Welcome to the World of Particles!
Ever wondered why ice cubes melt in your drink, or why a bicycle pump feels warm after you’ve used it? The answer lies in the Particle Model. In this chapter, we explore how everything around us is made of tiny "building blocks" called particles and how their movement explains almost everything we see in the physical world. This is a key part of your Paper 2 exam, so let's dive in!
1. States of Matter
Everything is made of atoms or molecules. We call these "particles." Depending on how much energy they have, they behave in three main ways:
• Solids: Particles are packed closely together in a regular, fixed arrangement. They can't move from place to place; they only vibrate around fixed positions. This is why solids keep their shape.
• Liquids: Particles are still close together but in an irregular arrangement. They have enough energy to slide over each other. This is why liquids can flow and take the shape of their container.
• Gases: Particles are far apart and move randomly at high speeds in all directions. They have the most energy and will expand to fill any space available.
Analogy Time!
Think of a crowded school hallway:
- Solid: Everyone is standing in a perfect line for assembly, just shuffling their feet.
- Liquid: Students are moving between classes, bumping into each other but staying in a group.
- Gas: It’s the end of the day, and everyone is sprinting across the playground in different directions!
Quick Review:
- Solids = Regular pattern, vibrating.
- Liquids = Random pattern, sliding.
- Gases = Far apart, fast and random.
2. Density: How Packed is it?
Density tells us how much "stuff" (mass) is packed into a certain amount of space (volume). A brick is denser than a sponge because there are more particles packed into the same size block.
The Equation:
\( \rho = \frac{m}{V} \)
Where:
- \( \rho \) (the Greek letter 'rho') is Density in kilograms per cubic metre (kg/m³).
- \( m \) is Mass in kilograms (kg).
- \( V \) is Volume in cubic metres (m³).
Why are densities different?
Solids are usually the densest because their particles are closest together. Gases have the lowest density because their particles are very spread out. Exception: Ice is actually less dense than liquid water, which is why ice cubes float!
Core Practical: Finding Density
1. For a regular solid (like a cube): Weigh it for the mass. Measure the sides and multiply (length × width × height) for the volume. Use the formula.
2. For an irregular solid (like a stone): Weigh it for the mass. Drop it into a displacement can (Eureka can) filled with water. The volume of the water that spills out equals the volume of the stone!
3. For a liquid: Place a measuring cylinder on a scale and zero it. Pour in the liquid to find the mass. Read the volume from the cylinder. Use the formula.
Key Takeaway: Density is just "Mass per unit Volume." High density means particles are tightly packed.
3. Changing State and Conserving Mass
When you heat a substance, the particles gain energy. Eventually, they might change state:
• Melting: Solid to Liquid
• Freezing: Liquid to Solid
• Evaporating/Boiling: Liquid to Gas
• Condensing: Gas to Liquid
• Sublimating: Solid directly to Gas (like dry ice)
Important Point: These are physical changes, not chemical ones. If you freeze steam back into water, it’s still water! Because no particles are added or removed, mass is always conserved (the weight stays the same).
4. Internal Energy and Temperature
Internal Energy is the total energy stored by the particles in a system. It includes the energy of their movement (kinetic) and the energy of their bonds (potential).
When you heat something, one of two things happens:
1. The temperature rises (the particles move faster).
2. The state changes (the particles break their bonds, but the temperature stays the same while melting or boiling).
Specific Heat Capacity (SHC)
This is the energy needed to raise the temperature of 1 kg of a substance by 1°C. Some things, like water, take a lot of energy to heat up!
The Equation:
\( \Delta Q = m \times c \times \Delta\theta \)
- \( \Delta Q \) = Change in thermal energy (Joules, J)
- \( m \) = Mass (kg)
- \( c \) = Specific heat capacity (J/kg°C)
- \( \Delta\theta \) = Change in temperature (°C)
Specific Latent Heat (SLH)
This is the "hidden" energy needed to change the state of 1 kg of a substance without changing its temperature.
- Latent Heat of Fusion: Melting or freezing.
- Latent Heat of Vaporisation: Boiling or condensing.
The Equation:
\( Q = m \times L \)
- \( Q \) = Thermal energy (J)
- \( m \) = Mass (kg)
- \( L \) = Specific latent heat (J/kg)
Common Mistake: Don't mix these up! Use SHC when the temperature is changing. Use SLH when the substance is melting or boiling (the temperature is flat on a graph).
5. Gas Pressure and the Kelvin Scale
What is Gas Pressure?
Gas particles are constantly flying around. When they hit the walls of a container, they exert a tiny force. Millions of these collisions every second create Pressure. Pressure acts at right angles (90°) to the surface.
Temperature and Pressure:
If you heat a gas, the particles move faster. This means:
1. They hit the walls more often.
2. They hit the walls with more force.
Result: Higher temperature = Higher pressure (if volume stays the same).
Absolute Zero and Kelvin
If you keep cooling a gas, the particles move slower and slower. At -273°C, they stop moving entirely! This is called Absolute Zero. Scientists use the Kelvin scale which starts here.
Conversion:
- To get Kelvin: Add 273 to the Celsius temperature.
- To get Celsius: Subtract 273 from the Kelvin temperature.
Example: 0°C = 273 K. 100°C = 373 K.
6. Boyle's Law: Pressure vs. Volume (Physics Only)
If you have a fixed mass of gas at a constant temperature, and you squash it into a smaller space (decrease volume), the pressure goes up. This is because the particles are more crowded and hit the walls more often.
The Equation:
\( P_1 \times V_1 = P_2 \times V_2 \)
This means the pressure multiplied by the volume at the start equals the pressure multiplied by the volume at the end.
Did you know?
When you use a bicycle pump quickly, it gets hot! This is because you are doing work on the gas. You are transferring energy to the gas particles by squashing them, which increases their kinetic energy and raises the temperature.
Quick Summary Table:
- Increase Temp: Pressure increases (faster particles).
- Decrease Volume: Pressure increases (more crowded).
- Absolute Zero: -273°C or 0 K (no movement).
Final Tips for Success
• Check your units: Always ensure mass is in kg and volume is in m³. If the question gives you grams, divide by 1,000!
• Learn the definitions: Examiners love asking for the definition of "Specific Heat Capacity" or "Density."
• Watch the graphs: On a heating graph, the "flat" parts are where the state is changing (Latent Heat). The "sloped" parts are where the temperature is rising (Specific Heat Capacity).
• Don't panic: If a formula looks scary, just write down the numbers you know from the question and see where they fit!