Problem 39 - Chemistry: The Central Science in SI Units, Global Edition

Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) Ice-cream melts at room temperature. (b) Potato slices become crisp when fried. \((\mathbf{c})\) Droplets of water appear on the lid of a food container containing freshly baked bread. (d) You can see your own breath on a cold day.

Step-by-Step Solution

Answer

a) Melting, endothermic. b) Vaporization, endothermic. c) Condensation, exothermic. d) Condensation, exothermic.

Step 1: a) Ice-cream melts at room temperature

When solid ice-cream absorbs heat from its surroundings and changes into a liquid state, the phase transition taking place is melting. Melting is an endothermic process, as it absorbs energy from the surroundings in the form of heat.

Step 2: b) Potato slices become crisp when fried

When potato slices are fried, they lose their water content in the form of steam and the texture becomes crisp. The phase transition occurring here is vaporization or boiling. As the potato slices are heated up, the water in them obtains enough energy to turn into a gaseous state. Vaporization is an endothermic process, since heat energy is absorbed to turn water into steam.

Step 3: c) Droplets of water appear on the lid of a food container containing freshly baked bread

The water droplets appearing on the lid are a result of the cooling down of water vapor present in the container filled with freshly baked bread. The phase transition taking place here is condensation. Water vapor cools down and transforms back into the liquid state as water droplets. Condensation is an exothermic process, as energy is released in the form of heat to the surroundings.

Step 4: d) You can see your own breath on a cold day

On a cold day, the breath we exhale is warmer than the cold surrounding air. It contains water vapor which, when comes in contact with the cold air, loses its energy, and is converted into tiny water droplets, forming a mist or cloud. The phase transition happening here is condensation, similar to the previous case. As the water vapor loses heat energy to the cold surroundings while turning into tiny droplets, it is an exothermic process.

Key Concepts

Melting

Melting is a phase transition where a substance changes from a solid state to a liquid state. Imagine an ice cube slowly transforming into a pool of water as it sits on a warm table. That's melting in action. This process happens as the solid absorbs heat from its surroundings, causing its molecules to gain energy and vibrate more vigorously.

• The solid structure breaks down, allowing the molecules to move more freely, resulting in a liquid form.
• Melting is considered an endothermic process because it requires an input of energy (heat) from the environment.

Whenever you witness ice cream melting into a creamy liquid on a sunny day, remember: energy is being absorbed to facilitate this transformation.

Vaporization

Vaporization is the process in which a liquid turns into a gas. This can occur through evaporation or boiling. For instance, when you heat a pot of water on a stove, the water transforms into steam. As with melting, vaporization absorbs energy from the surroundings.

• This means vaporization is also an endothermic process, as it demands energy to convert the liquid molecules into a gaseous state.
• Vaporization can be observed when frying potato slices—water inside the potatoes turns into steam, making the slices crispy.

Every time you see water boiling, that's vaporization fueled by the absorption of heat, demonstrating an impressive dance of molecules transitioning from liquid to gas.

Condensation

Condensation is the opposite of vaporization. During this phase transition, gas turns into a liquid. It usually comes into play when gas loses heat and energy, such as the formation of droplets of water on the outside of a cold beverage on a humid day.

• Condensation is an exothermic process, as it releases energy in the form of heat into the surroundings.
• You might notice condensation when steam from a freshly baked loaf traps under the lid of a container and converts back into liquid droplets.
• Similarly, when you see your breath on a cold day, the water vapor in your breath condenses into tiny droplets, forming a visible mist.

This release of energy to transition from a gaseous to a liquid state highlights the fascinating interplay between temperature and phase.

Endothermic and Exothermic Processes

Phase transitions can either absorb energy (endothermic) or release energy (exothermic). Here's how these two processes work:

• Endothermic Processes: Absorb heat from the surroundings. Examples include melting and vaporization. Substances need energy to break intermolecular bonds and transition to a higher energy state.
• Exothermic Processes: Release heat to the surroundings. Examples are condensation and freezing. These processes involve forming intermolecular bonds and shifting to a lower energy state.

Understanding how these processes transfer energy aids in predicting the behavior of substances as they change states, providing insight into the energy dynamics at play.