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Physics Heat

Theory
When an object is heated or cooled it changes state. The following diagram illustrates the changes of state that can occur:


Gas
To change state the particles need to either vibrate faster or vibrate slower. To do this they must absorb energy or release energy. Therefore a change of state can only occur when energy can be
released or absorbed.

Example
An Iodine crystal is being heated in a conical flask with a bung on it. Iodine is a deep purple steely grey. Purple vapour rises directly from the solid. Use a gentle, and not direct, Bunsen flame.



Explanation
Iodine is subliming in the conical flask. This means the iodine is changing directly from a solid to a gas. The vapour cools on the glass and reverse sublimes directly back to the solid. If this experiment is demonstrated take care as iodine vapour is toxic. The conical flask must remain stoppered until the iodine has reverse sublimed. Care on disposal of the iodine is also essential.

Heat Transfer

Conduction:
The transfer of heat from one part of a solid to another which is caused by particle collisions
between particles in the solid.

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Convection:
The transfer of heat energy from one place to another by the transfer of material e.g. rising and
falling water.

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Radiation:
The transfer of heat energy through a vacuum by electromagnetic radiation.

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