Notes

Substances exist in one of four phases: gas, liquid, solid, and plasma.
Substances change between the phases depending on their temperature. Boiling, melting, condensing, and freezing are all processes that change a substance’s state.
Molecules connect together to form all kinds of substances. Depending on the properties of the matter and what temperature it is, substances can be in one of four states, or phases: gas, liquid, solid, and plasma.

In a gas, there is a lot of space between the molecules. Liquids have less space between molecules and can change shape. Both gases and liquids are fluids, which means they can change shape according to where they are. Solids have very little space in between the molecules. They cannot flow (move like fluid) and they have a more fixed shape.

Plasma is a hot gas with different properties. It is considered a fourth phase of matter, but we do not interact with it as often as the other three phases.

Look at the example of water. At room temperature, water is a liquid. If you heat water up, it becomes a gas (steam). And if you make it cold enough, it freezes and becomes a solid (ice).

Figure 12.3: This picture shows water in all three phases.
This picture shows water in all three phases.
Substances melt from solid to liquid and boil from liquid to gas. The changes can go in the other direction too. Condensing is what happens when a gas changes to a liquid. Freezing is the process that changes a liquid into a solid (like freezing water into ice). Different substances change their state at different temperatures, depending on what they are made of. Water freezes at 32 degrees Fehrenheit (0 degrees Celcius) but other substances might freeze at different temperatures.

Atomic mass
The atomic mass of an atom is the number of protons plus the number of neutrons that it contains.
The mass of an atom stays the same even when its state changes.
In science, especially chemistry and physics, the word mass has a very specific meaning. Mass is a property of matter and does not change.

The atomic mass of an atom is the number of protons plus the number of neutrons that it contains. (Some of the numbers are partial numbers, or decimals. That’s because the numbers of neutrons in an atom can vary, so the atomic mass number is an average). The mass of any one atom is the same no matter what the temperature is, what state it is in, or whether or not the atom is part of a molecule.

People often confuse mass with weight, which is how heavy something is. This is mostly because in English we often use the word weight when we actually mean mass, and the other way around. Weight can change because it depends on gravity (you will cover gravity in your physics lessons). Mass, the amount of matter, does not change.

Important Facts
1
In chemistry, a substance is something with a defined chemical composition and properties.
2
Atoms consist of even tinier particles called protons, neutrons, and electrons.
3
Each element has its own symbol in the form of letters, and its own number that indicates the number of protons in its atom.
4
A chemical formula is a symbol for a substance that tells you which elements it is made of and how many atoms of each element it contains.
5
Substances change from state to another depending on their temperature.
6
The mass of an atom stays the same even when its state changes.
Vocabulary
Atom

Tiny particle made of protons, neutrons, and electrons

Atomic mass

The number of protons plus the number of neutrons in an atom

Atomic number

The number of protons in an atom

Attracted

Pulled together, like a magnet

Boil

The process of turning a liquid turns into a gas

Chemical composition

The specific elements that make up a substance

Chemistry

The study of what things are made of and how they change

Chemical formula

The symbol for a substance that tells you elements it is made of and how many atoms of each element its molecules contain

Composition

What something is made of

Compound

A substance made of atoms of more than one element

Condensing

The process of changing gas to liquid

Electrical charge

A force that contains electrical power and attracts an opposite electrical charge

Electrons

A part of an atom with a negative charge

Element

Substance made up of atoms of the same type

Freezing

The process of cooling liquid to change it into a solid

Mass

A measure of the amount of matter in a substance

Matter

Anything that has mass and volume

Melt

When a solid turns into a liquid

Molecule

More than one atom stuck together

Neutron

A part of an atom with no charge, or a neutral charge

Particle

A tiny, usually microscopic, piece

Properties

Qualities or characteristics of a substance, including how they look, feel, or behave

Proton

A part of an atom with a positive charge

Substances

All living and nonliving things; or (in scientific terms) matter with a defined chemical composition and properties

Volume

The size of something or how much space it takes up