(for test)

Evaporation and Boiling

Evaporation - the change in state from a liquid to a gas at a temperature below the boiling point.

• heat makes the molecules move faster as they gain energy

• fast moving molecules at the surface will overcome forces of attraction and escape

Boiling - the change in state from a liquid to a gasat the boiling point temperature.

• when heated enough, at the boiling point, particles have enough speed and energy to overcome forces and escape each other

• particles in throughout the liquid break away in big bubbles of gas

Differences between Evaporation and Boiling

• evaporation occurs at any temperature - not just the boiling point

• evaporation only happens at the surface of the liquid - not throughout the liquid like boiling

• evaporation doesn’t require a steady energy source. boiling requires a steady energy source

Factors affecting Evaporation -

• liquid evaporates more rapidly when it is hotter; with higher temperatures more particles of the liquid have energy to escape off its surface.

• liquid evaporates more rapidly when it has a greater surface area; increased surface area means more particles of the liquid are near the surface, and so they can escape easily.

• liquid evaporates more rapidly when a draught blows across its surface; particles of the liquid nearer to its surface are blown away when a draught flows (so that they cannot fall back).

Kinetic model of matter - model in which matter consist of particles - atoms, molecules or ions - in continuous motion. An evidence for this is The Brownian Motion for example.

Brownian Motion

Brownian motion - the motion of small particles suspended in a liquid or a gas, caused by repeated collisions with the moving liquid or gas molecules.

Brownian motion is named in the honor of its first investigator: Robert Brown.

Gases and Pressure -

Kinetic theory - tells us that gases consist of very small particles that are constantly moving in completely random directions. The particles have mass, so whenever they collide with something they exert a force on it.

In sealed containers, gas particles will smash against the walls of the container - creating an outward pressure.

(When talking about containers) -

If a smaller container is used, then there will be more collisions with the walls as the particles are being squashed closer together. The pressure will increase.

If the same amount of gas is put into a bigger container, there will be fewer collisions with the walls of the container, so the pressure will decrease.

(As the volume gets bigger, the gas pressure goes down)

Thermal Expansion

• When materials are heated, they expand.

• This expansion happens because the molecules start to move around (or vibrate) faster, which causes them to knock into each other and push each other apart.

• When this happens, it is the space taken up by the molecules that increases. The molecules themselves remain the same size.

When temperature is increased (at constant pressure) -

• Solids will tend to expand the least

• Gases expand the most

• Liquids fall in between the two

Uses and Consequences of Thermal Expansion -

Applications

• Thermometers rely on the expansion of liquids to measure temperature

• Temperature-activated switches work when a bimetallic strip, consisting of two metals that expand at different rates, bends by a predictable amount at a given temperature

Consequences

• The expansion of solid materials can cause them to buckle if they get too hot

• This could include; Metal railway tracks, Road surfaces and Bridges

• Things that are prone to buckling in this way have gaps built in, this creates space for the expansion to happen without causing damage

Transfer of Heat (Thermal Energy)

• Heat is transferred only when two objects are at different temperatures.

• Thermal energy (heat) always moves from warmer to cooler objects.

• The warmer object transfers heat to the cooler one. its temperature decreases and increases in the cooler one.

• Energy will continue to move from a warmer object to a cooler object until both have the same temperature (thermal equilibrium).

Conduction - Atoms or molecules in a material are always vibrating. when heat is applied they vibrate and move faster and so their kinetic energy increases. The vibrating atoms bump into their anearest atoms and pass on the energy.

• Conduction is the transfer of heat (thermal energy) by direct contact

• Conduction is the transfer of energy from one atom or molecule to another atom or molecule

Conductors - substances that transfer thermal energy rapidly, Insulators - substances that have difficulty to transfer thermal energy.

Thermal conductors

• Gold

• Copper

• Aluminium

Thermal Insulators

• Wood

• Plastic

• Styrofoam

Convection - the flow of currents in a liquid or gas. A current is created when the warmer (less dense and lighter) material rises, forcing the cooler (more dense) material to sink into the warm areas until it is warm enough to rise.

Radiation - the transfer of heat energy by electromagnetic radiation and specially speaking that by infrared radiation. The hotter the object is the more heat energy it radiates. All objects also recieve and emit radiation and the exchange of radiant energy is a continous process. Unlike Conduction and Convection, Radiation can occur in empty space, as well as in solids, liquids, or gases.

Dark matt surfaces are good absorbers (bad reflectors) and good emitters of radiation.

Light, shiny surfaces are poor absorbers (good reflectors) and poor emitters of radiation.