Density and Pressure

Density and Pressure

Density

• Density is a measure of how compact the mass in a substance or object is. It is calculated by dividing the mass of the substance by its volume.
• The formula for density is density = mass/volume.
• The SI unit of density is kilograms per cubic metre (kg/m³), but it can also be measured in grams per cubic centimetre (g/cm³) where 1 g/cm³ = 1000 kg/m³.
• More dense substances have particles that are more closely packed together.
• In the context of solids, liquids, and gases, solids generally have a higher density because their particles are closely packed together.
• Comparatively, gases usually have the lowest density as their particles are spread out.
• Heating a substance can also cause it to expand, which increases its volume and therefore reduces its density.
• Understanding density is essential for predicting whether an object will float or sink when placed in a fluid.

Pressure

• Pressure is the amount of force exerted per unit area.
• The formula for pressure is pressure = force/area.
• The SI unit of pressure is pascal (Pa), but it can also be measured in atmospheres (atm), and millimetres of mercury (mmHg).
• Pressure is experienced differently across solids, liquids, and gases.
• Solid materials distribute applied pressure evenly across their entire surface, while liquids and gases apply pressure equally in every direction.
• An increase in temperature of a gas will increase its pressure, if the volume is kept constant. This is known as Gay-Lussac’s law.
• A decrease in the volume of a gas will increase its pressure, if the temperature is kept constant, known as Boyle’s law.
• Atmospheric pressure decreases with height.

Remember, concepts of density and pressure are integral for understanding behaviours of solids, liquids, and gases and state changes between these.