Infrared waves are part of the electromagnetic spectrum, just past visible light.
Every single item emits radiation.
(Infrared) radiation can travel through a vaccum, that is how we get heat from the sun.
Dark surfaces are good absorbers and emitters.
Light and shiny surfaces are good reflectors.
There are 3 states of matter: solid (s), liquid (l) and gas (g).
In a solid, all the particled vibrate but are in a fixed position. Particles in a liquid touch, but move around at random, so they have more kinetic energy than a solid. A gas' particles are far apart and are not connected.
Liquids and gases are poor conductors but solids are usually good conductors, especially metals. If one end of a solid is heated, then that side of the solid gains kinetic energy. The particles within, vibrate and collide more often which then gives energy to other particles. All the particles in the solid gain energy which gives the result of conduction. Delocalised electrons also do this as they move freely through the solid whilst colliding with other particles.
A BAD CONDUCTOR = A GOOD INSULATOR (and vica versa)
When a liquid is heated, it rises as it becomes less dense. The hot water is then replace by cooler water. The switch between them both is known as the convection current.
Evaporation is when a liquid is heated and is turned into a gas. This happens because the top layer of water particles escape from the water's surface and go into the air as "water vapour". As the average kinetic energy has decreased, the temperate drops too. This causes cooling and is useful for when the human body sweats.