Problem 39 - Chemistry The Central Science In Si Units

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) Evaporation, endothermic. (c) Condensation, exothermic. (d) Condensation, exothermic.

Step 1: Identify the Phase Transition in (a)

When ice-cream melts at room temperature, it changes from a solid to a liquid state. This phase transition is called melting.

Step 2: Determine if (a) is Exothermic or Endothermic

Melting requires the absorption of heat from the surroundings, so it is an endothermic process.

Step 3: Identify the Phase Transition in (b)

When potato slices become crisp when fried, the moisture within them evaporates. This phase transition is evaporation or vaporization.

Step 4: Determine if (b) is Exothermic or Endothermic

Evaporation requires heat input to convert liquid moisture into vapor, making it an endothermic process.

Step 5: Identify the Phase Transition in (c)

Droplets of water appearing on the lid of a food container is due to the condensation of water vapor back into liquid water.

Step 6: Determine if (c) is Exothermic or Endothermic

Condensation releases heat to the surroundings as vapor turns into liquid, so it is an exothermic process.

Step 7: Identify the Phase Transition in (d)

Seeing your breath on a cold day is due to water vapor in your breath condensing into tiny droplets when it meets the cold air, a process known as condensation.

Step 8: Determine if (d) is Exothermic or Endothermic

Like other condensation processes, this involves releasing heat to the surroundings, making it exothermic.

Key Concepts

Melting

Melting occurs when a solid changes into a liquid. It's a common phase transition that many of us see everyday, like when ice turns into water. This process happens because the molecules in a solid gain enough energy to break free from their fixed positions.

• During melting, heat is absorbed from the environment.
• The absorbed heat increases the movement of molecules, weakening the forces that hold them together in a solid form.

This is why melting is considered an endothermic process. It requires energy to overcome the attractions between particles. A practical example of melting is ice cream becoming liquid on a warm day. The surrounding temperature provides enough heat for the ice cream to melt.

Evaporation

Evaporation is the process where liquid turns into gas or vapor. It happens on the surface of a liquid as molecules gain energy and escape into the air. This phase transition is visible when you see water evaporating from a puddle or your skin drying after a swim.

• Evaporation takes place at the surface of a liquid.
• It occurs at any temperature, though faster at higher temperatures.
• Heat is absorbed from the surrounding environment during this process.

Because it requires energy input, evaporation is also an endothermic process. In the kitchen, frying potato slices causes the water within them to evaporate, making the slices crispy.

Condensation

Condensation is the transition of a gas turning into a liquid. This generally happens when gas particles lose energy. As they cool down, they move less and come closer together, reverting to a liquid state. You might notice condensation when water droplets form on the outside of a cold glass, or on a bathroom mirror after a hot shower.

• This process involves removal of heat from the gas particles.
• The energy loss allows particles to come together, forming liquid drops.

In other words, condensation is an exothermic process. It releases energy to the surroundings. Simple daily examples include seeing drops of water on fresh food container lids or your breath becoming visible in cold air.

Endothermic Process

Endothermic processes involve the absorption of heat. They require energy to proceed and are often detected by a cooling sensation because they consume heat from the surroundings. This feature is what classifies actions like melting and evaporation.

• Heat is absorbed from the environment.
• Energy is needed to break molecular attractions.
• Examples include melting ice and boiling water.

These processes play critical roles in our environment and daily activities, making them fundamental concepts in understanding heat relations and energy cycles.

Exothermic Process

Exothermic processes release heat to their surroundings. During these processes, energy is released when molecules come closer together. This can even produce a noticeable warmth.

• Heat is given off to the environment.
• Energy release occurs when bond formations happen.
• Examples include condensation and freezing.

Exothermic reactions are fundamental in various natural and industrial processes. They show us how nature balances energy release and absorption, which is vital for life and technology.