Depending on temperature and pressure, matter exists in three states: solid, liquid, and gas. These changes of state are explained by the thermal motion of particles.
Basic Knowledge
The particles (atoms, molecules, and ions) that make up matter are always in thermal motion. The higher the temperature, the more vigorous the thermal motion.
Characteristics of each state:
・Solid: Particles are arranged regularly and vibrate in place. Shape and volume are constant.
・Liquid: Particles are close together but can move freely. The shape changes, but the volume is constant.
・Gas: Particles are far apart and move freely. Both shape and volume change freely.
Names of changes of state:
solid → liquid = melting, liquid → gas = evaporation (boiling), solid → gas = sublimation, gas → liquid = condensation, liquid → solid = solidification, gas → solid = deposition (or sublimation).
The melting point (the temperature at which melting occurs) and boiling point (the temperature at which boiling occurs) are values unique to pure substances and serve as indicators of purity.
Thermal motion (the continuous motion of particles; more vigorous at higher temperatures)
Melting and solidification (changes of state between solid and liquid; occur at the melting point)
Evaporation and condensation (changes of state between liquid and gas; boiling is evaporation from within a liquid)
Sublimation (a solid changes directly into a gas; iodine and dry ice)
Melting point and boiling point (temperatures unique to pure substances; at 1 atm, water is 0℃ and 100℃)
Absolute temperature (temperature expressed in K (kelvins); 0K=-273℃)
Further Study (Background and Significance)
During a change of state, the temperature remains constant. For example, while ice is changing into water, its temperature remains 0℃ even when it is heated, and the heat supplied is used as the heat of fusion to break the bonds between particles. Similarly, the energy required when water changes into water vapor is the heat of vaporization. The heat of vaporization is greater than the heat of fusion (for water: heat of fusion 6.0 kJ/mol, heat of vaporization 40.7 kJ/mol) because the particles must be separated almost completely when they become a gas.
Absolute zero (0 K = -273℃) is the limiting temperature at which the thermal motion of particles is at a minimum; no temperature lower than this is possible. The volume of a gas is approximately proportional to its absolute temperature (Charles's law). Sprinkling water cools the surroundings because the heat of vaporization of water is taken from them, and sweating lowers body temperature by the same principle.
- Three states = solid, liquid, and gas
- Solids are regular, liquids are close together and can move, and gases are far apart and move freely
- Temperature remains constant during a change of state (heat is used as heat of fusion or vaporization)
- Heat of vaporization > heat of fusion (40.7 vs 6.0 kJ/mol for water)
- Sublimation = solid → gas (iodine, dry ice, and naphthalene)
- Absolute zero 0 K = -273℃
- Cooling by sprinkling water and sweating is based on the principle of heat of vaporization
Points to Note (Easily Confused)
① Evaporation (from the liquid surface) and boiling (evaporation from within a liquid, occurring at the boiling point) are different concepts. ② Do not confuse the melting point (solid and liquid coexist) with the boiling point (liquid and gas coexist). ③ Absolute temperature T(K) = Celsius temperature t(℃) + 273. ④ The temperature does not rise during heating while a state change is occurring because the energy is used to break bonds.
Practice
- What is the change called when a solid changes directly into a gas without becoming a liquid?
- What is absolute zero in degrees Celsius?
- State the melting point and boiling point of pure water (at 1 atm).