Water freezes at 0℃ and boils at 100℃. We will learn about changes of state in substances and changes in temperature and volume during these changes.
Basic Knowledge
Substances have three states: solid, liquid, and gas. They change from one state to another as temperature changes. This is called a change of state.
・Solid→liquid: melting (melting point)
・Liquid→gas: evaporation and boiling (boiling point)
・Gas→liquid: condensation
・Liquid→solid: solidification
・Solid→gas (or the reverse): sublimation (dry ice, naphthalene, and so on)
During a change of state, mass remains unchanged, while only volume changes. In general, volume increases in the order solid→liquid→gas (water is an exception: ice has a larger volume than water). The melting point and boiling point are values specific to each substance, and a pure substance maintains a constant temperature during a change of state.
Melting point（the temperature at which a solid changes into a liquid. Water is 0℃）
Boiling point（the temperature at which a liquid boils. Water is 100℃)
Evaporation（the phenomenon in which a liquid changes into a gas at its surface. It occurs regardless of temperature)
Boiling（the phenomenon in which a liquid also changes into a gas inside the liquid. It occurs at the boiling point)
Sublimation（the phenomenon in which a substance moves directly between solid and gas. Dry ice is an example)
Distillation（a method of separating components of a mixture using differences in boiling points)
Further Exploration (Background and Significance)
The real reason changes of state occur is the difference in the intensity of motion of particles (atoms and molecules). In a solid, particles vibrate in fixed positions; in a liquid, they move while changing positions; and in a gas, they move freely. When heat is added, particle motion becomes more intense and the state changes.
Melting and boiling differ between pure substances and mixtures. A pure substance (water, ethanol, and so on) changes state at a constant temperature, while a mixture (seawater, air) changes state as its temperature changes. Distillation uses this property and is used to separate kerosene and gasoline from crude oil and to produce whiskey.
- Three states = solid, liquid, and gas
- Melting (melting point), solidification, evaporation/boiling (boiling point), condensation, and sublimation
- Mass remains unchanged and volume changes during a change of state
- Water is an exception: the solid (ice) has a larger volume
- Melting and boiling points are specific to each substance
- Distillation = separating a mixture using differences in boiling points
- Difference between evaporation and boiling: surface only or also inside
Points to Note (Easily Confused)
① Melting (solid→liquid) and dissolving are different phenomena. Ice becoming water is melting, while sugar dissolving in water is dissolving. ② Distinguish evaporation (surface only, regardless of temperature) from boiling (also from inside, at the boiling point). ③ Sublimation is direct movement between solid and gas. Dry ice (CO2) is a typical example. ④ During a change of state, mass remains unchanged; only volume changes.
Practice
- What is the change from a liquid to a solid called?
- What happens to the temperature while a pure substance is melting?
- What property does distillation use as a separation method?