Chemistry: Properties of materials and the particle model

All matter exists in one of three states: solid, liquid, or gas. The particle model explains the properties of these states by considering matter as being made of tiny particles. In solids, particles are packed tightly in a regular pattern (a lattice) and have low energy. They can only vibrate in fixed positions due to strong forces of attraction holding them together. In liquids, particles are still close together but are arranged randomly. They have more energy than solids, allowing them to move and slide past one another, which is why liquids can flow. The forces between liquid particles are weaker than in solids. In gases, particles have high energy and are far apart with no regular arrangement. They move randomly and rapidly in all directions. The forces between gas particles are negligible, which is why gases expand to fill any container and are easily compressed.

Substances can change state when they gain or lose thermal energy. These are physical changes because no new substance is formed. When a solid is heated, its particles gain energy and vibrate more. At the melting point, they gain enough energy to break free from their fixed positions and the solid melts into a liquid. If a liquid is heated further, its particles gain more energy and move faster. At the boiling point, they have enough energy to overcome the forces of attraction completely and escape as a gas (boiling). Evaporation is a similar process but occurs only at the surface of a liquid and at any temperature below the boiling point. The reverse processes happen on cooling. When a gas cools, it loses energy and condenses into a liquid. When a liquid cools, it freezes into a solid. Sublimation is the direct change from a solid to a gas, like with dry ice (solid CO2).

All substances can be classified as elements, compounds, or mixtures. An element is a pure substance consisting of only one type of atom (e.g., oxygen, iron). It cannot be broken down into anything simpler by chemical means. A compound is a pure substance formed when two or more different elements are chemically bonded together in a fixed ratio (e.g., water, H₂O; carbon dioxide, CO₂). The properties of a compound are completely different from its constituent elements. A mixture consists of two or more substances (elements or compounds) that are not chemically bonded. The components of a mixture keep their own properties and can be separated by physical methods like filtration or evaporation (e.g., air, salt water).

Particle diagrams are simple drawings used to show the difference between elements, compounds, and mixtures at a microscopic level. Different types of atoms are represented by different coloured or sized circles. For an element, the diagram shows only one type of particle (e.g., all identical circles). For a compound, the diagram shows identical units (molecules) where two or more different types of atoms are chemically joined together. For a mixture, the diagram shows two or more different types of particles (which can be atoms of elements or molecules of compounds) that are separate and not joined to each other.

An alloy is a mixture that contains a metal and at least one other element, which may be another metal or a non-metal. For example, steel is an alloy of iron and carbon, and brass is an alloy of copper and zinc. Alloys are not compounds; they are solid mixtures. Pure metals have a regular lattice structure where all atoms are the same size. This allows layers of atoms to slide over each other easily, making pure metals relatively soft and malleable. In an alloy, the different sized atoms of the added element(s) distort this regular lattice. This disruption makes it much more difficult for the layers of atoms to slide past one another. As a result, alloys are almost always harder, stronger, and less malleable than their pure metal components.

Physical properties are characteristics of a substance that can be observed without changing its chemical identity. These properties are used to classify substances, particularly to distinguish between metals and non-metals. Key properties include: appearance (shiny/lustrous or dull), electrical conductivity (conducts electricity or is an insulator), thermal conductivity (conducts heat or is an insulator), malleability (can be hammered into shape or is brittle), ductility (can be drawn into wires), and density (high or low). Typically, metals are lustrous, good electrical and thermal conductors, malleable, ductile, and have high densities and melting points. Non-metals are typically dull, poor conductors (insulators), brittle when solid, and have low densities and melting points.