Matter and Its Properties: A Comprehensive Guide

Pure Substances

A pure substance has a constant composition and distinct properties. These are further divided into:

1. Elements: The simplest form of matter that cannot be broken down further by chemical means.
2. Compounds: Substances composed of two or more elements chemically combined in a fixed ratio.

Mixtures

A mixture is a combination of two or more substances that are physically combined but not chemically bonded. Because they are not chemically bonded, they can be separated by physical means.

• Homogeneous Mixtures (Solutions): These have a uniform appearance and composition throughout. A common example is salt water, where the salt (solute) is dissolved in water (solvent).
• Heterogeneous Mixtures: These consist of visibly different substances or phases, such as sand in water.

Properties of Matter

We can describe matter using two different frameworks: how it behaves chemically and how its properties change with scale.

Physical vs. Chemical Properties

Physical properties can be observed or measured without changing the identity of the substance. Examples include color, hardness, melting point, boiling point, and density.

Chemical properties describe how a substance changes into a completely new substance through a chemical change. For example, the combustibility of wood or the way iron rusts are chemical properties because they result in new substances.

Extensive vs. Intensive Properties

When measuring physical properties, we distinguish between:

• Extensive Properties: These depend on the amount of matter present. Examples include mass (the amount of matter in an object) and volume (the space an object takes up).
• Intensive Properties: These do not depend on the amount of matter. Density is a prime example; it is a measure of how tightly packed atoms are within a substance. Whether you have a gram or a kilogram of copper, its density remains the same.

Math Tip: Calculating Density Density ($\rho$) is defined as mass ($m$) divided by volume ($V$): $\rho = \frac{m}{V}$ If you know the density and volume, you can find the mass: $m = \rho \cdot V$ Example: If a sample of copper has a volume of $32.6\text{ cm}^3$ and a density of $8.96\text{ g/cm}^3$, its mass is: $m = 8.96\text{ g/cm}^3 \cdot 32.6\text{ cm}^3 = 292.096\text{ g}$.

Separation Techniques

Because mixtures are physically combined, we can use their unique physical properties to separate them.

Distillation is particularly useful for separating liquids. Simple distillation works best when the boiling points of the liquids differ by more than $25^\circ\text{C}$. If the difference is less than $25^\circ\text{C}$, fractional distillation is required. This is how various components of crude oil, such as gasoline and paraffin wax, are separated.

For liquids that do not mix (immiscible liquids), such as oil and water, a separating funnel is used. The denser liquid settles at the bottom and can be drained out through a stopcock.

Exam Tip: When asked to identify a separation method, always look for the "clue" in the description. If the text mentions "boiling points," think distillation. If it mentions "different sizes," think filtration or sieving.