4.1Chapter 4: Chemical Reactions 1

What are chemical reactions? — Alhussan Chemistry - Stage 7 Revision Notes & Topical Questions

5 min read 12 parts Lesson 1 of 6 3 quizzes

📚 Alhussan Chemistry - Stage 7 Hub / Topical Revision

0%read

Lesson summary

4.1 What are chemical reactions?

Not easily reversed
Everywhere
Word equation
Different properties
Physical changes
Reactants and product

Section 1 of 12What a chemical reaction is

Chemical reaction

Chemical reaction a change that makes new substances, in which atoms are rearranged and join together differently.

Not easily reversed

Not easily reversed at the end it is difficult to get back to the starting substances.

Energy

Energy all chemical reactions transfer energy to or from the surroundings.

Everywhere

Everywhere chemical reactions happen in places such as engines, kitchens and inside your body.

Section 2 of 12Reactants and products

Reactants

Reactants the starting substances in a chemical reaction.

Products

Products the substances that are made.

Word equation

Word equation an arrow means "react to make", with reactants on the left and products on the right.
Hydrogen + oxygen → water.

Different properties

Different properties the products have different properties from the reactants because their atoms are rearranged into new substances.

Section 3 of 12Signs of a chemical reaction

Flames or sparks.

A smell, sweet or foul.

Substances getting hotter or colder.

A loud bang or gentle fizzing.

Colour change, gas, new solid (precipitate) or energy change.

Physical changes

Physical changes dissolving and changing state are not chemical reactions.

Section 4 of 12Burning magnesium

Reaction

Reaction a piece of shiny magnesium held in a flame burns brightly, joins with oxygen from the air and leaves white ash.

Reactants and product

Reactants and product the reactants are magnesium and oxygen, and the one product is magnesium oxide (the white ash).

Different properties

Different properties the reactants and the product look different.

Section 5 of 12Magnesium and acid

Observation

Observation magnesium added to hydrochloric acid bubbles.

Product

Product the bubbles are hydrogen gas, one of the products.

Displacement

Displacement magnesium reacts with iron oxide to make magnesium oxide and iron.

Section 6 of 12Working safely

Flammable

Flammable magnesium burns quickly and has the hazard symbol "flammable".

Hazard symbol

Hazard symbol a symbol that shows how a substance can cause harm, and hazard symbols are the same everywhere.

Reducing risks

Reducing risks keep the roll of magnesium away from flames.
Hold the magnesium with tongs because it burns with a hot flame.
The bright white flame can damage eyesight, so do not look at it directly.

Section 7 of 12Types of reaction

Types of reaction

Combustion

Combustion a fuel burns in oxygen, giving out energy, for example methane + oxygen → carbon dioxide + water.

Thermal decomposition

Thermal decomposition one substance breaks down on heating, for example calcium carbonate → calcium oxide + carbon dioxide.

Oxidation

Oxidation a substance gains oxygen, for example magnesium + oxygen → magnesium oxide.

Displacement

Displacement a more reactive metal pushes out a less reactive one from a compound.

Neutralisation

Neutralisation an acid and alkali react to form a salt and water.

Section 8 of 12Energy changes

Exothermic

Exothermic a reaction that gives out energy to the surroundings and the temperature rises, such as burning, respiration and neutralisation.

Endothermic

Endothermic a reaction that takes in energy and the temperature falls, such as thermal decomposition and photosynthesis.

Uses

Uses hand warmers use exothermic reactions and instant cold packs use endothermic ones.

Section 9 of 12Chemical or physical change

Chemical change

Chemical change new substances form and it is usually hard to reverse, for example burning paper.

Physical change

Physical change no new substances form and it can be reversed easily, such as melting, boiling or dissolving.

Section 10 of 12Speed of reactions

Rusting is slow, burning is fast and an explosion is very fast.

Everyday reactions

Everyday reactions baking, digestion, photosynthesis and fireworks, where metal compounds burn with coloured flames.

Starting a reaction

Starting a reaction many need activation energy, which is why a match must be struck.

Catalyst

Catalyst it makes a reaction go faster without being used up.

Atoms

Atoms the total number of atoms stays the same because reactions rearrange atoms only.

Section 11 of 12Key terms

Term	Meaning
Chemical reaction	A change that makes new substances.
Reactant	A starting substance in a chemical reaction.
Product	A substance made in a chemical reaction.
Hazard symbol	A symbol showing how a substance can cause harm.
Exothermic	Giving out energy to the surroundings.
Endothermic	Taking in energy from the surroundings.