Chemistry: Chemical Reactions, States of Matter, and Atomic Structure

Chemical Reaction Observable Clues

Observable evidence that a chemical reaction may have occurred: color change, formation of a gas (bubbles), temperature change, formation of a solid (precipitate), or light or sound produced.

Reactant

A starting substance that is changed during a chemical reaction. It is written on the left side of the arrow.

Product

A new substance formed during a chemical reaction. It is written on the right side of the arrow.

Physical Change

A change in form or state that keeps the same substance (for example, ice melting), without creating new substances.

Chemical Change

A process that forms new substances with different properties.

Solid Particle Motion

The particle behavior in a solid, where particles vibrate in fixed positions.

Liquid Particle Motion

The particle behavior in a liquid, where particles are close together and slide past each other.

Gas Particle Motion

The particle behavior in a gas, where particles move fast and far apart in random directions.

Melting (Particle Behavior)

The phase change where particles gain thermal energy, vibrate faster, break away from fixed positions, and spread slightly apart, while the mass stays the same.

Boiling (Particle Behavior)

The transition where particles move faster and spread much farther apart, forming a gas that fills the container with random motion.

Protons

Subatomic particles located in the nucleus with a positive ($+$) charge.

Neutrons

Subatomic particles located in the nucleus with no charge.

Electrons

Subatomic particles with a negative ($-$) charge that move around the nucleus in the electron cloud.

Atomic Number

The number of protons in an atom, which identifies the element.

Element

A pure substance made of only one kind of atom.

Compound

A substance made of two or more different elements chemically bonded in a fixed ratio (for example, $H_2O$).

Mixture

Substances combined physically rather than chemically bonded, whose proportions can vary (for example, salt water).

Subscript

Numbers in a chemical formula that indicate the number of atoms of each element in one molecule (for example, the $2$ in $H_2O$).

Coefficient

Numbers in a chemical equation (such as the $2$, $1$, and $2$ in $2H_2 + O_2 \rightarrow 2H_2O$) that indicate how many molecules of each substance are involved.