Matter is defined as anything that has mass and occupies space. All matter is composed of tiny particles, such as atoms and molecules, which are constantly in motion. The physical form matter takes is its state, and the difference between these forms is determined by the energy level of its particles. The three most commonly observed states of matter in everyday life are solid, liquid, and gas.
Macroscopic Properties of Solids, Liquids, and Gases
The most observable differences between the three states of matter can be described by their shape, volume, and compressibility. A solid is characterized by a fixed shape and a fixed volume, meaning it holds its form and size regardless of the container it is placed in. Because the particles are tightly packed, solids are not easily compressed, making them rigid.
A liquid, in contrast, maintains a fixed volume but possesses a variable shape. A liquid will flow to take the exact shape of its container, such as water conforming to the shape of a glass. Like solids, liquids are considered nearly incompressible because the particles are already very close together.
Gases differ from both solids and liquids by having neither a fixed shape nor a fixed volume. Gas particles will expand to completely fill any container they occupy, assuming both its shape and volume. This state is also highly compressible, allowing its volume to be significantly reduced when pressure is applied.
How Particle Movement Dictates State
Temperature is a measure of the average kinetic energy, or energy of motion, of the constituent particles. In a solid, particles possess the lowest kinetic energy. The attractive forces between them are strong enough to lock them into fixed positions, allowing them only to vibrate in place. This accounts for the material’s fixed shape and rigidity.
Particles in a liquid have a moderate amount of kinetic energy, enough to partially overcome the strong attractive forces. This energy allows the particles to slide past one another while remaining close together, giving liquids their ability to flow. The close proximity of the particles is why liquids maintain a constant volume and are resistant to compression.
A gas represents the state with the highest kinetic energy, where particles move rapidly and randomly with minimal attractive forces between them. The vast empty space between gas particles allows the substance to be easily compressed and to expand freely.
Transitioning Between States
A phase transition occurs when a substance either gains or loses thermal energy, causing the kinetic energy of its particles to shift. The transition from a solid to a liquid is called melting and requires the absorption of heat energy. The reverse process, where a liquid loses heat energy and becomes a solid, is known as freezing.
When a liquid absorbs enough heat energy to become a gas, the process is called vaporization. The opposite process, condensation, happens when a gas loses thermal energy, causing the particles to slow down and form a liquid.
Some substances can transition directly between the solid and gas states, bypassing the liquid phase entirely. Sublimation is the process where a solid absorbs energy and transforms directly into a gas. The reverse process, deposition, occurs when a gas loses energy and changes directly into a solid.