Part 1 of 3 · Materials, Particles and Substances

TaskRead the passage, then answer the questions.

There are hundreds of thousands of different materials in the world. All of them are made up of tiny pieces called particles. Particles are far too small to see, even with the best light microscope. There are about 8 400 000 000 000 000 000 000 000 particles in a single glass of water.

Many materials are mixtures. Wood is a mixture, and so are milk and the air you are breathing. A mixture contains two or more different types of particle, so its properties are not the same all the way through.

Some materials are not mixtures. They consist of just one substance. A substance is made of only one type of material and has the same properties all the way through. Gold, water and oxygen are all substances.

In a pure substance every particle is the same. One gold particle is identical to every other gold particle. However, every substance has its own type of particle, so a gold particle is not the same as a water particle.

Part 2 of 3 · How the Particle Model Explains Properties

TaskRead the passage, then answer the questions.

The properties of a substance describe what it looks like and how it behaves. Every substance has its own properties, and those properties depend on its particles.

The particle model is a way of thinking about how a substance behaves in terms of its tiny moving or vibrating particles. Three things about the particles decide the properties of a substance: what its particles are like, how its particles are arranged, and how its particles move around.

An important idea is that a single particle does not have the properties of the substance. A gold ring is yellow and is a solid at room temperature, but one individual particle of gold is not yellow and is not a solid. Those properties only appear when many particles are together.

TaskComplete the passage using the words below.

Part 3 of 3 · Using the Particle Model to Explain Density

TaskStudy the data, then answer the questions.

The density of a substance is how much matter there is in a certain volume, or how heavy it is for its size. The particle model explains why different substances have different densities: it depends partly on how heavy each particle is, and partly on how closely the particles are packed.

Gold has a higher density than aluminium because one gold particle has a much greater mass than one aluminium particle. Look at the relative masses in the table: a gold particle is about seven times heavier than an aluminium particle, so the same volume of gold weighs far more.

Liquid aluminium has a lower density than solid aluminium. The particles themselves are identical, but in the liquid state they are less closely packed together, so the same volume contains fewer particles and therefore has less mass.

Part 1 of 3 · The Three States of Matter

TaskRead the passage, then complete the practical table.

Water can exist as a solid, a liquid or a gas. These are the three states of matter. The particles of water in ice, in liquid water and in steam are identical, but the properties of the three are very different.

To decide which state a substance is in, chemists ask four questions. Does it have a fixed shape? Can it be compressed — squashed into a smaller volume? Does it take the shape of its container? Can it flow or be poured?

TaskComplete the results table.

Part 2 of 3 · Particle Arrangement in Solids, Liquids and Gases

TaskStudy the diagram, then read the passage.

In the solid state the particles are arranged in a fixed pattern and touch their neighbours. They cannot move around and can only vibrate on the spot. There is no space between them to squash into, so solids cannot be compressed, and because the particles cannot move past each other, solids cannot flow.

In the liquid state the particles still touch their neighbours, but they can slide over each other. This means a liquid can flow and has no fixed shape, taking the shape of the bottom of its container. There is still almost no space between the particles, so liquids cannot be compressed.

In the gas state the particles are spread far apart and move freely in all directions. There is a great deal of empty space between them, so a gas can be compressed, and because the particles move freely a gas flows and spreads out to fill its whole container.

TaskComplete the summary table.

Part 3 of 3 · Using the Particle Model to Explain Properties

TaskRead the passage, then answer the six-mark question.

Being able to explain a property using the particle model is a key chemistry skill. A good explanation names the state, describes how the particles are arranged and how they move, and then links that to the property being explained.

For example: a gas can be compressed. In a gas, the particles are spread far apart with lots of empty space between them, and they move freely in all directions. When you push the plunger of a syringe, the particles are forced closer together into that empty space, so the volume of the gas gets smaller.

Notice that the explanation never says the particles themselves get smaller. It is the arrangement of the particles in space that changes, not the particles.

Part 1 of 3 · Melting and Freezing

TaskRead the passage and study the diagram.

When a substance changes from the solid state to the liquid state, it melts. Melting is a change of state. Freezing is the opposite change of state, from liquid to solid. Liquid water freezes if it is cooled to 0 °C.

The particle model explains melting. First, the surroundings transfer energy to the solid. The particles gain kinetic energy and vibrate faster and faster. Eventually they vibrate so much that they break away from their places in the fixed pattern. As more particles leave the pattern, more of the solid melts.

Freezing is the reverse. The particles transfer energy to the surroundings, so they move more and more slowly. They move closer together and settle into a regular pattern, where they can only vibrate on the spot. Eventually all of the liquid has frozen.

TaskComplete the two particle descriptions below.

Part 2 of 3 · Melting Points and Purity

TaskRead the passage, then use the data table.

The temperature at which a substance melts is its melting point. Melting points are useful because they tell you the state of a substance at a given temperature. Below its melting point a substance is in the solid state; above its melting point it may be a liquid or a gas.

Melting points have two important uses. They help chemists to identify a substance, because different substances have different melting points. They also help chemists to check purity. A pure substance has a sharp melting point — it melts at one single temperature. An impure substance melts over a range of temperatures instead.

Part 3 of 3 · Practical — Cooling Stearic Acid

TaskRead the method and complete the practical.

The graph is called a cooling curve. The temperature falls steadily at first as the liquid cools. Then it stops falling and stays constant for a while, before beginning to fall again. The flat section, or plateau, is where the substance is freezing — so that temperature is the melting point of stearic acid, at about 69 °C.

The temperature stays constant during freezing because all the particles have to rearrange themselves into the regular pattern before the temperature can change again. Energy has to be transferred out of the whole substance, not just a few particles.

Part 1 of 3 · What Happens When a Liquid Boils

TaskStudy the diagram, then read the passage.

When water boils, bubbles of steam form all the way through the liquid, not just at the surface. In the liquid, water particles touch their neighbours. Inside a bubble, the water particles are spread far apart, because the bubble contains water in the gas state.

As the water boils, the steam bubbles rise to the surface of the liquid and escape into the air. The total mass of the steam and the remaining water is exactly the same as the mass of the water at the start. Chemists say that mass is conserved during boiling — no particles are created or destroyed, they simply spread out.

To boil, the water must be given enough energy. As the temperature rises, thermal energy is transferred to the water particles and they move faster and faster. Boiling only happens once enough energy has been transferred. Different substances need different amounts of energy, which is why different substances boil at different temperatures.

The temperature at which a substance boils is its boiling point. For water at sea level this is 100 °C. Boiling point depends on how high above the Earth's surface you are: at Mount Everest Base Camp, 5364 m up, water boils at just 82 °C.

Part 2 of 3 · Practical — Measuring the Boiling Point of Water

TaskRead the method and carry out the practical.

TaskIf you did not obtain a full set of readings, use the data below.

The temperature rises steadily, then levels off, and after a while begins to rise again. The flat section is at 100 °C, which is the boiling point. During the plateau the particles are gaining energy and separating from each other to become a gas, but the temperature does not rise until all of them have gained enough energy.

Part 3 of 3 · Predicting the State of a Substance

TaskStudy the diagram, then use the data table.

If you know the melting point and the boiling point of a substance, you can work out its state at any temperature. The easiest way is to draw a number line. Below the melting point the substance is a solid; between the melting and boiling points it is a liquid; above the boiling point it is a gas.

TaskComplete the table to predict the state of each substance.

Part 1 of 3 · Evaporation

TaskStudy the diagram, then read the passage.

In a liquid, some particles have more energy than others. The particles with the most energy can escape from the liquid surface and move away. Once they have escaped, they spread out to form a gas and mix with the air particles. This process is evaporation.

A substance can change from the liquid state to the gas state in two different ways: by evaporating or by boiling. The difference matters. Evaporation happens only at the surface and at any temperature, whereas boiling happens throughout the liquid and only at the boiling point.

TaskComplete the table comparing evaporation and boiling.

Evaporation is useful. Sweating cools you down: sweat comes out of the pores in your skin, water from the sweat evaporates, and the escaping water particles need energy to move away as a gas. They take that energy from your skin, which cools you down.

A hairdryer speeds up evaporation in two ways. It transfers energy to the water, which helps particles leave the liquid surface, and it blows the just-evaporated water particles away from your hair so more can escape.

Part 2 of 3 · Condensation and Sublimation

TaskRead the passage, then answer the questions.

Think about why the inside of a bedroom window is wet after a cold night. At bedtime, water particles were mixed with the air particles, spread out as a gas. During the night, those water particles hit the cold glass of the window. They lost energy, moved closer to other water particles until they were touching, and formed liquid water.

The change of state from gas to liquid is called condensation. It is the reverse of boiling and evaporation. Condensation happens when a gas is cooled, because the particles lose energy and can no longer stay spread apart.

Most substances change from solid to liquid to gas as they are heated, but a few skip the liquid state entirely. Solid carbon dioxide, often called dry ice, is solid at temperatures below −78.5 °C. At that temperature and above, it changes state directly to become a gas. It does not normally exist as a liquid at all.

The change of state directly from solid to gas is called sublimation. It is what produces the thick white stage smoke used in theatres and concerts, which is made using dry ice.

Part 3 of 3 · Practical — Growing the Biggest Crystals

TaskRead the passage, then plan and carry out the practical.

Crystals of salts form when the solution they are dissolved in evaporates. The size of the crystals that form depends on how quickly the evaporation happens: slower evaporation forms bigger crystals. Your task is to compete with the rest of the class to grow the biggest copper sulfate crystals.

The control sample is important. Without it you would have nothing to compare your crystals against, so you could not tell whether your change had made any difference at all.

Part 1 of 3 · What Is Diffusion?

TaskStudy the diagram, then read the passage.

Diffusion is the random movement of particles spreading out from an area of high concentration to an area of low concentration. Concentration means how many particles of a substance there are in a given space.

Diffusion explains everyday smells. When household waste rots at a landfill site, gases with an unpleasant smell are produced. Those gas particles start out at a very high concentration at the tip and spread out through the air until they can be detected a long way away.

Diffusion also happens in liquids. If you pour water into a glass and then add some squash, the colour of the squash spreads through the whole liquid. The squash particles, which cause the colour, mingle with the water particles until they are evenly mixed.

Part 2 of 3 · What Affects the Speed of Diffusion?

TaskRead the passage, then complete the practical.

Diffusion does not always happen at the same speed. Three factors affect how fast a substance diffuses: the temperature, the size of the particles, and the state of the diffusing substance.

Diffusion happens fastest in gases, because the particles are far apart. A particle in a gas travels a long way before it hits another particle. In a liquid the particles are much closer together, so a particle collides much sooner and diffusion is slower. Diffusion does not happen in solids at all, because the particles cannot move from place to place.

Part 3 of 3 · Evidence That Particles Exist

TaskStudy the diagram, then read the passage.

Particles are far too small to see, so how do chemists know they exist? Diffusion in gases and liquids is one piece of evidence: something must be moving to carry a smell across a room.

In 1827, Robert Brown found a second piece of evidence, although he did not realise it at the time. Brown was looking through a microscope at pollen grains floating in water. He noticed that the pollen grains were moving around in a jerky, random way, and he could not explain why.

Many years later, Albert Einstein explained what Brown had seen. Fast-moving water particles were colliding with the pollen grains and pushing them in different directions. This random movement is now called Brownian motion, named after Robert Brown, and it is strong evidence that particles exist and are constantly moving.

You can see the same effect at the cinema: specks of dust in the beam of the projector jiggle about randomly as air particles collide with them.

Part 1 of 3 · Mass, Volume and Density

TaskRead the passage, then answer the questions.

Mass is the amount of matter, or 'stuff', in an object. It is measured in grams (g) or kilograms (kg). The volume of an object is the amount of space it takes up, measured in cubic centimetres (cm³) or cubic metres (m³).

The density of a substance is its mass in a certain volume. It tells you how heavy something is for its size. An iron dumbbell has a mass of 20 kg. An aluminium barbell of exactly the same size has a mass of only 7 kg. Iron has a greater density than aluminium, so the iron one is much heavier even though they take up the same space.

The particle model explains why different substances have different densities. Look at the data below: iron has a greater density than aluminium partly because an iron particle has a greater mass than an aluminium particle. The density of a substance depends on the mass of its particles as well as on how closely they are packed.

Part 2 of 3 · How the State Affects Density

TaskStudy the data, then read the passage.

The same substance has different densities in its three states, even though the particles themselves are identical. The difference comes from how closely the particles are packed.

Solid gold has a greater density than liquid gold, because the particles are packed together more tightly in the solid. In the gas state the particles are far apart, so gold gas has a much lower density than liquid gold. Most substances follow this pattern: their density is greatest in the solid state, lower in the liquid state, and lowest of all in the gas state.

TaskComplete the passage using the words below.

Water is unusual. Below 4 °C its density is greater in the liquid state than in the solid state, because the water particles pack together more closely as a liquid. This is why ice floats on liquid water — and it is why ponds freeze from the top down rather than the bottom up.

Part 3 of 3 · Calculating Density

TaskRead the passage, then work through the example.

You can calculate the density of a substance if you know its mass and its volume.

The unit of density depends on the units you use for mass and volume. If the mass is in grams and the volume in cm³, the density is in g/cm³. If the mass is in kilograms and the volume in m³, the density is in kg/m³.