Particle model of matter flashcards

Key points

• Density is the mass per unit volume: how much matter is packed into a space.
• Mass in kg and volume in m³ give kg/m³; grams and cm³ give g/cm³.
• Water's density is 1 g/cm³, which is 1000 kg/m³. Anything denser sinks in it.

Formula

A 300 g stone with a volume of 120 cm³

Key points

• Find the mass with a balance every time.
• Regular solid: measure its sides with a ruler and work out the volume.
• Irregular solid: lower it into a full eureka can. The volume of water pushed out is its volume.
• Liquid: measure a volume in a measuring cylinder, and find its mass on a balance.

A 240 g block with a volume of 30 cm³

Key points

• Solid: particles close together in a fixed, regular pattern. They only vibrate, so it keeps its shape.
• Liquid: particles still close, but they can move past each other, so it flows.
• Gas: particles far apart, moving quickly and randomly, so it fills its container and is far less dense.
• Stronger forces between particles need more energy to overcome, so the melting point is higher.

Key points

• Internal energy is the total kinetic and potential energy of all the particles in a substance.
• Heating raises it in one of two ways.
• The particles move faster: more kinetic energy, so the temperature rises.
• Or the forces between them are overcome: more potential energy, so the state changes.

Key points

• Melting (solid to liquid), freezing (liquid to solid), boiling or evaporating (liquid to gas), condensing (gas to liquid).
• Sublimation: a solid turning straight into a gas, as dry ice does.
• No new substance forms, so a change of state is physical and can be reversed.
• Mass is conserved: 50 g of ice melts into 50 g of water.

Key points

• Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.
• Water's is high, about 4200 J/kg °C, so it stores a lot of energy and cools slowly: good for radiators.
• Use it only while the substance stays in the same state.

Formula

9000 J warms a 1 kg block by 10 °C

Key points

• Specific latent heat is the energy needed to change the state of 1 kg of a substance with no change in temperature.
• Latent heat of fusion: melting or freezing. Latent heat of vaporisation: boiling or condensing.
• The energy breaks the forces between particles, so the temperature stays the same.

Formula

Melting 0.5 kg of ice at 0 °C

Key points

• A sloping part is a temperature change: use E = mcΔθ.
• A flat part is a change of state, melting or boiling: use E = mL.
• On a flat part the energy goes into breaking forces between particles, not speeding them up.

Ice at −10 °C heated to water at 20 °C

Key points

• Gas particles move randomly and collide with the walls of their container, pushing on them.
• Temperature measures the particles' average kinetic energy: the hotter the gas, the faster they move.
• Heated at a fixed volume, the particles hit the walls harder and more often, so the pressure rises.
• That is why a sealed can heated in a fire can burst.

Key points

• Squash a gas into a smaller volume and the particles hit the walls more often, so the pressure rises.
• At a constant temperature, pressure × volume stays the same: halve the volume and the pressure doubles.

Formula

200 kPa in 0.5 m³, squashed to 0.25 m³