# Solids, Liquids and Gases: Density and Pressure

## Solids, Liquids and Gases: Density and Pressure

- Density is the amount of mass per unit of volume. This can be calculated by dividing mass by volume, hence the formula: density = mass/volume.
- A solid has a higher density than a liquid or gas due to its particles being packed closely together. A gas has the lowest density due to its particles being far apart.
- The density of an object determines whether it will float or sink in a liquid. Objects with lower density than the liquid will float, whereas objects with higher density will sink.
- The density may change depending on the temperature. For example, when a substance is heated, typically it expands and its density decreases.
- Pressure is the force applied per unit area. The formula for pressure is pressure = force/area.
- Gas pressure is caused by the gas molecules colliding with the walls of its container. The more regularly these collisions occur or the stronger they are, the greater the pressure.
- The pressure a liquid exerts is determined by its depth and the density of the liquid. The greater the depth or the greater the density, the more pressure the liquid exerts.
- Pressure in solids can be applied in specific areas to change the shape of the solid, for instance, cutting with a knife (high pressure, small area) or flattening dough with a rolling pin (moderate pressure, large area).
- Underwater pressure increases due to the weight of the water above increasing with depth, hence why ears may ‘pop’ when diving or ascending quickly.
- Atmospheric pressure decreases with altitude. At sea level, the atmospheric pressure is greatest because there’s more air above pressing down.

Remember, for any topic, understanding the key concepts, definitions, and relations between them is vital. Being familiar with units, being able to perform calculations involving density and pressure, and understanding their real-world applications, are crucial for mastering these topics.