States of Matter

Matter exists in different physical states depending mainly on temperature and pressure. The three most familiar states are solid, liquid and gas, while plasma, found naturally in stars and lightning, is considered a fourth common state. In a solid, particles are packed closely together and mainly vibrate around fixed positions, giving the material a definite shape and volume.

In a liquid, the particles remain close but can move past one another, allowing the substance to flow while keeping nearly the same volume. In a gas, particles are much farther apart and move freely, so a gas expands to fill any container. Although the particles behave differently in each state, their chemical identity usually remains unchanged during a change of state, provided no chemical reaction occurs.

Changes between states happen when matter gains or loses thermal energy. Melting transforms a solid into a liquid, freezing changes a liquid into a solid, evaporation and boiling produce a gas from a liquid, while condensation turns a gas back into a liquid. Some substances can also pass directly between the solid and gaseous states through sublimation and deposition.

Pressure can also influence these changes by affecting how closely particles are packed together. Understanding the states of matter helps explain many everyday phenomena, from ice melting in a drink and steam rising from boiling water to the formation of clouds and the operation of refrigerators. These concepts are also essential in chemistry, physics, engineering and many industrial processes.