1. Mass and energy are both conserved
Mass and energy are both conserved. Neither mass nor energy can be created or destroyed.
When an ice cube melts, the mass of the liquid water is the same as the mass of the ice cube. Mass is conserved. Energy can change forms DURING physical and chemical changes, but the total amount of energy present before and after a change is the same. The amount of energy in a substance can change, but this added energy must come from another source.

2. Mass cannot be created or destroyed.
In chemical changes, as well as in physical changes, the total mass of the substances undergoing the change stays the same before and after the change. In other words, mass cannot be created or destroyed, which is the law of conservation of mass

3. For instance, when you burn a match, it seems to lose mass
For instance, when you burn a match, it seems to lose mass. The ash has less mass than the match. But the oxygen that reacts with the match, the tiny smoke particles, and the gases formed in the reaction also have mass. The total mass of the reactants (the match and oxygen) is the same as the total mass of the products (the ash, smoke, and gases).

4. Energy cannot be created or destroyed.
Energy may be changed to another form during a physical or chemical change, but the total amount of energy present before and after the change is the same. In other words, energy cannot be created or destroyed, which is the
law of conservation of energy

5. Starting a lawn mower may seem to violate the law of conservation of energy, but that is not the case. It is true that for the small amount of energy needed to start the mower, a lot of energy results. But the mower needs gasoline to run. Gasoline has stored energy that is released when it is burned. When the stored energy is considered, the energy present before you start the lawn mower is equal to the energy that is produced. Some of the energy from the gasoline is transferred to the surroundings as heat, which is why the lawn mower gets hot. The total amount of energy released by the gasoline is equal to the energy used to power the lawn mower plus the energy transferred to the surroundings as heat.

6. Temperature is constant during changes of state.
When a substance loses or gains energy, either its temperature changes or its state changes. But the temperature of a substance does not change during a change of state. If you add energy to ice at 0 °C, the temperature will not rise until all of the ice has melted.

7. Energy and changes of state
The identity of a substance does not change during a change of state, but the energy of a substance does change.
If energy is added to a substance, its particles move faster. If energy is removed, the substance’s particles move slower. For instance, the particles in steam have more energy than the particles in liquid water. A transfer of energy known as heat causes the temperature of a substance to change. If enough energy is added or removed, the substance will change state.

8. Melting
A solid changes to a liquid by melting. Heating a solid transfers energy to the particles, which vibrate faster as they gain energy. Eventually, they break from their fixed positions, and the solid melts. The melting point is the temperature at which a substance changes from solid to liquid. Melting point depends on the pressure.

9. Evaporation
The change of a substance from a liquid to a gas. Boiling is evaporation that occurs throughout a liquid at a specific temperature and pressure. The temperature at which a liquid boils is the liquid’s boiling point.

10. Sublimation
Solids can also change directly into gases in a process called sublimation. Dry Ice changes into gaseous carbon dioxide. Ice cubes in a freezer will eventually get smaller, as the ice sublimes. Note that melting, evaporation, and sublimation all require energy.

11. Why do people sweat?
The molecules in sweat onyour skin gain energy fromyour body and move faster.Eventually, the fastest moving particles break away, and the sweat evaporates. This loss of energy by the body makes the body feel cooler.
H2O Vapor
nitrogen
Oxygen
Sweat

12. Condensation
When water vapor in the air becomes a liquid, energy is released from the water to its surroundings. This process is an example of condensation, which is the change of state from a gas to a liquid. For a gas to become a liquid, large numbers of gas particles clump together. Energy is released from the gas, and the particles slow down.

13. Freezing Point
Energy is also released during freezing, which is the change of state from a liquid to a solid. The temperature at which a liquid changes into a solid is the substance’s freezing point.

14. Use arrows to label the change of state.
Use a dotted arrow if the process REQUIRES energy.
Use a solid arrow if the process RELEASES energy.

15. Describe the following changes of state, and explain how particles behave in each state.
freezing
sublimation
boiling
melting