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1.1.1 States of matter and the particle model

Properties of solids, liquids and gases

Definition

Solid

A state of matter that has a fixed shape and a fixed volume because its particles are held in fixed positions.

Definition

Liquid

A state of matter that has a fixed volume but no fixed shape, so it flows and takes the shape of its container.

Definition

Gas

A state of matter that has no fixed shape and no fixed volume, so it spreads out to fill the whole of any container.

  1. Every substance exists in one of three states of matter: solid, liquid or gas.
  2. You can tell the three states apart by four properties: their shape, their volume, whether they can flow and whether they can be compressed.
  3. Solids
    1. A solid has a fixed shape, so it keeps the same shape when it is moved from one container to another.
    2. A solid has a fixed volume, so the same mass of solid always takes up the same amount of space at a given temperature.
    3. A solid does not flow, which is why an ice cube keeps its shape on a plate until it melts.
    4. A solid is very difficult to compress, so squeezing it does not make its volume noticeably smaller.
  4. Liquids
    1. A liquid has no fixed shape, so it takes the shape of the part of the container that it fills.
    2. A liquid has a fixed volume, so 100 cm3100\ \text{cm}^3100 cm3 of water still measures 100 cm3100\ \text{cm}^3100 cm3 after it is poured from a beaker into a measuring cylinder.
    3. A liquid flows, which is why it can be poured.
    4. A liquid is very difficult to compress, just like a solid.
  5. Gases
    1. A gas has no fixed shape and no fixed volume, because it spreads out to fill the whole of any container it is placed in.
    2. A gas flows easily, which is why gases can be carried through pipes.
    3. A gas is easily compressed, which is how a large volume of air can be squeezed into a bicycle tyre or a diving cylinder.
  6. The same substance can exist in all three states, so water is solid ice below 0 ∘C0\,^{\circ}\text{C}0∘C, liquid water between 0 ∘C0\,^{\circ}\text{C}0∘C and 100 ∘C100\,^{\circ}\text{C}100∘C, and gaseous steam above 100 ∘C100\,^{\circ}\text{C}100∘C at normal atmospheric pressure.

Particles in solids, liquids and gases

Definition

Particle model

A model in which all matter is made of tiny particles whose separation, arrangement and motion decide whether a substance is a solid, a liquid or a gas.

  1. The particle model explains the properties of each state by describing three features of its particles: their separation, their arrangement and their motion.
  2. Particles in a solid
    1. Separation: the particles are very close together and touch their neighbours.
    2. Arrangement: the particles are packed in a regular arrangement, in neat repeating rows.
    3. Motion: the particles vibrate about fixed positions and do not move from place to place.
    4. The particles cannot leave their fixed positions, so a solid keeps its shape and cannot flow.
    5. There is almost no space between the particles, so a solid cannot be compressed.
  3. Particles in a liquid
    1. Separation: the particles are close together, and most of them still touch their neighbours.
    2. Arrangement: the particles are in an irregular arrangement, with no fixed pattern.
    3. Motion: the particles move randomly and slide past one another.
    4. The particles can slide past each other, so a liquid flows and takes the shape of its container.
    5. The particles stay close together, so a liquid keeps a fixed volume and is very difficult to compress.
  4. Particles in a gas
    1. Separation: the particles are far apart, with large empty spaces between them.
    2. Arrangement: the particles are randomly arranged, with no pattern at all.
    3. Motion: the particles move quickly and randomly in all directions, travelling in straight lines until they hit another particle or the walls of the container.
    4. The particles move freely in every direction, so a gas spreads out to fill its whole container.
    5. There are large spaces between the particles, so a gas can be squeezed into a smaller volume.
  5. In a particle diagram, solid particles are drawn touching in neat rows, liquid particles are drawn touching but jumbled, and gas particles are drawn widely spaced with only a few in the box.

Particle model diagram showing the arrangement and separation of particles in a solid, liquid, and gas.

Exam technique

Describing particles

  • When a question asks you to describe the particles in a state, give all three features: separation, arrangement and motion.
  • Write that the particles in a solid vibrate about fixed positions, because the statement that they "do not move" is wrong and gains no credit.
  • Use the word random for the arrangement of gas particles and for the motion of both liquid and gas particles.
  • Draw the particles the same size in all three states, and change only how close together they are and how they are arranged.
Common Mistake

Do not say that the particles themselves get bigger when a solid melts or a liquid boils, because the particles stay the same size and only the spaces between them change.

Melting and freezing

Definition

Melting

The change of state from a solid to a liquid that takes place when a solid is heated to its melting point.

Definition

Freezing

The change of state from a liquid to a solid that takes place when a liquid is cooled to its freezing point.

Definition

Melting point

The temperature at which a solid changes into a liquid.

  1. A change of state is a change from one state to another in which the substance itself stays the same, so ice, liquid water and steam are all H2O\text{H}_2\text{O}H2​O.
  2. A change of state can be reversed by heating or cooling the substance again.
  3. Melting takes place when a solid is heated until it reaches its melting point.
  4. During melting, the particles stop vibrating about fixed positions and begin to slide past one another, so the regular arrangement of the solid is lost.
  5. Freezing is the reverse of melting, and it takes place when a liquid is cooled until it reaches its freezing point.
  6. During freezing, the particles stop sliding past one another and settle into fixed positions in a regular arrangement.
  7. For a single substance, the freezing point is the same temperature as the melting point, so water freezes at 0 ∘C0\,^{\circ}\text{C}0∘C and ice melts at 0 ∘C0\,^{\circ}\text{C}0∘C.
  8. Different substances have different melting points, so iron melts at about 1538 ∘C1538\,^{\circ}\text{C}1538∘C while candle wax melts at about 60 ∘C60\,^{\circ}\text{C}60∘C.

Boiling, evaporation and condensation

Definition

Boiling

The rapid change of state from a liquid to a gas that takes place throughout the liquid at its boiling point.

Definition

Evaporation

The change of state from a liquid to a gas that takes place only at the surface of the liquid, at temperatures below its boiling point.

Definition

Condensation

The change of state from a gas to a liquid that takes place when a gas is cooled.

Definition

Boiling point

The temperature at which a liquid boils and changes into a gas throughout the liquid.

  1. Boiling and evaporation both change a liquid into a gas, but they happen in different ways.
  2. Boiling
    1. Boiling takes place only at the boiling point of the liquid, so pure water at normal atmospheric pressure boils at 100 ∘C100\,^{\circ}\text{C}100∘C.
    2. Boiling takes place throughout the liquid, so bubbles of gas form inside the liquid, rise to the surface and escape.
    3. Boiling is fast and continues only while the liquid is being heated.
  3. Evaporation
    1. Evaporation takes place at any temperature below the boiling point, which is how a puddle dries on a cool day.
    2. Evaporation takes place only at the surface of the liquid, so no bubbles form.
    3. Evaporation is much slower than boiling.
  4. During both boiling and evaporation, particles leave the close, irregular arrangement of the liquid and become widely spaced, randomly moving gas particles.
  5. Condensation
    1. Condensation is the reverse of boiling and evaporation, and it turns a gas back into a liquid when the gas is cooled.
    2. During condensation, the widely spaced gas particles come close together and take up the irregular arrangement of a liquid.
    3. Steam from a kettle condenses into tiny droplets of water when it meets a cold window, which is why the glass mists over.
  6. The five changes of state therefore form two reversible pairs: melting and freezing link solid and liquid, while boiling or evaporation and condensation link liquid and gas.
Common Mistake
  • Do not use boiling and evaporation as if they mean the same thing.
  • Boiling happens throughout the liquid at one fixed temperature, while evaporation happens only at the surface and at any temperature below the boiling point.
Example

Predicting the state of a substance

  • Room temperature is taken as 25 ∘C25\,^{\circ}\text{C}25∘C, so compare 25 ∘C25\,^{\circ}\text{C}25∘C with the melting point and the boiling point of the substance.
  • A substance is a solid at room temperature when its melting point is above 25 ∘C25\,^{\circ}\text{C}25∘C.
  • A substance is a liquid at room temperature when its melting point is below 25 ∘C25\,^{\circ}\text{C}25∘C and its boiling point is above 25 ∘C25\,^{\circ}\text{C}25∘C.
  • A substance is a gas at room temperature when its boiling point is below 25 ∘C25\,^{\circ}\text{C}25∘C.
  • Bromine melts at −7 ∘C-7\,^{\circ}\text{C}−7∘C and boils at 59 ∘C59\,^{\circ}\text{C}59∘C, so at 25 ∘C25\,^{\circ}\text{C}25∘C it has melted but not yet boiled, and it is a liquid.
  • Chlorine boils at −34 ∘C-34\,^{\circ}\text{C}−34∘C, so at 25 ∘C25\,^{\circ}\text{C}25∘C it is far above its boiling point, and it is a gas.
  • Sodium chloride melts at 801 ∘C801\,^{\circ}\text{C}801∘C, so at 25 ∘C25\,^{\circ}\text{C}25∘C it is far below its melting point, and it is a solid.

Gas volume, temperature and pressure

Definition

Gas pressure

The force that a gas exerts on each unit area of the walls of its container.

  1. The volume of a gas changes far more than the volume of a solid or a liquid when its temperature or its pressure changes.
  2. To see each effect clearly, you must keep the mass of gas fixed and change only one condition at a time.
  3. Effect of temperature at constant pressure
    1. When a fixed mass of gas is heated, its volume increases and the gas expands.
    2. When a fixed mass of gas is cooled, its volume decreases and the gas contracts.
    3. A balloon carried from a warm room into a freezer shrinks, then swells back to its original size when it warms up again.
  4. Effect of pressure at constant temperature
    1. When the pressure on a fixed mass of gas is increased, its volume decreases.
    2. When the pressure on a fixed mass of gas is decreased, its volume increases.
    3. Pushing in the plunger of a sealed syringe full of air increases the pressure on the air and squeezes it into a smaller volume.
    4. Releasing the plunger lets the pressure fall back, so the air expands and pushes the plunger out again.
Practical

Effect of temperature on the volume of a gas

  • Aim: to measure how the volume of a fixed mass of air changes as its temperature rises at constant pressure.
  • Apparatus: a dry 100 cm3100\ \text{cm}^3100 cm3 conical flask fitted with a rubber bung and a short delivery tube, a 100 cm3100\ \text{cm}^3100 cm3 gas syringe with a freely moving plunger, a large beaker to act as a water bath, crushed ice, a thermometer reading from −10 ∘C-10\,^{\circ}\text{C}−10∘C to 110 ∘C110\,^{\circ}\text{C}110∘C, a stirring rod, a Bunsen burner, tripod, gauze, heatproof mat, two clamp stands and eye protection.
  • Method:
    • Push the plunger of the gas syringe fully in, then connect the syringe to the delivery tube so that the air in the flask is sealed inside the apparatus.
    • Clamp the flask so that it sits as deep as possible in the beaker, then fill the beaker with a mixture of water and crushed ice until the flask is almost completely covered.
    • Clamp the gas syringe horizontally so that the plunger can slide out freely without being pulled by gravity or held back by friction.
    • Stir the water, wait until the syringe reading stops changing, then record the temperature of the water and the volume reading on the syringe.
    • Heat the water gently with the Bunsen burner, and remove the flame each time the temperature has risen by about 10 ∘C10\,^{\circ}\text{C}10∘C.
    • Stir the water and wait about two minutes, so that the air inside the flask reaches the same temperature as the water around it.
    • Record the temperature and the syringe reading each time, and continue until the water reaches about 80 ∘C80\,^{\circ}\text{C}80∘C.
    • At the end, fill the flask completely with water and pour the water into a measuring cylinder to find the volume of the flask.
    • Add the volume of the flask to each syringe reading to give the total volume of air at each temperature.
    • Plot a graph of volume of air / cm3\text{cm}^3cm3 on the vertical axis against temperature / ∘C^{\circ}\text{C}∘C on the horizontal axis and draw a line of best fit.
  • Variables: the independent variable is the temperature of the air, the dependent variable is the volume of the air, and the controlled variables are the mass of air, which stays fixed because the apparatus is sealed, and the pressure, which stays equal to atmospheric pressure because the plunger moves freely.
  • Results: the plunger moves steadily outwards as the water is heated, and the points on the graph lie close to a straight line that slopes upwards.
  • Conclusion: at constant pressure, the volume of a fixed mass of gas increases as its temperature increases.
  • Sources of error: a sticking plunger raises the pressure inside and makes the volume readings too small, a leak at the bung lets air escape, and taking a reading before the air has reached the temperature of the water makes the volume too small.
  • Safety: wear eye protection, take care with hot water and the hot tripod, clamp the glassware firmly, and never heat the flask with the syringe plunger fixed in place, because the trapped air would build up pressure.
Self review
  • State three properties that distinguish a gas from a solid.
  • Describe the separation, arrangement and motion of the particles in a liquid.
  • State two differences between boiling and evaporation.
  • Name the change of state that turns a gas into a liquid.
  • State what happens to the volume of a fixed mass of gas when its pressure is increased at constant temperature.
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1.1.1 States of matter and the particle model Revision Guide

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Revision notes for CIE IGCSE Chemistry 1.1.1 States of matter and the particle model: explanations and worked examples.

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