Question

Identify each of the following changes of state as melting, freezing, sublimation, or deposition: a. Dry ice in an ice-cream cart disappears. b. Snow on the ground turns to liquid water. c. Heat is removed from \(125 \mathrm{~g}\) of liquid water at \(0{ }^{\circ} \mathrm{C}\). d. Frost (ice) forms on the walls of a freezer unit of a refrigerator.

Short answer

a. Sublimation; b. Melting; c. Freezing; d. Deposition.

Step-by-step solution

Identify Change of State for Dry Ice

Dry ice is solid carbon dioxide. When it disappears, it goes directly from the solid state to the gas state without becoming liquid. This process is known as sublimation.

Identify Change of State for Snow

Snow is frozen water. When it turns to liquid water, it undergoes the process of melting as it changes from solid to liquid.

Identify Change of State for Liquid Water with Removed Heat

When heat is removed from liquid water at 0°C, it freezes and changes from liquid to solid. This process is known as freezing.

Identify Change of State for Frost Forming

Frost forming on the walls of a freezer happens when water vapor in the air turns directly into ice without becoming liquid first. This process is called deposition.

Key concepts

sublimation

In chemistry, sublimation is the transition of a substance directly from the solid phase to the gas phase, bypassing the liquid phase entirely. This may seem strange since we usually think of a solid turning into a liquid before becoming a gas, but some substances, like dry ice (solid carbon dioxide), undergo sublimation.
For example, when you observe 'dry ice in an ice-cream cart disappearing,' it means that the dry ice is sublimating. In simpler terms:

• The solid carbon dioxide is changing directly into carbon dioxide gas.
• There is no liquid carbon dioxide in between these stages.

Sublimation often requires specific conditions, such as low pressure or high temperature, making it a fascinating concept in the world of chemistry.

melting

Melting is the process where a solid turns into a liquid when it is heated to its melting point. This is a common change of state that we see every day. For example, when 'snow on the ground turns to liquid water,' it undergoes melting.
To understand melting, think of these points:

• A solid, like ice or snow, absorbs heat.
• As it absorbs heat, its internal energy increases.
• Once enough energy is absorbed, the solid becomes a liquid.

Melting requires the substance to reach a specific temperature, known as the melting point. Different materials have different melting points, such as:

• Ice melts at 0°C (32°F).
• Gold melts at approximately 1064°C (1947°F).

These conditions are critical in understanding why and how melting occurs.

freezing

Freezing is the process in which a liquid turns into a solid when it loses enough heat. This is the reverse of melting. For instance, when 'heat is removed from 125g of liquid water at 0°C,' the water undergoes the freezing process.
Key aspects of freezing include:

• The removal of heat from the liquid (cooling down).
• Internal energy of the liquid decreases.
• The liquid reaches its freezing point and changes into a solid.

The temperature at which freezing occurs for water is 0°C (32°F). Just like melting points vary amongst substances, so do freezing points. For example:

• Water freezes at 0°C (32°F).
• Mercury freezes at approximately -38.83°C (-37.89°F).

These points help explain why freezing is a crucial change of state in chemistry.

deposition

Deposition is the process where a gas transitions directly into a solid, skipping the liquid phase. It's fascinating because it's less commonly observed compared to the other changes of state. An example of deposition is 'frost (ice) forming on the walls of a freezer unit.' This happens when water vapor directly forms solid ice.
To grasp deposition, consider these points:

• Gas particles lose energy quickly.
• They arrange themselves into a solid without becoming a liquid first.

Deposition typically occurs in environments with extremely low temperatures, enhancing the speed of energy loss among gas particles. Practical examples of deposition include:

• Frost forming on surfaces during cold weather.
• Formation of soot in chimneys.

Understanding deposition helps in studying various natural and industrial processes.