Cambridge Science Learner's Book for Year 8

Solids have closely packed particles with limited movement, resulting in fixed shape and volume. Liquids have particles that are close but can move past each other, allowing them to flow and take the shape of their container, while maintaining a fixed volume. Gases have widely spaced particles with rapid, random movement, making them compressible and able to fill any available space.

An element consists of only one type of atom (e.g., oxygen). A compound is formed from two or more elements chemically bonded in a fixed ratio (e.g., water). A mixture is a combination of substances that are physically combined but not chemically bonded (e.g., salad). To determine if a substance is a pure compound or a mixture, measure its melting and boiling points; pure compounds have sharp, specific values, while mixtures melt and boil over a range of temperatures.

Convection is heat transfer through fluids via the movement of hotter, less dense regions rising and cooler, denser regions sinking, creating currents. Temperature differences cause density variations, driving these currents. An example is in weather systems, where warm air rises and cool air sinks, influencing wind patterns and cloud formation.