Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Density, amount of a substance contained within a specific area. In physics, density is the ratio of the mass of a substance to its volume, and it can be calculated by dividing the mass by the volume. Density is often expressed in units such as grams per cubic centimeter or pounds per cubic foot.

The density of a substance can vary under different conditions. Substances expand and contract as their temperature changes, and as a result their density also changes. Precise measurements of density therefore include the temperature at which they were taken. For example, 1 cu cm of water at 4°C (39°F) weighs 1 g, so water’s density at that temperature is 1 g/cm3. Hot air balloons are able to fly because the density of air changes as its temperature changes. The balloon rises because the heated air inside its bag is less dense than the cooler air outside. Pressure also affects the density of gases. If a given amount of gas is contained within a smaller volume, its density increases. Air at sea level, for example, is denser than air at the top of Mount Everest because the air pressure at sea level is higher and squeezes the air into a smaller volume.

Density can be measured in a number of ways. Solid objects can be weighed to determine their mass and then immersed in a liquid to determine their volume. The volume of liquid displaced by the object is equal to the object’s volume, and the mass divided by the volume is its density. The density of a liquid may be determined similarly. The liquid’s mass can be found by first weighing an empty container, then weighing the container with the liquid in it, and then subtracting the empty weight from the full weight. The liquid’s volume may be determined by instruments similar to the transparent measuring cups used in cooking. Gases may be weighed in airtight containers of known volume and weight. Since gases are more sensitive to changes in temperature and pressure than are liquids or solids, the temperature and pressure must be included in any measurement of the density of a gas.