Question

Iodine has \(\mathrm{mp}=113.7^{\circ} \mathrm{C}\) and \(\mathrm{bp}=184.4^{\circ} \mathrm{C}\). What, if any, phase changes take place under the following conditions at \(1.0\) atm pressure? (a) The temperature of a solid sample is held at \(113.7{ }^{\circ} \mathrm{C}\) while heat is added. (b) The temperature of a sample is lowered from \(452 \mathrm{~K}\) to \(389 \mathrm{~K}\).

Short answer

(a) Melting occurs; (b) No phase change.

Step-by-step solution

Analyze Condition (a)

The given condition #1 states that a solid sample of iodine is held at its melting point of \(113.7^{\circ} \mathrm{C}\) while heat is added. This indicates that the temperature corresponds exactly to the melting point, where the phase change from solid to liquid occurs. Since heat is added at the melting point, iodine undergoes melting.

Determine Phase Change for Condition (a)

Given iodine's melting point at \(113.7^{\circ} \mathrm{C}\), heat addition at this exact point implies iodine transitions from solid to liquid by absorbing heat energy during the phase change.

Convert Temperatures for Condition (b)

Condition #2 involves reducing the temperature from \(452 \mathrm{~K}\) to \(389 \mathrm{~K}\). Convert these temperatures to degrees Celsius for comparison:- Initial Temperature: \(452 \mathrm{~K} = 452 - 273.15 = 178.85^{\circ} \mathrm{C}\).- Final Temperature: \(389 \mathrm{~K} = 389 - 273.15 = 115.85^{\circ} \mathrm{C}\).

Analyze Temperature Changes for Condition (b)

Compare the initial and final temperatures with iodine's melting and boiling points:- Initial Temperature: \(178.85^{\circ} \mathrm{C}\) (Below boiling point, above melting point) - Iodine exists in the liquid state initially.- Final Temperature: \(115.85^{\circ} \mathrm{C}\) (Above melting point) - Iodine remains in liquid state as it is still above the melting point.

Determine Phase Change for Condition (b)

Since lowering the temperature from \(178.85^{\circ} \mathrm{C}\) to \(115.85^{\circ} \mathrm{C}\) stays above the melting point \(113.7^{\circ} \mathrm{C}\) and doesn't go below it, no phase change occurs, and iodine remains a liquid.

Key concepts

Melting Point

The melting point of a substance is the temperature at which it transitions from a solid to a liquid state. For iodine, this occurs at 113.7°C. When a substance reaches its melting point, the particles have enough energy to overcome the forces holding them together in a fixed position.
To better understand this:

• The solid structure breaks down as heat is absorbed.
• The particles start moving more freely, turning the solid into a liquid.

An interesting thing about the melting point is that even if you continue to add heat, the temperature will not rise until all of the solid has melted. This is because all energy is used in shifting phase rather than increasing temperature.

Boiling Point

The boiling point marks the temperature at which a liquid becomes a gas. For iodine, this occurs at 184.4°C. At this point, the energy input into the liquid is enough to allow the liquid particles to break free from each other and enter a gaseous state.
Key points to note about the boiling point include:

• The added heat causes the molecules to move faster.
• Molecules gain enough energy to escape the liquid's surface.

During boiling, bubbles of vapor form throughout the liquid, which rise to the surface and release gas. Like melting, while iodine boils, its temperature remains constant until all the liquid has converted into gas.

Iodine Phase Transition

Iodine, like other substances, undergoes phase transitions under certain conditions. Knowing its melting and boiling points helps us predict these changes. When iodine reaches its melting point of 113.7°C, it begins to melt, transitioning from a solid to a liquid as it absorbs heat. Conversely, when its temperature is lowered to between this melting point and its boiling point (184.4°C), iodine remains in a liquid state. It won't boil until it reaches 184.4°C.
Here's why understanding iodine's phase transitions is important:

• Knowing these points helps in controlling its application in various scientific processes.
• Predicting its behavior under temperature changes is essential for practical uses.

In essence, understanding phase changes allows us to better utilize iodine in real-world settings while avoiding unwanted phase transitions during applications.