What quantity is mass per unit volume?

• A. Volume
• B. Mass
• C. Density
• D. Temperature

Answer: (C)

Density is mass per unit volume. It tells you how much matter is packed into a given amount of space. By definition, density = mass / volume. In SI units it's kilograms per cubic meter, but grams per cubic centimeter (or per milliliter) are commonly used as well. For example, water has a density of about 1 g/mL (which equals 1 kg/L), so a liter of water weighs roughly 1 kilogram. Metals like iron or lead have high densities, meaning a lot of mass fits into a small volume, while gases have low densities because the same mass spreads out over a much larger volume. Temperature and pressure can affect density, especially for gases, since changing conditions changes volume while mass stays the same.