Everything in the Universe is changing all the time: stars are born and die, rivers change direction, mountains change shape, and our bodies grow older. Chemists are most interested in changes that involve the formation of new substances. These types of change are called CHEMICAL REACTIONS.
Chemical reactions happen around us all the time, even at home! Whenever materials are changed and new substances are created then a chemical change has taken place.
Everyday examples include growing plants, cooking food, burning fuels, rusting iron, making plastics, etc.
The chemistry lab is an obvious place to find examples of chemical reactions. There are many ways of telling if a chemical reaction has happened:
Effervescence
If a chemical reaction fizzes then it normally means that a gas (the new substance) has been produced. The fizzing is called
EFFERVESCENCE. Examples of reactions that produce effervescence are the reaction of lumps of marble (calcium carbonate) with acid, and the reaction of calcium granules with water.
Precipitation
Evidence for a chemical reaction may be a PRECIPITATE forming (a solid formed from the reaction between two solutions). An example of a precipitation reaction is when sodium carbonate reacts with calcium nitrate to produce a white precipitate of calcium carbonate.
Colour change
In some reactions a permanent colour change indicates the formation of new substances. For example, when green copper carbonate is heated it quickly changes into a black powder of copper oxide, with carbon dioxide also produced. The colour change is permanent, i.e. it cannot be reversed.
TOPIC 7.2: PHYSICAL AND CHEMICAL CHANGE 1

Energy changes
Most chemical reactions involve an energy change. EXOTHERMIC reactions give out heat, whereas ENDOTHERMIC reactions take in heat. Exothermic reactions include burning fuels and fireworks. For the thermal decomposition of copper carbonate (see ‘colour change, above), heat must be absorbed for the reaction to happen, so that is a good example of an endothermic reaction.
In a chemical reaction, the substances we start with are called the REACTANTS, and the new substances produced are called the PRODUCTS. Chemical reactions are described using chemical reactions, which show the direction of chemical change. In a WORD
EQUATION the names of the reactants are written on the left, and the names of the products are named on the right, e.g.
Reactants → Products
The arrow ( → ) shows the direction of chemical change. It is important you understand just what is being shown above. When a chemical reaction is complete then the reactants
no longer exist; they have been changed into products (new substances). So a word equation shows the substances that are present ‘before’ and ‘after’ the reaction happened.
Consider the reactions described earlier:
1) Calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water.
In this reaction the reactants are calcium carbonate and hydrochloric acid; the products are calcium chloride, carbon dioxide and water.
2. Calcium nitrate + sodium carbonate → calcium carbonate + sodium nitrate
In this reaction the reactants are calcium nitrate and sodium carbonate; the products are calcium carbonate and sodium nitrate.
3. Copper carbonate → copper oxide + carbon dioxide
In this reaction the reactant is copper carbonate; the products are copper oxide and carbon dioxide.
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So, to summarise, when a chemical reaction happens the reactants are turned into products, and the products are completely different from the reactants that made them.
Chemical reactions are very difficult, and often impossible, to reverse. For example, it’s easy to turn flour, eggs, water, butter and sugar into a cake, but it’s impossible to turn a cake back into raw ingredients! Consider also what happens when you strike a match: you create lots of ash, and gases that escape. From this charred ash and gases it’s not possible to make a new match, so the reaction is irreversible.
Physical changes differ from chemical changes in that no new substances are produced during the change. A physical change may involve MIXING substances together e.g.
DISSOLVING, or it may involve a CHANGE OF STATE (e.g. a solid becoming a liquid). The key thing to remember is that the actual substance(s) involved are the same; it’s just that they might now be in a different physical form. As a result, physical changes are usually easy to reverse.
Mixing substances
Mixing substances together does not produce new substances, and mixtures are quite easy to separate.
Salt can dissolve in water forming a salt solution. Both the water and the salt are still water and salt, it’s just that now they’re mixed together in the same container. The salt and water can be separated by evaporating off the water, leaving the salt crystals behind.
Powdered iron and sulphur can be mixed together without reacting. It’s easy to separate the iron from the sulphur by using a magnet (iron is magnetic, whereas sulphur is not).
Other important mixtures include:
 Sea water: a mixture of water, salt and other dissolved substances
 Air: a mixture of gases such as oxygen, nitrogen, argon and carbon dioxide
 Rocks: a mixture of different minerals
 Soil: a mixture of rock pieces, water, air, dissolved minerals and humus.
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Matter is a scientific word used to describe all the different types of substances found in our universe. The three states of matter are SOLID, LIQUID and GAS.
SOLID
A solid has a fixed shape and a fixed volume. A force must be applied to change the shape of a solid.
LIQUID
A liquid has a fixed volume but no fixed shape. It can flow and take the shape of any container.
GAS
A gas has no fixed shape or volume. A gas can spread out in all directions and will fill any container.
Most substances can exist in all three states, but not all at once! The state of a substance depends on the temperature, so changes of state happen by heating and cooling. The changes between the different states are summarised below.
In a pure substance changes of state happen at fixed temperatures (except by evaporation). The BOILING POINT of a liquid is the temperature at which it quickly changes into a gas. The MELTING POINT of a solid is the temperature it changes into a liquid.
The freezing point of a substance is the same as its melting point. In other words, the temperature at which a solid changes into a liquid (melting) is the same at which the liquid changes back into a solid (freezing).
For example, solid ice melts into liquid water at 0 °C, and liquid water freezes into ice at
0 °C.
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The particles in a solid are held closely together by strong forces of attraction. The particles cannot move freely, but they do vibrate about a fixed position. This means that solids have a fixed volume and shape.
The particles in a liquid have more energy than the particles in a solid. The forces of attraction are weaker, and so the particles are further apart and can move past each other freely. This means that liquids have fixed volumes and take the shape of their container.
Scientists know that all matter is made up tiny particles called atoms or molecules, which are in constant motion. Between these different types of particle are forces of attraction that hold the particles together, and these forces vary in the different states of matter. This model of matter is called PARTICLE THEORY.
The difference between solids, liquids and gases can be explained in terms of particle theory.
The particles in a solid are held closely together by strong forces of attraction. The particles cannot move freely, but they do vibrate about a fixed position. This means that solids have a fixed volume and shape.
The particles in a liquid have more energy than the particles in a solid. The forces of attraction are weaker, and so the particles are further apart and can move past each other freely. This means that liquids have fixed volumes and take the shape of their container.
The particles in a gas have much more energy than those in the liquid, so only have very weak forces of attraction between them. This means that the particles are very far apart and move about in all directions to fill their container.
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Being able to explain simple physical changes is really important in our understanding of the nature of matter.
Melting
When a solid is heated, energy is supplied to the particles of the solid, making them vibrate faster. As the temperature increases, the speed of movement of the particles increases. Eventually the particles will have enough energy to break free from each other and move freely. This is melting and a liquid is formed.
Boiling
When a liquid is heated the particles in the liquid move faster. As the temperature rises the particles move faster, until they are moving fast enough that they escape from the liquid to the gas state. This is how boiling happens.
Evaporation
Evaporation is often confused with boiling, but they are not the same thing. Particles in a liquid have different energies, so some move more quickly than others. As a result, there will always be some particles that are moving fast enough to escape from the surface of the liquid to join the gas above it. Evaporation is therefore a type of mixing that happens between the liquid’s surface and the gas directly above it. This is why puddles dry up even on a cold day.
Condensation and freezing
When a gas is cooled the particles of the gas move more slowly. If the temperature falls to a certain value then the particles move slowly enough to come back together again and form a liquid. This is condensation.
Similarly, when a liquid is cooled the particles in the liquid begin to move more slowly.
Eventually they will move slowly enough so that they cannot move freely, and a solid is made. This is what happens during freezing.
Sublimation
Some substances, such as iodine and graphite (the material in pencils), do not melt when heated. Instead, they change from a solid directly into a gas, without going through the liquid state. This change is reversible, just like melting and boiling. The opposite process, i.e. when a gas changes straight into a solid, is also called sublimation.
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The graph above shows what happens to the temperature of a substance as it changes state.
1.
Starting from the solid state, the temperature increases steadily
2.
When the temperature reaches the melting point for that particular substance the particles in the solid start separate and a liquid is formed. The temperature does not change because all the energy being supplied is being used to separate the particles from each other. This is the melting point and is shown by the first flat section of the graph.
3.
Once the entire solid has turned into a liquid then the energy being supplied increases the speed of the particles. Now the graph shows the temperature of the liquid rising steadily.
4.
The temperature of the liquid continues to rise until it reaches the boiling point for that particular substance. Here, the particles in the liquid begin to separate and a gas if formed. The temperature does not change during boiling because the energy being supplied is used to separate the particles in the liquid. This is the boiling point and is shown by the second flat section of the graph.
5.
Once all the liquid has turned into a gas then the energy being supplied makes the particles in the gas move with increasing speed. Now the graph shows the gas becoming steadily hotter.
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TOPIC SUMMARY
A reaction in which a new substance is produced is called a ___________ reaction.
Such reactions are not easily ____________.
A chemical reaction can be identified by a _________ change; the production of a gas known as _____________ or a change in ____________ (exothermic or endothermic).
___________ is a process in which a solid material is produced by the reaction of two solutions.
In a chemical reaction, the substances we start with are called the reactants, and the new substances produced are called the _______________.
In an equation the reactants are shown on the ______ hand side of the arrow.
In the equation hydrogen + oxygen  water the hydrogen and oxygen are used up and there is ______ left at the end. The water which is formed was ______ there at the start.
A physical change does not produce a new chemical substance but may involve
_________ substances together or could also involve a change of ________.
When a gas turns into a liquid the process is called _____________.
______________ is when a solid turns directly into a gas.
In a solid the particles are arranged in a ________ pattern and cannot ______ places. When a solid melts its particles become free to __________.
In a _____ the particles are able to move in all directions and can ______ the container they are in. condensation reversed chemical energy precipitation none mixing state change colour left not sublimation fixed effervescence products move gas fill
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