Heat Capacity and Specific Heat Capacity
The amount of of temperature change, when thermal energy is added to the particles is another property that
particles in different materials have. Different materials will increase or decrease their average energy depending
on how much thermal energy is provided.
Heat Capacity is the amount of thermal energy that warms
or cools an object by 1oC (it depends on the mass and the
type of particle the object is made of).
Specific Heat Capacity is the amount of thermal energy that
warms or cools 1 gram, of a specific type of particle, by 1oC.
Changes of State
Some substances like water (or wax) can undergo observable changes through all three states
of matter - solid liquid and gas.
Some substances, like hydrogen, require high pressures and low temperatures (-253oC) to make the particles slow down enough for them to change their state
from a gas to a liquid.
Any pure substance can exist in all three states of matter.
Melting and Boiling Points
When heat is transferred in a space the average energy of the particles - the temperature of
the substance - is affected, by increasing or decreasing. A substance will change it's state when it reaches certain
temperatures - called boiling and melting points. Table 3 (p. 221) At everyday temperatures on Earth, most substances
are either gases or solids.
What Happens When A Liquid Evaporates?
In a liquid, the particles are moving very quickly. At the surface, some of the particles are able to escape into the air, while others do not have enough energy to escape
and remain in the liquid. As high energy particles escape, the average energy of the remaining particles is less
and so the liquid cools. The cool liquid then cools the surface on which it is resting. This is called evaporative
cooling. It is common and useful in many situations:
Joggers cooling down as their sweaty clothes dry out
Water cools down a roof on hot summer day
A wet cloth is placed on your forehead when you have a fever
Why The Temperature Stays The Same
During a phase change, the average energy of the particles remains the same, but, the
particles are rearranging themselves. Particles become less organized as their energy increases, so the substance
changes from a solid to a liquid to a gas. As the energy of the particles becomes less, the particles rearrange
themselves more orderly, so a gas changes to a liquid and then to a solid. The total energy of the particles changes
- by increasing or decreasing, because the particles are not increasing or decreasing their speed, just their arrangement.
The average energy doesn't change. The energy change is hidden from a thermometer and is called 'hidden
heat' or 'latent heat'.
Topic Review p. 225