Materials (Junior Cert Science): Revision Notes

Materials

Introduction

Everything around us is made of matter. We define matter as anything that occupies space and has mass. The chair you sit on, the water you drink, and even the air you breathe are all examples of matter. All matter exists in different forms, which we call the states of matter.

States of matter

There are three main states of matter that we encounter in everyday life: solids, liquids, and gases. These three forms of matter behave differently from each other because of how their tiny particles are arranged and how much they can move.

Space and mass

While all matter occupies space and has mass, different states occupy space in different ways. A solid like wood occupies a fixed amount of space. A liquid like water can be poured and takes the shape of its container. A gas spreads out to fill whatever space is available. Even though we cannot see air, we can prove that gases occupy space and have mass by simple experiments, such as comparing the mass of an empty balloon to an inflated one.

The particle theory

Scientists developed the particle theory to help us understand how matter behaves. This theory explains the properties of solids, liquids, and gases based on how their particles are arranged and move.

Particles in solids

In a solid, the particles are packed very closely together in fixed positions. They cannot move from place to place, but they do vibrate slightly. Strong forces hold the particles together, keeping them in their positions. This is why solids keep their shape and are difficult to compress.

Particles in liquids

In a liquid, the particles are still close together, but they are not in fixed positions. The forces between the particles are not as strong as in solids, which allows the particles to slide over each other. This is why liquids can flow and take the shape of their container, while still maintaining a definite volume.

Particles in gases

In a gas, the particles are much farther apart than in solids or liquids. They move around freely at high speeds in all directions. The forces between gas particles are very weak, so they can easily move away from each other. The particles constantly collide with each other and with the walls of their container. This is why gases spread out to fill all available space.

Properties of the states of matter

Each state of matter has characteristic properties that we can observe and measure. Understanding these properties helps us identify which state a substance is in and predict how it will behave.

Properties of solids

Solids have several distinctive properties. They have a definite shape that does not change unless a force is applied. This is because the particles are held in fixed positions and cannot move around. Solids also have a definite volume, meaning they cannot be easily squeezed into a smaller space. You cannot compress a solid much because the particles are already very close together. Finally, solids do not flow because the particles cannot slide past each other.

Properties of liquids

Liquids behave quite differently from solids. A liquid has no definite shape and will take the shape of whatever container it is in. This happens because the particles can slide over each other. However, liquids do have a definite volume, just like solids. If you pour water from one glass to another, the shape changes but the amount of water stays the same. Liquids are also difficult to compress because their particles are already close together. Unlike solids, liquids flow easily, allowing them to be poured from one container to another.

Common examples of liquids include water, milk, oil, and petrol.

Properties of gases

Gases have very different properties from both solids and liquids. A gas has no definite shape and no definite volume. Gases expand to completely fill whatever container they are placed in. Unlike solids and liquids, gases are easy to compress. This is because there is a lot of empty space between the gas particles, so they can be pushed closer together. Gases also diffuse to fill all available space, spreading out evenly throughout any container.

Examples of gases include oxygen, nitrogen, carbon dioxide, and water vapour (steam).

Comparison of properties

The table below summarises the key differences between solids, liquids, and gases:

Property	Solids	Liquids	Gases
Shape	Definite shape	No definite shape	No definite shape
Volume	Definite volume	Definite volume	No definite volume
Compressibility	Difficult to compress	Difficult to compress	Easy to compress
Flow	Do not flow	Flow easily	Diffuse to fill all available space

Fluids

Liquids and gases share an important property: they both can flow. Because of this, we often refer to liquids and gases together as fluids. This term is useful when we want to talk about the common behaviours of liquids and gases, such as their ability to move and flow through pipes or containers.

Diffusion

Diffusion is the process by which particles spread out from an area of high concentration to an area of low concentration. This spreading happens because the particles are constantly moving. Diffusion occurs in both liquids and gases, though it happens much faster in gases because gas particles move more quickly and have more space to move through.

The rate of diffusion is faster in gases than in liquids because gas particles move faster and are farther apart. In solids, diffusion is extremely slow because the particles can only vibrate in fixed positions and cannot move around.

Change of state

When we heat or cool a substance, it can change from one state to another. We call this a change of state. These changes are physical changes, not chemical changes, because the substance itself remains the same – only its state changes.

Melting

Melting is the change from a solid to a liquid. When a solid is heated, thermal energy is transferred to its particles. This makes the particles vibrate faster and faster. Eventually, the particles gain enough energy to break free from their fixed positions and begin to slide over each other. At this point, the solid has melted to form a liquid.

The melting point is the specific temperature at which a solid changes to a liquid. Different substances have different melting points. For example, ice melts at $0°\text{C}$, while steel has a melting point of about $1500°\text{C}$. When molten steel is poured into moulds, it can be shaped into various objects and sizes.

Freezing

Freezing is the reverse of melting – it is the change from a liquid to a solid. The freezing point is the temperature at which this change occurs. For any given substance, the freezing point and melting point are the same temperature. For example, water freezes at $0°\text{C}$ and ice melts at $0°\text{C}$.

Evaporation

Evaporation is the change from a liquid to a gas that occurs at the surface of the liquid. It can happen at any temperature, not just at the boiling point. When particles near the surface of a liquid gain enough energy, they can escape from the liquid into the air. This is why puddles dry up on a warm, sunny day even though the water temperature is well below $100°\text{C}$.

Boiling

Boiling is also the change from a liquid to a gas, but it is different from evaporation. When a liquid boils, the change to gas occurs throughout the entire liquid, not just at the surface. During boiling, bubbles of gas form within the liquid and rise to the surface.

The boiling point is the temperature at which a liquid changes to a gas throughout the liquid. Different liquids have different boiling points. Water boils at $100°\text{C}$, while ethanol (found in alcoholic drinks) boils at $78°\text{C}$. The only liquid whose boiling point you need to remember for your course is that of water.

Condensation

Condensation is the change from a gas to a liquid. When a gas is cooled, the particles slow down and move closer together. Eventually, they slow down enough that forces between them can pull them together to form a liquid. You can often see condensation in a kitchen when steam from boiling water meets the cooler glass of a window. The water vapour condenses to form tiny droplets of liquid water on the glass. On a cold day, you can see your breath because the warm water vapour in your breath condenses when it meets the cold air.

Sublimation

Some substances can change directly from a solid to a gas without becoming a liquid first. This process is called sublimation. Solid carbon dioxide (dry ice) is an example of a substance that sublimes. It changes directly from a solid to a gas at room temperature. This property makes it useful for creating special effects in theatre productions and scientific demonstrations.

Heating curves

A heating curve is a graph that shows how the temperature of a substance changes as heat is continuously added to it. These graphs help us understand what happens during changes of state.

Let's look at a heating curve for ice being heated to form water and then steam:

The flat sections of a heating curve show the temperatures at which changes of state occur. These are the melting point ($0°\text{C}$ for water) and the boiling point ($100°\text{C}$ for water).

Measuring melting and boiling points

Scientists need to be able to measure melting points and boiling points accurately. These measurements help identify unknown substances and check the purity of materials.

To measure a melting point, a small amount of solid is placed in a special tube and heated gradually. A thermometer is used to record the temperature at which the solid begins to melt. The melting point is reached when the solid starts to change to a liquid.

To measure a boiling point, a liquid is heated in suitable apparatus with a thermometer positioned in the vapour above the liquid. The temperature is recorded when the liquid begins to boil throughout. Care must be taken to ensure accurate temperature readings and safe heating.

Remember!