do not write carbon dioxide for a hydroxide or a nitrate.
6 Interaction with Oxygen: Oxidation
Oxidation is a chemical reaction in which a substance reacts with oxygen.
- Examples of oxidation reactions: combustion, cellular respiration and rusting.
7 Combustion
Combustion is a chemical reaction in which a substance reacts with oxygen in air in the presence of heat.
- During combustion, one or more new substances may be produced.
- Complete combustion occurs in the presence of sufficient oxygen.
- Incomplete combustion occurs when there is insufficient oxygen.
heat
propane + oxygen → carbon dioxide + water
propane + oxygen → carbon dioxide + water
- Sulfur is an impurity in fossil fuels, so burning them also produces sulfur dioxide.
- Hydrocarbons are compounds made up of only carbon and hydrogen.
- When a hydrocarbon burns in insufficient oxygen, it produces carbon monoxide and/or carbon (soot) instead of carbon dioxide.
- Water is usually produced as well, because hydrocarbons contain hydrogen.
- The signs are a yellow, smoky flame and black sooty deposits.
heat
propane + oxygen → carbon monoxide + water
propane + oxygen → carbon monoxide + water
Exam tip: the reason for incomplete combustion is always insufficient oxygen. Carbon monoxide is colourless and odourless, causes respiratory problems, and can result in death.
8 Cellular Respiration
Cellular respiration is a chemical reaction in which the cells of living organisms take in oxygen and break down glucose to release energy needed for cellular activities.
glucose + oxygen → carbon dioxide + water
- The chemical energy stored in glucose is converted into other forms of energy that body cells can use.
- The energy released is used for growth, movement and keeping warm.
- Plants respire as well, day and night.
9 Rusting
Rusting is a chemical reaction in which iron reacts with water and oxygen in the air to produce red-brown hydrated iron(III) oxide, which is rust.
iron + water + oxygen → iron(III) oxide
- Both water and oxygen must be present — dry air alone will not rust iron, and water with no air in it will not either.
- Rust flakes off, exposing fresh iron underneath, so the damage works its way inwards.
| Method | How it prevents rusting |
|---|---|
| Painting | Coats iron or steel objects such as cars, ships and bridges, so air and water cannot reach the metal. |
| Oiling or greasing | Coats iron or steel tools and parts of machinery; the oil or grease also acts as a lubricant to reduce wear and tear due to friction. |
| Galvanising | Coats the object with a layer of zinc. Zinc is preferentially oxidised (due to its higher reactivity) instead of the iron underneath, so the iron is protected even where the coating is scratched. |
| Drying | Dries the iron or steel object in an oven to prevent contact with water. |
Exam tip: name the method and what it keeps away from the iron — air, water, or both.
10 Interaction with Electricity
Electrolysis
Electrolysis is a chemical reaction in which a single compound decomposes (breaks down) into simpler substances when an electric current is passed through it.
- The compound must be molten or dissolved, so that its ions are free to move.
electricity
water → hydrogen + oxygen
water → hydrogen + oxygen
- Bubbles of gas are seen at both electrodes, because both hydrogen and oxygen are gases.
electricity
lead(II) bromide (molten) → lead + bromine
lead(II) bromide (molten) → lead + bromine
- Silvery beads of lead form at one electrode, while red-brown bromine vapour is released at the other.
- Electrolysis can be used to extract a metal from its molten compound — the metal collects at one electrode and the non-metal is released at the other.
Electroplating is a chemical reaction in which a substance is coated with a metal when an electric current is passed through the set-up.
- The set-up uses the object to be coated and a bar of the coating metal, in a solution of a compound of that metal.
- For example, a ring can be coated with silver using a silver bar and aqueous silver nitrate.
- Chromium-plated taps and tin-plated food cans are everyday examples.
11 Interaction with Light: Photochemical Reactions
- Photosynthesis is an example of a photochemical reaction.
Photosynthesis is a chemical reaction in which green plants take in carbon dioxide and water to produce glucose and oxygen in the presence of light, using chlorophyll.
light
carbon dioxide + water → glucose + oxygen
carbon dioxide + water → glucose + oxygen
- Light energy is stored in the chemical bonds of glucose; respiration later releases that energy.
- It happens only in green parts of a plant, and stops in the dark.
X-ray imaging
- X-ray films are coated with a layer of silver bromide, which decomposes when exposed to X-rays.
- X-rays that are not blocked by the skeleton reach the film and produce silver, forming dark spots.
- Areas blocked by the skeleton are not exposed, so they remain light areas.
- The contrast between the dark and light areas allows the skeleton to be seen.
x-rays / light
silver bromide → silver + bromine
silver bromide → silver + bromine
12 Interaction with Other Substances (Mixing)
- Many chemical changes happen when two or more reactive substances are mixed together.
- Acids and alkalis are two common groups of reactive substances: acids are found in citrus fruits and vinegar, while alkalis are found in soap and liquid detergents.
| 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 |
Testing the gases produced
| 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. |
Exam tip: red litmus paper must be damp for it to turn blue in the presence of ammonia (NH3) — the alkaline properties are shown only once the gas has dissolved in water.
Synthesis (combination reaction) is a chemical reaction in which two or more simple substances combine to produce a single compound.
Synthesis (combination reactions)
| Reactants | Examples |
|---|---|
| Between elements | iron + sulfur → iron sulfide zinc + oxygen → zinc oxide |
| Between compounds | sodium oxide + water → sodium hydroxide lead(II) oxide + sulfur trioxide → lead(II) sulfate |
| Between element(s) and compound(s) | carbon + carbon dioxide → carbon monoxide oxygen + sulfur dioxide → sulfur trioxide |
Exam tip: in synthesis, any number of substances go in but only one single compound comes out. Thermal decomposition is the mirror image — one single compound goes in and two or more simpler substances come out.
13 Observations during Chemical Changes
- During chemical changes, one or more of the following may be observed:
| Observation | What to write |
|---|---|
| Change in colour | Say which colour to which colour, and which substance changed. |
| Formation of insoluble solids | 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 / emission of light | The mixture gets hotter or colder, or a flame or glow appears. |
Exam tip: an observation is what you can see or measure, not the conclusion. “Bubbles of a gas that turned limewater milky” is an observation; “carbon dioxide is produced” is the conclusion.
Part C · Effects of chemical changes
14 Beneficial and Harmful Effects
- Chemical changes can bring about both beneficial and harmful effects.
| Beneficial effects | Harmful effects |
|---|---|
| Cooking of food | Rusting |
| Cellular respiration | Air pollution |
| Combustion of fuels | Decay of food |
| Production of medicines | Global warming |
| Manufacture of daily products | Ocean acidification |
15 Air Pollution
Air pollution occurs when unwanted and harmful substances, known as air pollutants, are introduced into the atmosphere.
- Natural processes: lightning, volcanic eruptions.
- Human activities: burning of fossil fuels, industrial activities.
| Air pollutant | Source | Effect |
|---|---|---|
| Carbon monoxide | Forest fires; incomplete combustion of fuels in vehicle engines | Causes respiratory problems; can result in death |
| Sulfur dioxide | Volcanic eruptions; combustion of sulfur impurities in fuels such as coal and petroleum in power stations and factories | Irritates the eyes, nose and lungs, which can cause respiratory problems; formation of acid rain, which harms aquatic life and corrodes buildings |
| Oxides of nitrogen | Lightning; combustion of fuels in vehicle engines, power stations and factories at high temperatures | Irritates the eyes, nose and lungs, which can cause respiratory problems; formation of acid rain, which harms aquatic life and corrodes buildings |
How acid rain is formed
- The sulfur dioxide and oxides of nitrogen released into the air dissolve in rainwater and react with oxygen and water.
- The rain becomes a dilute solution of sulfuric acid and nitric acid. Rain that is more acidic than normal is called acid rain.
sulfur dioxide + oxygen + water → sulfuric acid
nitrogen dioxide + oxygen + water → nitric acid
- Acid rain harms aquatic life in rivers and lakes, corrodes buildings and metal structures, and damages leaves so that plants grow poorly.
- Because the pollutants travel with the wind, the country that suffers the acid rain is often not the country that released the gases.
16 Global Warming
How the greenhouse effect works
- Sunlight passes through the atmosphere and warms up the Earth's surface.
- The warmed surface gives out heat, and certain gases in the air absorb some of that heat and send it back down instead of letting it escape into space.
- These gases are called greenhouse gases. The heat they trap keeps the Earth warm enough for life — this is the greenhouse effect.
- When the amount of greenhouse gases in the air increases, more heat is trapped and less escapes, so the Earth warms up.
- This rise in the Earth's average temperature is global warming.
What puts greenhouse gases into the air
| Greenhouse gas | Where it comes from |
|---|---|
| Carbon dioxide | Respiration and aerobic decomposition; the complete combustion of fuels such as coal, petroleum and natural gas; deforestation |
| Methane | Decaying plant matter in landfills and paddy fields; cattle and other livestock |
- Forests take carbon dioxide out of the air, so clearing them does two things at once: fewer trees remove the gas, and the carbon already stored in the trees is released as carbon dioxide.
What global warming leads to
- Ice caps and glaciers melt, so sea levels rise and low-lying coastlines and islands are at risk of flooding.
- The oceans become warmer, which bleaches and kills coral reefs.
- Ocean temperature, humidity and the speed and direction of the wind all change, giving climate change — longer droughts, heavier storms and more extreme heat.
- Some regions get too much rain and others too little, so harvests fall and food becomes harder to grow.
17 Ocean Acidification
- The oceans absorb much of the extra carbon dioxide that human activity releases into the air.
- Carbon dioxide dissolves in seawater and reacts with water to form carbonic acid, so the oceans slowly become more acidic. This is ocean acidification.
carbon dioxide + water → carbonic acid
- Carbonic acid is an acid, so it reacts with the carbonates in the shells and skeletons of marine organisms such as oysters, clams and corals — the same acid–carbonate reaction you saw in Topic 12.
- The shells and skeletons are weakened or broken down, and the organisms are injured or die.
Why it matters
- Corals with damaged skeletons die off, so the reef crumbles.
- Fish and other animals that feed on or shelter among these organisms are affected, so the whole food chain suffers.
- Shellfish and reef fish become harder to catch, which harms fishing and tourism.
- Reefs also break the force of waves; as they crumble, coastlines are eroded more quickly.
Exam tip: global warming and ocean acidification share one cause — more carbon dioxide in the air — but the mechanisms differ. Global warming comes from the heat the extra carbon dioxide traps; ocean acidification comes from the carbonic acid it forms once it dissolves in seawater.
MAPConcept Map
Chapter 11: Chemical Changes — the ideas that hold the chapter together
Is it a chemical change?
only if
New substances have been formed. A change in colour, size, state or shape on its own is not enough.
Why does the mass stay the same?
because
The atoms are rearranged, never created or destroyed — every atom you started with is still there at the end.
What starts the change?
five triggers
heat → thermal decomposition
oxygen → combustion, cellular respiration, rusting
electricity → electrolysis, electroplating
light → photosynthesis, X-ray film
mixing → neutralisation, synthesis
oxygen → combustion, cellular respiration, rusting
electricity → electrolysis, electroplating
light → photosynthesis, X-ray film
mixing → neutralisation, synthesis
Two reactions that mirror each other
↔
Synthesis: two or more substances → one compound
Thermal decomposition: one compound → two or more simpler substances
Thermal decomposition: one compound → two or more simpler substances
Two ways a fuel can burn
depends on oxygen
Complete combustion (enough oxygen): carbon dioxide + water
Incomplete combustion (too little): carbon monoxide and/or soot
Incomplete combustion (too little): carbon monoxide and/or soot
Two problems, one cause
extra carbon dioxide
Global warming — the heat it traps in the atmosphere
Ocean acidification — the carbonic acid it forms in seawater
Ocean acidification — the carbonic acid it forms in seawater
Why chemical changes matter
both ways
Beneficial: cooking, cellular respiration, fuels, medicines
Harmful: rusting, air pollution, decay, global warming, ocean acidification
Harmful: rusting, air pollution, decay, global warming, ocean acidification