Question

Supply the term that describes each of the following changes of physical state: (a) gas to liquid (b) liquid to solid

Short answer

(a) Condensation; (b) Freezing.

Step-by-step solution

Understanding the Change from Gas to Liquid

When a substance changes its state from a gas to a liquid, this process is termed 'condensation'. Condensation typically happens when the temperature of the gas is decreased to its dew point, causing gas molecules to lose energy and form a liquid.

Understanding the Change from Liquid to Solid

When a substance changes its state from a liquid to a solid, this process is referred to as 'freezing'. Freezing occurs when the temperature of the liquid is lowered to its freezing point, and the kinetic energy of its molecules decreases to a point where they take on a fixed, orderly structure characteristic of a solid.

Key concepts

Condensation

Condensation is a key phase change in which a substance transitions from a gas to a liquid state. This process occurs when gas molecules lose thermal energy. Imagine you have a gas at a high temperature: the molecules are moving very fast. When you cool it down, they slow enough to bond as a liquid.
This transformation usually requires a drop in temperature or an increase in pressure. For example, on a cool morning, you might notice droplets of water forming on the outside of a cold glass. This happens because the water vapor in the air is cooling down, losing energy, and changing to liquid form on the glass surface.
Here's a quick look at the steps involved in condensation:

• Gas temperature decreases to the dew point.
• Gas molecules lose energy and slow down.
• Molecules stick together to form a liquid.

Condensation is crucial for processes like the water cycle, where it forms clouds as water vapor in the air changes into liquid droplets.

Freezing

Freezing is a process where a liquid turns into a solid. This phase change happens when the liquid's temperature falls to its freezing point. At this temperature, molecules arrange into a more fixed and orderly structure, creating a solid.
A classic example is water turning into ice. When water is cooled to 0°C (32°F), it becomes ice. The molecules lose kinetic energy, slowing down and packing closely until they form a rigid structure.
In summary, freezing involves the following steps:

• Lowering the temperature of the liquid.
• Decreasing the kinetic energy of molecules.
• Molecules organizing into a solid structure.

Freezing is an essential concept in both natural phenomena and everyday life, from forming ice in the winter to making frozen foods.

Physical States of Matter

Matter can exist in different states: solid, liquid, and gas. Each state has unique properties because of the arrangement and movement of its molecules.
- **In Solids**: Molecules are tightly packed in a fixed structure. Solids have a definite shape and volume.
- **In Liquids**: Molecules are less tightly packed than in solids and can move around. Liquids have a definite volume but take the shape of their container.
- **In Gases**: Molecules are far apart and move freely. Gases have neither a definite shape nor volume and expand to fill their container.
Understanding the physical states of matter is crucial for comprehending phase changes. For instance, when considering condensation and freezing, we observe transitions between these physical states.
Learning about these states helps us explain a wide array of natural and industrial processes, such as weather patterns and food preservation.