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Solid ice, liquid water and gaseous steam all in one shot - same particles, just different arrangements and energy
Everything around you is a solid, a liquid or a gas. These are the three states of matter. A chocolate bar, the juice in a carton and the air you breathe are made of particles, and the state depends on how those particles are arranged and how they move.
Chemists show this with a simple particle model. In this model, each particle is drawn as a small solid sphere (a little ball). The particles could be atoms, ions or molecules, but the model draws them all the same way.
Particles are packed closely in a regular pattern. They vibrate about fixed positions. A solid keeps its own shape and cannot be squashed.
Particles are still close together but arranged randomly. They can move around each other. A liquid flows and takes the shape of its container.
Particles are far apart and arranged randomly. They move quickly in all directions. A gas spreads out to fill its container and can be squashed.
When you draw these in an exam, make the solid a neat grid of touching circles, the liquid a jumbled group of mostly touching circles, and the gas a few circles spread well apart. Keep the circles the same size in all three, because it is the same substance.
Key terms:
This chocolate is melting at its melting point - the same temperature at which it would freeze again
There are four changes of state you need to know:
Here is the key fact from the spec: melting and freezing take place at the melting point, and boiling and condensing take place at the boiling point. A substance's freezing point is the same temperature as its melting point. Pure water, for example, melts at 0 °C and also freezes at 0 °C.
Changes of state are physical changes. No new substance is made. The particles stay the same, only their arrangement and movement change.
Particle theory explains changes of state using energy.
Energy is transferred to the substance, usually by heating. The particles gain energy and this energy is used to overcome the forces holding them together. In melting, enough forces are overcome for the particles to move around each other. In boiling, the forces are overcome so the particles can move far apart.
Energy is transferred from the substance to the surroundings as it cools. The particles lose energy, and the forces between them pull them back closer together. Gas particles condense into a liquid, and liquid particles freeze into a fixed pattern.
The spec says the amount of energy needed to change from solid to liquid and from liquid to gas depends on the strength of the forces between the particles.
Which forces are involved depends on the type of bonding and the structure of the substance. You met ionic, covalent and metallic bonding in earlier lessons. In giant structures, melting means breaking lots of strong bonds, so melting points are high. Substances made of small molecules have only weak forces between the molecules, so they melt and boil at low temperatures. You will look at each structure in detail in Properties of Ionic Compounds and Small Molecules, Polymers and Giant Covalent Structures, and Properties of Metals and Alloys.
Properties such as melting point, colour, hardness or being able to conduct electricity are bulk properties. They belong to a large amount of a substance, made of a huge number of particles together.
A single atom does not have these properties. One iron atom does not have a melting point, because melting is about many particles changing how they are arranged. One atom cannot be "solid" or "liquid" on its own. The properties come from how the particles are arranged and how strongly they are held together.
The simple model is useful, but it has limitations. The spec lists three:
So when the spec talks about solid inelastic spheres which have no forces between them, the key point is this: a model with no forces cannot explain why energy must be transferred at the melting and boiling points.
You can work out the state of any substance at a given temperature if you know its melting point and boiling point. Use this rule:
A number line helps. Mark the melting point and boiling point, then see where your temperature falls.
The table shows data for four substances.
What state is each at 25 °C?
A: 25 is above −114 but below 78, so liquid.
B: 25 is below 801, so solid.
C: 25 is above −183, so gas.
D: 25 is below 44, so solid.
Which substance has the strongest forces between its particles? B, because it has the highest melting point and boiling point.
What state is D at 100 °C? 100 is between 44 and 280, so liquid.
In chemical equations, we show the state of each substance with a state symbol in brackets after its formula.
Some examples:
You need to add the right state symbols to equations for the reactions you meet across the course, so look out for them in later lessons.
Key terms:
Water itself is (l), never (aq). Aqueous means dissolved in water, so the water can't be dissolved in itself. Also, check the conditions in the question. Water in a beaker is H2O(l), but steam is H2O(g).
Substance X has a melting point of −7 °C and a boiling point of 59 °C. Substance Y has a melting point of 1085 °C and a boiling point of 2562 °C.
(a) Give the state of X at 20 °C. [1 mark]
(b) Explain, in terms of forces and energy, why Y has a much higher melting point than X. [3 marks]
(a) Liquid (20 °C is between −7 °C and 59 °C).
(b) The forces between the particles in Y are much stronger than in X (1). Melting needs energy to overcome these forces (1). So more energy is needed to melt Y, which means a higher temperature (1).
In "explain" questions about melting or boiling points, always link three things: the strength of the forces, the energy needed to overcome them, and the temperature. Missing out energy loses a mark.