Topic 3 - Particle model of matter

• Density

  The mass per unit volume, where density = mass / volume
• Density formula

  ρ = m / V (where ρ is density in kg/m3, m is mass in kg, and V is volume in m3)
• States of matter and their densities

  • Solids and liquids have similar densities as the space between particles does not change significantly
  • Gases have a far lower density compared to solids/liquids
• Usually liquids have a lower density than solids (main exception is ice and water)
• If questions involve change in state and ask for new volume/pressure, the mass is the same
• Mass is conserved during a change of state
• If 20g of liquid evaporates, the gas produced will also weigh 20g
• Physical changes during changes of state are reversible, and not chemical changes
• Internal Energy is the energy stored by particles (atoms and molecules) within a system
• Forms of energy within a system

  • Kinetic Energy (vibration of atoms etc.)
  • Potential Energy (between the particles)
• Heating a system increases the energy the particles have, raising the temperature or producing a change of state
• Specific Heat Capacity is the amount of energy required to raise the temperature of 1kg of a substance by 1°C
• Specific Latent Heat

  The amount of energy needed to change the state of 1kg of a substance without a change in temperature
• Specific Latent Heat of fusion

  Energy required to melt/freeze
• Specific Latent Heat of vaporisation

  Energy required to boil/condense
• Energy is absorbed when melting and evaporating, and energy is released when freezing and condensing
• Sublimation is when a solid goes straight to gas, like "dry ice" (solid CO2 does this)
• Graph shows the temperature changes during the phase transitions of ice
• Graph shows the temperature of ice
• The molecules of a gas are in constant random motion
• The temperature of the gas is related to the average kinetic energy of the molecules
• The higher the temperature, the greater the average kinetic energy and so the faster the average speed of the molecules
• When gas molecules collide with the wall of their container, they exert a force on the wall
• The total force exerted by all gas molecules inside the container on a unit area of the walls is the gas pressure
• Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas (Pressure law)
• Changing the volume of a gas affects the pressure
• A gas can be compressed or expanded by pressure changes
• Increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure (Boyle’s law)
• For a gas at fixed mass and temperature, the Pressure-Volume relationship is given by 𝑃1𝑉1 = 𝑃2𝑉2
• Increasing the volume of a container will lead to a decrease in pressure
• Doing work on a gas increases its temperature
• Adding more particles to a fixed volume increases pressure and temperature
• Particles gain momentum when colliding with a wall moving inward
• As particles gain momentum, the pressure increases as collisions with the walls become more frequent
• Temperature also increases as the kinetic energy of each particle increases