A state change is a change of a substance from one form to another.
There are 4 main states of matter:
Solid: Fixed volume, fixed shape.
Liquid: Fixed volume, changeable shape.
Gas: changeable volume, indeterminate shape.
Plasma: changeable volume, indeterminate shape.
All state changes are physical changes because the identity of the substance does not change.
What Causes State Changes?
Substances change state when they are heated up or cooled down.
When heat is added or subtracted from a substance, energy is added or removed from its particles and they begin to behave differently than they were.
In each state of matter, the particles that make up the substance move and behave differently; they have different amounts of energy.
Relative energy of states:
Plasma: Highest Energy State of Matter
Gas: High Energy State
Liquid: Moderate Energy State
Solid: Low Energy State
Specific Changes of State
Melting: Solid to Liquid
Occurs when you add energy to a substance.
Particles move from being rigid and stuck in place to being more freely moving and energetic.
Solid substances will only melt when they hit a specific temperature, called the melting point.
One important fact regarding melting point (etc.) is that, all thermal energy added to a frozen object is directed toward melting the object.
Therefore, no matter how long an object is exposed to heat, its temperature will never increase above its melting point until all the solid has changed state.
For example: if you were to leave a block of ice in a room for 20 minutes and return to measure its temperature, the portion of ice that remained in the solid state would still be at a temperature at or below 0 degrees.
Freezing: Liquid to solid.
Occurs when you remove energy from a substance.
Particles move from being freely moving to being more tightly compressed and rigid.
Substances only freeze when they reach their freezing point.
It is important to note that the freezing point is the same as the melting point for a substance!
Liquid to Gas State Changes
Liquids can become gases in several ways:
Evaporation is the change of a substance from a liquid to a gas.
This occurs at the surface of a liquid that is below its boiling point, and can occur at any temperature.
For example, when you sweat, your body is cooled via evaporation: the water in your sweat absorbs energy from your skin as it evaporates, making you feel cooler as temperature is transferred from your body
Boiling, unlike evaporation, occurs at a specific temperature.
The temperature at which a liquid boils is its boiling point.
Adding temperature to a liquid causes the particles inside of the liquid to move around very quickly and turn into gas.
Boiling is the change of a liquid to vapor throughout the liquid, not just at the top of a liquid.
Adding temperature to a liquid causes the particles inside of the liquid to move around very quickly and turn into gas. This gas is formed in small pockets that rumble through the liquid – it’s the bubbles you see when you boil liquids!
Boiling occurs when the pressure inside the bubbles (vapor pressure) equals the pressure outside the bubbles (atmospheric pressure).
The boiling point of a liquid can change, depending on the atmospheric pressure of the air at which the water is boiling.
The higher above sea level you go, the lower the atmospheric pressure, and therefore the lower the temperature required to build up pressure inside of the liquid.
Condensation: Gas to Liquid
Condensation: the change of state from a gas to a liquid.
It is the opposite of evaporation.
The Condensation point is the temperature at which gas becomes a liquid.
Large numbers of gas particles clump together to lock into a loose group and become water again.
Sublimation: the change of a solid directly into a gas.
For this to occur: the particles of a solid must move from being very tightly packed to being spread very apart. So, the attractions between these particles must be completely overcome.
