Question

Indicate whether the following changes of physical state require heating or cooling: (a) solid to liquid (b) gas to liquid

Short answer

(a) solid to liquid requires heating; (b) gas to liquid requires cooling.

Step-by-step solution

Understanding Physical States

Let's establish the basic states of matter: solid, liquid, and gas. A solid has a fixed shape and volume, a liquid has a fixed volume but takes the shape of its container, and a gas fills the volume and shape of its container.

Transition from Solid to Liquid

The process of changing from a solid to a liquid is called melting. Melting occurs when a substance gains thermal energy sufficient enough to overcome its rigid structure, leading to a more fluid state. This requires the addition of heat.

Transition from Gas to Liquid

The process of changing from a gas to a liquid is called condensation. During condensation, a substance loses thermal energy as the gas molecules slow down and attract each other to form a liquid, which necessitates cooling.

Key concepts

Melting Process

When a solid changes into a liquid, we call this transformation the melting process. This change involves heating because the solid needs to absorb thermal energy. Let's explore this with an example. Think about ice cubes turning into water. The solid ice cubes absorb heat energy, which helps in breaking the bonds that hold the molecules in their fixed positions.
As the temperature rises to the melting point, the solid becomes fluid and flows more freely.

• Absorption of Heat Energy: The solid needs to absorb enough thermal energy to break free from its structured form.
• Temperature at the Melting Point: Each substance has a specific temperature at which it melts, known as its melting point.

This process is essential for understanding many everyday phenomena and plays a crucial role in natural and industrial processes.

Condensation Process

The condensation process describes a gas transforming into a liquid. For condensation to occur, the gas must cool down and release thermal energy. Let's consider how steam from a boiling kettle turns into water droplets on a cooler surface. As steam encounters a cooler surface, it loses energy. This is because the gas particles slow down and come closer together to form a liquid.
The cooling results in decreased motion in the molecules, ultimately causing them to stick together.

• Loss of Thermal Energy: Condensation requires cooling, meaning the gas releases energy to the surrounding environment.
• Temperature Influence: The condensation process is influenced by the temperature of the environment, leading to the liquid phase formation when the temperature drops below the condensation point.

This process is commonly observed in nature as dew or fog and is fundamental to understanding weather patterns and systems.

Thermal Energy Changes

Understanding how thermal energy changes during state transitions is vital. Thermal energy, which is related to temperature, affects the state of matter by either being absorbed or released.

• Energy Absorption and Release: When a substance changes state, it either absorbs or releases thermal energy. During melting, energy absorption is necessary, while in condensation, energy is released to the environment.
• Energy Metrics: The specific quantity of energy required to change the state of one gram of a substance is known as the latent heat of fusion for melting, and latent heat of vaporization for condensation.

These changes significantly impact both everyday life and industrial applications, demonstrating how closely intertwined energy and physical state changes are.