Welcome to Change of State! Mastering Solids, Liquids, and Gases

Hello future physicist! This chapter is all about how matter moves between its three main forms: solid, liquid, and gas. Don't worry if Physics feels tricky sometimes—we'll break down these processes step-by-step using everyday examples like melting ice cream and boiling water. Understanding these concepts helps you explain everything from why puddles disappear to how refrigeration works!

We will use the kinetic particle theory (how particles move) to explain why these changes happen when energy is added or removed.

Section 1: Quick Review – The Three States of Matter

Before we look at changing state, let's quickly remind ourselves how particles behave in each state:

1.1 State and Particle Movement

  • Solid: Particles are held tightly in fixed positions by strong bonds. They can only vibrate in place. This gives solids a fixed shape and volume.
  • Liquid: Particles are still close together but have enough energy to overcome some of the bonds. They can slide past each other. This gives liquids a fixed volume but allows them to flow and take the shape of the container.
  • Gas: Particles have lots of energy and are far apart. The bonds between them are completely broken. They move randomly and rapidly in all directions. Gases have no fixed shape or volume and are easily compressed.

Quick Review Box: Energy Check
Particles in a gas have the most kinetic energy. Particles in a solid have the least kinetic energy.

Section 2: The Four Main Changes of State

A change of state occurs when you add or remove enough thermal energy (heat) to change how the particles are held together.

2.1 Solid <--> Liquid Transitions

1. Melting (Fusion)

  • Process: Solid turning into Liquid (e.g., ice turning into water).
  • Energy required: Heat energy must be supplied.
  • Particle Explanation: The energy supplied increases the vibration of the particles until they have enough energy to break free of their fixed positions and slide past each other.
  • Key Term: The Melting Point is the specific temperature at which a pure solid turns into a liquid (at standard pressure). For pure ice, this is \(0 \text{°C}\).

2. Freezing (Solidification)

  • Process: Liquid turning back into Solid (e.g., water turning into ice).
  • Energy required: Heat energy must be removed.
  • Particle Explanation: As energy is removed, the particles slow down until the bonds are strong enough to lock them back into fixed, rigid positions.
  • Key Term: The Freezing Point is the temperature at which a pure liquid turns into a solid. For pure substances, the melting point and freezing point are the same temperature.

2.2 Liquid <--> Gas Transitions

3. Boiling (Vaporisation)

  • Process: Liquid turning into Gas (e.g., water boiling into steam).
  • Energy required: Heat energy must be supplied.
  • Particle Explanation: Energy supplied gives the particles enough speed to completely break all bonds holding them to the liquid structure. They escape as fast-moving gas particles.
  • Key Term: The Boiling Point is the specific temperature at which a liquid turns into a gas throughout the entire volume (at standard pressure). For pure water, this is \(100 \text{°C}\).

4. Condensation

  • Process: Gas turning back into Liquid (e.g., steam turning into water droplets on a cold mirror).
  • Energy required: Heat energy must be removed.
  • Particle Explanation: As energy is removed, the gas particles slow down. When they bump into a cold surface, they lose energy, and the forces of attraction pull them close enough to form a liquid.
Did You Know? Evaporation vs. Boiling

Both processes turn liquid into gas, but they are different:

  • Boiling happens throughout the liquid, only occurs at the specific boiling point, and requires external heating.
  • Evaporation happens only at the surface of the liquid, occurs at any temperature below the boiling point, and speeds up with higher temperatures or wind.

Key Takeaway (Section 2): To break bonds (Solid -> Liquid -> Gas), you must add energy. To form bonds (Gas -> Liquid -> Solid), you must remove energy.

Section 3: Energy and Heating Curves

When you heat a substance, you expect its temperature to rise. But during a change of state, something strange happens: the temperature stays constant, even though you are still supplying heat!

3.1 Understanding the Heating Curve

Imagine heating a block of ice from \(-20 \text{°C}\) all the way up to steam at \(110 \text{°C}\). If we plot Temperature against Time (which represents energy supplied), we get a graph with flat sections called plateaus.

Step-by-Step Heating Process (Ice to Steam):

  1. Heating the Solid (Ice): Temperature rises from \(-20 \text{°C}\) to \(0 \text{°C}\). The energy is increasing the kinetic energy of the particles (they vibrate faster).
  2. Melting Plateau (Phase Change): At \(0 \text{°C}\), the temperature stops rising. All the supplied energy is used to break the bonds between the ice particles. Both solid and liquid exist here.
  3. Heating the Liquid (Water): Temperature rises from \(0 \text{°C}\) to \(100 \text{°C}\). The energy is once again increasing the kinetic energy of the particles (they move faster).
  4. Boiling Plateau (Phase Change): At \(100 \text{°C}\), the temperature stops rising again. All the supplied energy is used to break the remaining bonds so the particles can escape as gas. Both liquid and gas exist here.
  5. Heating the Gas (Steam): Temperature starts rising above \(100 \text{°C}\). The energy is now increasing the kinetic energy of the gas particles.

Common Mistake to Avoid: During a plateau, students often think the heater must have stopped working. It hasn't! The energy is just being used internally, not to speed up the particles.

Analogy: Think of a renovation project. Energy added in Step 1 and 3 is used to buy new furniture (increasing movement/temperature). Energy added in Step 2 and 4 is used to knock down internal walls (breaking bonds/changing state). Knocking down walls takes time and energy, but the temperature of the furniture doesn't change until the walls are down!

Key Takeaway (Section 3): During a change of state (the plateaus), the energy supplied does not increase temperature. It is used to break or form intermolecular bonds.

Section 4: Latent Heat – The Hidden Energy

The energy absorbed or released during a change of state is called Latent Heat. 'Latent' literally means 'hidden', because you can't see the effect of this energy on a thermometer until the transition is complete.

4.1 Specific Latent Heat (\(L\))

Because the amount of energy needed depends on the mass of the substance, we use the term Specific Latent Heat.

Definition: The Specific Latent Heat (\(L\)) is the amount of thermal energy required to change the state of 1 kg of a substance without changing its temperature.

The units for Specific Latent Heat are joules per kilogram (\(J/kg\)).

There are two main types:

Type 1: Specific Latent Heat of Fusion (\(L_f\))

This is the energy required to melt 1 kg of a substance (Solid \(\rightarrow\) Liquid) OR the energy released when 1 kg of the substance freezes (Liquid \(\rightarrow\) Solid).

Memory Aid: Fusion starts with F, like Freezing or the melting point plateau.

Type 2: Specific Latent Heat of Vaporisation (\(L_v\))

This is the energy required to boil 1 kg of a substance (Liquid \(\rightarrow\) Gas) OR the energy released when 1 kg of the substance condenses (Gas \(\rightarrow\) Liquid).

Important Note: \(L_v\) is almost always much larger than \(L_f\). It takes far more energy to break *all* the bonds (turn liquid into gas) than it does to loosen them (turn solid into liquid).

4.2 The Latent Heat Formula

To calculate the total energy (\(E\)) required for a change of state, you use this formula:

$$E = m \times L$$

Where:

  • \(E\) = Energy transferred (Joules, J)
  • \(m\) = Mass of the substance (Kilograms, kg)
  • \(L\) = Specific Latent Heat (\(L_f\) or \(L_v\)) (\(J/kg\))

Don't worry if this seems tricky at first—it’s just multiplication! Identify the mass and look up the correct value for L.

Why Condensation is Hot (A Practical Example)

When steam hits your skin and condenses into water, it doesn't just cool down from \(100 \text{°C}\). First, every kilogram of steam releases a massive amount of Specific Latent Heat of Vaporisation (as it changes state) before the temperature even starts to drop. This is why burns from steam are so severe—you are hit by both the high temperature and the enormous latent heat released.

Quick Review Box: Latent Heat
Latent Heat breaks bonds (going up the curve: melting/boiling) or forms bonds (going down the curve: freezing/condensing). It never changes the temperature.

Chapter Summary and Review

You have successfully covered the complex process of phase changes! You now know why adding energy doesn't always result in a temperature increase.

Summary Checklist:

  • I can describe the differences between particles in a solid, liquid, and gas.
  • I can define and explain melting, freezing, boiling, and condensation using the particle model.
  • I understand that temperature remains constant during a change of state (the plateau on a heating curve).
  • I know that Latent Heat is the energy used to break/form bonds.
  • I can distinguish between the Specific Latent Heat of Fusion (\(L_f\)) and Specific Latent Heat of Vaporisation (\(L_v\)).
  • I can use the formula \(E = mL\) to calculate energy required for a phase change.

Keep practising those heating curves! They are the key to understanding this chapter.