Any pure substance can exist any of four states (phases) of matter:

(coldest) solid, liquid, gas, plasma (hottest)

Solids have a definite shape and a definite volume.

The shape remains constant because the atoms are stuck in place and cannot move past each other.

The volume remains constant because the atoms are in contact with each other

Liquids do not have a definite shape (they take the shape of the container), but do have a definite volume.

In a liquid, the atoms can move past each other, but remain in contact with each other.

Gasses do not have a definite shape (they take the shape of the container), , and do not have a definite volume (they fill the volume of the container).

In a gas, the atoms can move past each other, and do not remain in contact with each other.

A plasma is a gas in which the atoms collide with such force that electrons are knocked off the atoms.

Matter in the plasma state glows and moves in response to electric/magnetic fields.

(ex: hydrogen in the sun, neon in a light).

Click here for a simulation of atomic behavior of solids, liquids, and gases: Link link link (all of these rock!)

The melting point is the temperature at which a substance changes between the solid and liquid states of matter. Think of it like an atomic speed limit, above it and the atoms are fast enough to behave like a liquid, below it and they are slow enough to act as a solid.

A substance can either melt or freeze at its melting point, depending on whether the substance is gaining or loosing energy.

The boiling point is also an atomic speed limit, between the liquid and gas states.

There are five state changes that occur when substances are heated and cooled past their melting points and boiling points:

Melting – a solid is heated past its melting point and becomes a liquid

Freezing – a liquid is cooled below its melting point and becomes a solid

Boiling – a liquid is heated above its boiling point and becomes a gas

Condensation – a gas is cooled below its boiling point and becomes a liquid

Sublimation – a substance makes a direct change from solid to gas

A substance can not be a solid above the melting point, and it cannot be a liquid above the boiling point.

When a substance is at the melting point, so of the atoms are in a solid state and some are in a liquid state. If energy is added to the substance, more atoms go into the liquid state, if energy is removed, more atoms go into the solid state.

Example:

Zinc melting point = 693 degrees Celsius boiling point = 1180 degrees Celsius

At what temperature does zinc change from a gas to a liquid? 1180 degrees Celsius

Would Zinc be a solid, liquid, or gas a 690 degrees Celsius? (solid)

Would Zinc be a solid, liquid, or gas a 1188 degrees Celsius? (gas)

The graph below is called a heating curve. It shows the change in a sample’s temperature as it is heated over time.

On any heating curve you get two plateaus, the lower one at the melting point and the higher one at the boiling point.

The curve below is for water. This can be inferred from the fact that the melting point is 0 ^oC and the boiling point is 100 ^oC

A volatile substance is one that evaporates easily. Note that this does not mean that is dangerous or explosive (though volatile liquids often are).

Evaporation differs from boiling in two important ways:

Evaporation happens at any temperature, boiling only happens at the boiling point

Evaporation only occurs at the surface of a liquid, boiling happens throughout

In order to understand why evaporation happens it is important to remember that

Temperature is a measure of the average kinetic energy (speed) of the atoms in a substance.

Evaporation occurs because at any given temperature some of the atoms in the liquid are moving at speeds that exceed the boiling point, even though the average atoms are much slower. If these faster atoms are inside the liquid, they will bump into a slower atom and slow down, but if these faster atoms are at the surface, then they will fly out of the liquid, producing evaporation

Evaporation causes cooling because only the fast atoms leave, which causes the average speed of the remaining atoms to be slower (slower atoms = colder substance)

Click here to see a simulation of evaporative cooling at the molecular level: Link