Physics Particle Model of Matter

What is density?

The mass per unit volume of a substance.

What is the equation for density?

ρ = m/V

What is the unit of density?

kg/m³

What is the unit of mass in the density equation?

kg

What is the unit of volume in the density equation?

m³

What does the density of a substance depend on?

The spacing of its atoms/particles.

Why do solids and liquids have similar densities?

The space between their particles does not change significantly.

Which usually has a lower density: a solid or a liquid?

A liquid, although water is an important exception because ice is less dense than liquid water.

Why are gases much less dense than solids and liquids?

Gas particles are much further apart, so the same mass occupies a much larger volume.

What happens to mass during a change of state?

Mass is conserved.

If 20 g of liquid evaporates, how much gas is produced?

20 g.

Are changes of state physical or chemical changes?

Physical changes.

Why are changes of state not chemical changes?

The material retains its original properties when the change is reversed.

What is melting?

The change from solid to liquid.

What is freezing?

The change from liquid to solid.

What is evaporation?

The change from liquid to gas.

What is condensation?

The change from gas to liquid.

What is sublimation?

The change directly from solid to gas.

What is an example of a substance that undergoes sublimation?

Dry ice (solid carbon dioxide).

What is internal energy?

The energy stored by the particles (atoms and molecules) within a system.

What two forms does internal energy take?

Kinetic energy and potential energy.

What does the kinetic energy of particles include?

The energy associated with the motion/vibration of particles.

What does the potential energy of particles relate to?

The energy between the particles.

What happens to internal energy when a system is heated?

Its internal energy increases.

What can an increase in internal energy cause?

An increase in temperature or a change of state.

What is specific heat capacity?

The energy required to raise the temperature of 1 kg of a substance by 1°C.

What is the equation for change in thermal energy?

ΔE = mcΔT

What does ΔE represent?

Change in thermal energy, measured in joules (J).

What does m represent in ΔE = mcΔT?

Mass, measured in kilograms (kg).

What does c represent in ΔE = mcΔT?

Specific heat capacity, measured in J kg⁻¹ °C⁻¹.

What does ΔT represent?

Temperature change, measured in °C.

What is specific latent heat?

The energy needed to change the state of 1 kg of a substance without changing its temperature.

What must happen before a substance can change state?

It must reach the appropriate temperature for the change of state.

What is specific latent heat of fusion?

The energy required to melt or freeze 1 kg of a substance.

What is specific latent heat of vaporisation?

The energy required to boil or condense 1 kg of a substance.

What is the equation for energy transferred during a change of state?

E = mL

What does E represent in E = mL?

Energy transferred, measured in joules (J).

What does m represent in E = mL?

Mass, measured in kilograms (kg).

What does L represent in E = mL?

Specific latent heat, measured in J/kg.

When is energy absorbed during changes of state?

During melting and evaporation.

When is energy released during changes of state?

During freezing and condensation.

What happens to temperature during a change of state?

The temperature remains constant while the change of state occurs.

What happens to the temperature while ice is being heated before it melts?

It increases until it reaches 0°C.

Why does the temperature remain constant while ice melts?

The energy supplied is being used for the change of state rather than increasing temperature.

What state is the substance in from A to B on the heating graph?

Solid.

What happens at B on the heating graph?

The ice reaches 0°C.

What happens from B to C?

The ice melts and the temperature remains constant.

What state is the substance in from C to D?

Liquid.

What happens from D to E?

The water boils and the temperature remains constant.

Why does boiling take longer than melting on the graph?

Evaporation requires more energy.

What happens from E to F?

The gas is heated and its temperature increases.

What is the motion of gas molecules like?

They are in constant random motion.

What is the relationship between gas temperature and average kinetic energy?

Higher temperature means greater average kinetic energy.

What happens to the average speed of gas molecules when temperature increases?

Their average speed increases.

How do gas molecules create pressure?

They collide with the walls of their container and exert forces on them.

What is gas pressure?

The total force exerted by gas molecules per unit area of the container walls.

What happens to the pressure of a gas when its temperature increases at constant volume?

The pressure increases.

What is the pressure law?

For a fixed mass of gas at constant volume, changing the temperature changes the pressure.

What happens when the volume of a gas increases at constant temperature?

The pressure decreases.

What is Boyle's law?

For a fixed mass of gas at constant temperature, increasing volume decreases pressure.

Why does increasing gas volume decrease pressure?

There are fewer collisions with the container walls per unit area.

What is the equation for Boyle's law?

P₁V₁ = constant.

What are the units of pressure?

Pascals (Pa).

What are the units of volume in the gas equations?

m³.

What happens to gas pressure when the same amount of gas is compressed?

The pressure increases.

Why does compressing a gas increase its pressure?

Particles collide with the walls more frequently.

What happens when more gas particles are pumped into a fixed volume?

The pressure increases.

Why does adding more gas particles increase pressure?

There are more particles and therefore more collisions with the container walls per unit time.

What happens to the temperature when more gas is added to a fixed volume?

The gas heats up because energy is transferred to the particles.

What happens to gas particles when a container is compressed?

The particles collide with the inward-moving wall and gain momentum.

Why does compressing a gas increase particle speed?

The particles rebound from the moving inward wall with greater velocity.

What happens to the pressure when particles move faster?

The pressure increases because collisions with the walls occur more frequently.

What happens to the temperature when a gas is compressed?

The temperature increases because the kinetic energy of the particles increases.

What happens to the time between collisions when a gas is compressed?

It decreases.