States of matter

Welcome to the Building Blocks of the Universe!

Ever wondered why an ice cube melts in your hand, or why the steam from a kettle can burn you so badly? It all comes down to the states of matter. In this chapter, we are going to look at the "Building Blocks" of our world. We’ll explore how particles move, why some things are heavier than others even if they are the same size, and how energy changes everything.

Don’t worry if some of this science feels like a lot to take in at first. We are going to break it down piece by piece. Think of this as learning the "rules of the game" for how every object in the universe behaves!

1. The Particle Model

To understand matter, we use a simple particle model. Imagine every substance is made of tiny, solid spheres (like marbles). How these marbles are arranged tells us if something is a solid, a liquid, or a gas.

The Three States

Solid: Particles are packed very closely together in a regular pattern. They can't move around; they only vibrate in fixed positions. This is why solids keep their shape!
Liquid: Particles are still close together, but they are not in a pattern. They can slide past each other. This is why you can pour a drink and it takes the shape of the glass.
Gas: Particles are far apart and move very quickly in random directions. They zoom around and fill up whatever container they are in.

Physical vs. Chemical Changes

When a substance changes state (like ice melting into water), it is a physical change.
• Mass is conserved: This means if you melt 10g of ice, you get exactly 10g of water. Nothing disappears!
• It’s reversible: You can freeze that water back into ice. The chemical properties stay the same.
• This is different from a chemical change (like burning wood), where you can’t easily get the original material back.

Higher Tier Only: Limitations of the Model

Don't worry if this seems tricky! In reality, atoms aren't solid spheres. The simple model is limited because:
1. Atoms, molecules, and ions have different shapes and aren't always "balls."
2. The model doesn't show the forces between the particles, which are very important in real life.

Quick Review: The Particle Model

Solid: Vibrate in place.
Liquid: Flow and slide.
Gas: Fast and random.

Key Takeaway: Matter exists in three states based on how its particles are arranged and how much they move. Changes between these states are physical and reversible.

2. Density: How much "stuff" is in there?

Have you ever picked up a small box that felt surprisingly heavy? That’s density. It tells us how much mass is packed into a certain volume.

The Formula

We calculate density using this equation:
\(density = \frac{mass}{volume}\)
Or in symbols: \(\rho = \frac{m}{V}\)

• Density (\(\rho\)) is measured in kilograms per metre cubed (\(kg/m^3\)).
• Mass (\(m\)) is measured in kilograms (\(kg\)).
• Volume (\(V\)) is measured in metres cubed (\(m^3\)).

Density in Different States

Because particles in a solid are packed tight, solids are usually very dense. Gases have particles spread far apart, so they have very low density.
Analogy: Think of a crowded elevator (Solid) versus an empty football field (Gas). There is much more "people-density" in the elevator!

Required Practical: Measuring Density

To find the density of an object:
1. Use a balance to find its mass.
2. Find its volume.
- For a regular box, just measure length × width × height.
- For an irregular object (like a stone), use the displacement technique: Drop it into a container of water and see how much the water level rises. That "extra" water volume is the volume of your object!

Key Takeaway: Density is mass per unit volume. Solids are usually the densest because their particles are packed the tightest.

3. Gas Pressure

Why do balloons stay inflated? It's all about gas pressure. Inside the balloon, gas molecules are constantly zooming around in constant random motion.

How it works

1. Gas particles move and collide with the walls of their container.
2. Every time a particle hits the wall, it exerts a tiny force.
3. The total force of all these millions of collisions over a certain area is what we call pressure.

Temperature and Pressure

If you heat up a gas (at a constant volume), the particles gain kinetic energy. They move faster!
• Faster particles hit the walls more often.
• They also hit the walls with more force.
• This leads to an increase in pressure.

Did you know? This is why you should never leave an aerosol can in a hot car. The pressure inside could get so high that the can explodes!

Key Takeaway: Gas pressure is caused by particles hitting the walls of a container. Higher temperature = faster particles = higher pressure.

4. Internal Energy and Heating

Energy is stored inside a system by the particles that make it up. This is called internal energy. When you heat a substance, you are adding to this energy store.

What happens when you heat something?

Two things can happen:
1. The temperature increases (the particles move faster).
2. The state changes (the particles break free from their bonds).

How much energy is needed to melt or boil something depends on the strength of the forces between the particles. Stronger forces mean higher melting and boiling points.

Specific Heat Capacity (SHC)

This is the energy needed to raise the temperature of 1 kg of a substance by 1°C.
The formula is: \(\Delta E = m \times c \times \Delta \theta\)
• \(\Delta E\) = Change in thermal energy (Joules, \(J\))
• \(m\) = Mass (\(kg\))
• \(c\) = Specific heat capacity (\(J/kg ^\circ C\))
• \(\Delta \theta\) = Temperature change (\(^\circ C\))

Specific Latent Heat

Have you noticed that when ice is melting, its temperature stays at 0°C even though you are still heating it? That's because the energy is being used to change state, not raise the temperature. This "hidden" energy is latent heat.

The formula is: \(E = m \times L\)
• \(L\) is the Specific Latent Heat (\(J/kg\)).
• Latent Heat of Fusion: Energy to change between solid and liquid.
• Latent Heat of Vaporisation: Energy to change between liquid and gas.

Common Mistake to Avoid

Students often mix these up! Remember:
- Use Specific Heat Capacity when the temperature is changing.
- Use Latent Heat when the state is changing (temperature stays the same!).

Key Takeaway: Internal energy is the total energy stored by particles. Heating can raise temperature or change state. Latent heat is the energy used specifically to change state without changing temperature.

5. Purity

In science, the word "pure" has a very strict meaning that is different from how we use it at the supermarket.

Everyday "Pure": Usually means nothing "bad" has been added (like "pure" orange juice, which actually contains water, sugar, and citric acid).
Scientific "Pure": A substance that contains only one type of element or compound. For example, pure water contains ONLY \(H_2O\) molecules.

How to test for purity

Pure substances have very specific, "sharp" melting and boiling points.
• Example: Pure water boils at exactly 100°C.
• If water is impure (like salty water), it will boil over a range of temperatures and usually at a higher temperature than 100°C.

Key Takeaway: A scientifically pure substance consists of only one type of particle and has a fixed, exact melting and boiling point.