Problem 8 - Introductory Chemistry Atoms First

Question

Molten iron cooling to solid iron is an example of (a) sublimation (b) condensation (c) freezing (d) melting

Step-by-Step Solution

Answer

The correct answer is (c) Freezing, as molten iron cooling to solid iron involves the transition from a liquid to a solid state.

Step 1: Option (a) Sublimation

Sublimation is the process in which a substance changes directly from a solid to a gas without passing through the liquid state. This does not fit our scenario, as molten iron is in a liquid state, and it is turning into a solid state. Therefore, sublimation is not the correct option.

Step 2: Option (b) Condensation

Condensation is the process in which a substance changes from a gas to a liquid state. This does not fit our scenario, as molten iron is initially in a liquid state and it is turning into a solid state, not a gas. Therefore, condensation is not the correct option.

Step 3: Option (c) Freezing

Freezing is the process in which a substance changes from a liquid to a solid state, usually due to a decrease in temperature. This option fits our given scenario, as molten iron is in a liquid state and when cooled, it turns into solid iron. Therefore, freezing is the correct option that describes molten iron cooling to solid iron.

Step 4: Option (d) Melting

Melting is the process in which a substance changes from a solid to a liquid state, usually due to an increase in temperature. This does not fit our scenario, as molten iron is initially in a liquid state and it is turning into a solid state, not the other way around. Therefore, melting is not the correct option. Based on the analysis, the correct answer is: \( \boxed{(c) \text{ Freezing}} \)

Key Concepts

Freezing

When we think about freezing, we're talking about turning a liquid into a solid. This happens when the temperature drops and the molecules in a liquid slow down. As they slow down, they pack closely together to form a solid.
This is exactly what happened with the molten iron turning into solid iron. Freezing is a very common process and occurs in everyday life.

• Water freezing into ice at 0°C (32°F)
• Liquid wax becoming solid

In each case, when the temperature gets low enough, the liquid's molecules reduce their movement and form strong bonds, which result in a solid structure. It’s an impressive transformation that shows how temperature affects matter!

Sublimation

Sublimation is quite an interesting phase change. It's when a solid turns directly into a gas, without becoming a liquid first. This might sound weird because most of the time, we're used to seeing solids melt into liquids before becoming gas. But sublimation isn't just a rare phenomenon. There are substances that readily undergo sublimation, such as:

• Dry ice, which is solid carbon dioxide. It turns directly into gas at room temperature.
• Naphthalene, found in mothballs, which sublimates easily at normal temperatures.

In these cases, the solid absorbs enough energy that it completely skips the liquid phase and jumps directly to gas. Isn’t it amazing how nature allows for such variations in phase changes?

Condensation

Condensation is all about turning gas back into a liquid. It's often seen when there's a temperature drop or when gas touches a cooler surface. Remember seeing water droplets on the outside of a cold glass? That’s condensation! This process is vital in many natural and industrial activities.

• It's part of the water cycle where it forms clouds.
• In technical systems, like air conditioners, condensation helps regulate temperatures.

Condensation happens because the gas molecules lose energy, slow down, and come closer together. Eventually, they intermingle and form a liquid. So, next time you notice dew on grass, you'll know it's the work of condensation.

Melting

Melting represents the transition from a solid to a liquid. It occurs when a substance absorbs heat, causing its molecules to speed up and spread apart. This everyday change happens in many materials.

• Ice turning into water at 0°C (32°F).
• Butter becoming liquid when heated.

The beauty of melting lies in its simplicity: sufficient heat breaks the rigid bonds of a solid, turning it into a more flexible liquid. It’s a key process in cooking, metallurgy, and even geology. Clearly, melting plays a crucial role in our world, bridging the divide between solid structures and fluidity.