1. Matter, Properties, and Changes

Matter (anything that has mass and takes up space) is the material in every sample. Matter has mass. It also takes up space. A sample can be described by its properties and by the changes it can undergo.

A sample has a chemical identity (what substance it is based on its particles and how their atoms are connected). A physical property (a characteristic observed or measured without changing chemical identity) can be checked without making a new substance. Color, mass, volume, and density are examples. The substance keeps the same chemical identity.

A chemical property (a characteristic describing the ability to form new substances) describes what a substance can do when it interacts with other substances. A chemical property is about a possible chemical behavior, not just what a sample looks like now.

2. Physical Properties

An intensive property (a property that does not depend on sample amount) stays the same when the amount changes. Density and melting point are intensive. An extensive property (a property that depends on sample amount) changes when the amount changes. Mass and volume are extensive.

Density (mass per unit volume) compares how much mass is packed into a certain volume. Use the equation density = mass ÷ volume. Common units are g/mL for liquids and g/cm³ for solids.

Solubility (the maximum amount of a substance that dissolves under stated conditions) depends on the substances and conditions such as temperature. Conductivity (ability to transfer electric current or heat) tells how well matter carries electricity or heat.

Melting point (temperature at which a solid changes to a liquid at stated pressure) can help identify a pure substance. At 1 atm, water melts or freezes at 0 °C and boils at 100 °C.

3. Chemical Properties

Reactivity (tendency to form new substances when interacting with another substance) depends on the substances and conditions. Iron can react with oxygen gas, written O2. Acids can react with some metals. A substance may be reactive even when no reaction is happening at that moment.

Flammability (ability to burn through a reaction with oxygen) is another chemical property. Burning requires oxygen. A fuel can be flammable while it sits safely in a closed container because the property describes what can happen under the right conditions.

Chemical properties are measured by observing whether a substance can form different substances. The test may use heat, oxygen, water, acid, or another substance. Safety rules matter because these tests can release energy or harmful materials.

4. Physical Changes

A physical change (a change in form, size, or state without changing chemical identity) does not make a new substance. Cutting foil, crushing a can, and many dissolving processes are physical changes.

A phase change (a change between solid, liquid, and gas states) is a physical change caused when energy moves into or out of a sample. During melting, a solid becomes a liquid. During freezing, a liquid becomes a solid. Vaporization (a phase change from liquid to gas) and condensation (a phase change from gas to liquid) are also phase changes.

Particles move and spread differently during a phase change, but the particles keep the same chemical identity. For example, H2O is H2O as ice, liquid water, and water vapor.

5. Chemical Changes and Reactions

A chemical change (a process that forms one or more new substances) happens through a chemical reaction (a rearrangement of atoms that changes which atoms are connected). The atoms are still present, but they are connected in new ways.

A reactant (a starting substance in a chemical reaction) is present before the change. A product (a new substance formed by a chemical reaction) is present after the change. In a chemical equation, reactants are on the left side of the arrow, and products are on the right side.

The conservation of matter (the rule that atoms are not created or destroyed in ordinary chemical reactions) means the same atoms must be accounted for before and after a reaction. In a closed system (a system where matter does not enter or leave), the total mass of reactants equals the total mass of products.

Possible evidence of a chemical change includes gas production that is not boiling, formation of a precipitate (a solid that forms from substances in solution), an unexpected temperature or light change, or a lasting color or odor change. Evidence supports a conclusion, but one sign alone does not prove a chemical change. Decide whether new substances formed.