Question

How does a sample of gas differ in its behavior from a sample of liquid in each of the following situations? (a) The sample is transferred from one container to a larger one. (b) The sample is heated in an expandable container, but no change of state occurs. (c) The sample is placed in a cylinder with a piston, and an external force is applied.

Short answer

Gases expand to fill larger containers, expand more when heated in expandable containers, and compress easily under force. Liquids retain volume and shape in larger containers, expand slightly when heated, and are relatively incompressible.

Step-by-step solution

Transferring from one container to a larger one

When a gas sample is transferred from one container to a larger one, the gas will expand to fill the entire volume of the new container, spreading out evenly. On the other hand, when a liquid sample is transferred to a larger container, the liquid will retain its volume and shape, adjusting only to the base of the container without spreading out to fill the entire volume.

Heating in an expandable container without state change

Upon heating a gas in an expandable container, the gas will expand significantly because gas molecules move more rapidly with increased temperature, leading to higher pressure and volume. For a liquid, heating in an expandable container also increases the kinetic energy of the molecules, causing some expansion, but it is much less compared to a gas since liquid molecules are more closely packed and have stronger intermolecular forces.

Applying an external force via a piston in a cylinder

When an external force is applied to a gas sample in a piston-cylinder setup, the gas will compress easily due to the significant amount of empty space between the gas molecules. For a liquid sample, compression is minimal as liquid molecules are already closely packed together, making liquids relatively incompressible compared to gases.

Key concepts

Gas Expansion

When we talk about gas expansion, we are referring to the ability of gas particles to spread out and fill any available space. Imagine moving gas from one container to a larger one. In this scenario, gas molecules will always expand to occupy the entire volume of the new container. This happens because they move freely and rapidly in all directions.
For example, when you inflate a balloon, the gas inside it expands to fill the balloon's volume.
This contrasts with liquids, which retain their volume when transferred to a larger container. They won't spread out across the entire container but will adjust to the shape of the bottom.

Liquid Retention

Liquid retention refers to the fact that liquids keep their volume constant, even when transferred to different containers. Unlike gases, liquid molecules are closely packed together and have strong intermolecular forces holding them in a fixed volume.
For instance, if you pour water from a glass into a bowl, the water will not spread out to fill the entire bowl. Instead, it will stay at the base, retaining its original volume.

• Gas molecules are free to move and expand.
• Liquid molecules are tightly packed and retain their volume.

Liquid retention ensures that the amount of liquid remains unchanged despite changes in the shape of the container.

Thermal Expansion

Thermal expansion occurs when a substance increases in volume as a result of temperature increase. In gases, heating causes molecules to move more quickly, leading to significant expansion. In an expandable container, this results in higher volume and pressure. Imagine heating air in a hot air balloon—it expands, making the balloon rise.
In liquids, heating also increases the kinetic energy of molecules, causing some expansion. However, this expansion is much less drastic because liquid molecules are more closely packed than gas molecules.
Though liquids do expand when heated, the amount is relatively small compared to gases.

Compressibility

Compressibility is the measure of how much a substance can be compacted. Gases are highly compressible because they have large amounts of empty space between molecules. Applying an external force, like pressing down a piston, can easily compress a gas, as the gas molecules can be pushed closer together.
Conversely, liquids are much less compressible. Their molecules are already close together, leaving very little empty space. For example, it's difficult to compress water in a syringe by applying pressure, as the molecules are tightly packed. Generally, liquids are considered nearly incompressible compared to gases.
This difference in compressibility has crucial implications in many practical applications, from hydraulic systems to aerosol cans.