Key Points

A pure substance consists of a single type of particle (element or compound) with a fixed chemical nature. A mixture contains two or more pure substances physically mixed in any proportion.

A mixture with a uniform composition throughout is called a homogeneous mixture or a solution. Examples include salt water and air. Its components cannot be seen separately.

A mixture with a non-uniform composition and physically distinct parts is called a heterogeneous mixture. Examples include a mixture of sand and water, or oil and water.

A solution has two components: a solvent and a solute. The solvent is the component that dissolves the other (usually in a larger amount), and the solute is the component that is dissolved (usually in a lesser amount).

Particles in a solution are smaller than $1$ nm, cannot be seen with naked eyes, and do not scatter light (no Tyndall effect). Solutions are stable and cannot be separated by filtration.

A suspension is a heterogeneous mixture where solute particles do not dissolve but remain suspended. Its particles are visible to the naked eye, it is unstable (particles settle down), and it can be separated by filtration.

A colloid is a heterogeneous mixture that appears homogeneous. Its particles are larger than solution particles but smaller than suspension particles. Colloids are stable and show the Tyndall effect.

The Tyndall effect is the scattering of a beam of light by particles in a colloid or a fine suspension, which makes the path of the light beam visible. True solutions do not exhibit this effect.

A saturated solution is one in which no more solute can be dissolved at a given temperature. An unsaturated solution is one in which more solute can be dissolved at the same temperature.

The concentration is the amount of solute present in a given amount of solution. One common method to express it is mass by mass percentage: $\text{Concentration} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$.

A physical change alters the physical properties of a substance without changing its chemical identity. No new substance is formed. Examples include melting of ice and boiling of water.

A chemical change results in the formation of one or more new substances with different properties and chemical composition. It is also called a chemical reaction. Examples include rusting of iron and burning of wood.

An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Elements are classified as metals, non-metals, and metalloids.

A compound is a substance composed of two or more elements chemically combined in a fixed proportion. The properties of a compound are entirely different from those of its constituent elements.

Mixtures have variable composition and show properties of their constituents, which can be separated by physical means. Compounds have a fixed composition, new properties, and can only be separated into elements by chemical reactions.

Alloys are homogeneous mixtures of two or more metals, or a metal and a non-metal. They are considered mixtures because they show the properties of their constituents and can have variable composition. An example is brass (copper and zinc).