Matter Study Guide

Density is a measure of how much mass is contained within a given volume. It helps us understand how tightly packed the particles (atoms or molecules) are in a substance. The three common states of matter are solid, liquid, and gas, each exhibiting different arrangements of particles, which directly impacts their density. In a solid, particles are closely packed in a regular, fixed arrangement. This tight packing results in solids typically having the highest densities because there is a large number of particles in a small volume. Liquids have particles that are also closely packed, but they are arranged randomly and can move past one another. This allows liquids to flow, and generally, their densities are slightly lower than solids because the particles are not held in a perfectly rigid structure, allowing for marginally more space between them. Gases have particles that are widely spread out and move randomly and rapidly. There are large distances between gas particles, meaning a much smaller number of particles occupy the same volume compared to solids or liquids. Consequently, gases have significantly lower densities than solids and liquids. For example, ice (solid water) is less dense than liquid water, which is an unusual exception; most substances are densest in their solid state. This difference in density explains why solids sink in their corresponding liquids, while gases float above both solids and liquids if not contained.

In chemistry, changes are broadly categorised into physical and chemical changes. A physical change involves altering the form or appearance of a substance, but not its chemical composition. During a physical change, no new chemical substances are formed. Examples include melting ice, boiling water, dissolving sugar, or tearing paper. The key characteristic distinguishing a physical change is its reversibility; if the conditions are altered back to their original state, the substance will recover its original properties. For instance, if ice melts to water, cooling the water will turn it back into ice, and it is still H₂O in both states. Similarly, evaporating salt water leaves solid salt, which can be redissolved. These changes typically involve changes in state (solid, liquid, gas) or dissolving, where the particles themselves remain chemically unchanged, merely rearranging or separating. In contrast, a chemical change results in the formation of one or more new substances with different chemical properties from the original materials. These changes are often difficult or impossible to reverse using simple physical means.