Problem 53 - Introductory Chemistry Atoms First

Question

When you eat sugar \(\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)\), it is eventually converted into \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\), which you exhale. If you exhale onto a cold window, the window fogs up. (a) What chemical change is being mentioned here? (b) What physical change is being mentioned here? (c) \(\mathrm{CO}_{2}\) is a solid that sublimes at \(-78^{\circ} \mathrm{C}\). Temperatures on planet Earth almost never get this cold. What does that tell you about the fog on the window?

Step-by-Step Solution

Answer

(a) The chemical change mentioned is the conversion of sugar (C12H22O11) into carbon dioxide (CO2) and water (H2O) due to metabolism and cellular respiration. (b) The physical change mentioned is the fogging up of the cold window due to the condensation of water vapor in the exhaled breath, forming tiny water droplets on the window. (c) The fog on the window is primarily formed by the condensation of water vapor (H2O) in the exhaled breath, not the sublimation of CO2, since Earth's temperatures rarely reach -78°C.

Step 1: (a) Identifying chemical change

The chemical change mentioned here is the conversion of sugar (C12H22O11) into carbon dioxide (CO2) and water (H2O). This occurs in the body as a result of metabolism and cellular respiration, wherein the body breaks down sugar molecules in order to release energy.

Step 2: (b) Identifying physical change

The physical change being mentioned is the fogging up of the cold window when exhaled breath, which contains CO2 and H2O, comes into contact with it. The warm, moist breath cools down when it meets the cold surface, causing the water vapor in the breath to condense into tiny water droplets that form fog on the window.

Step 3: (c) Understanding the relationship between CO2 and fog on the window

CO2 is a solid that sublimes at -78°C, meaning that it turns directly from a solid to a gas without passing through the liquid state. The fog on the cold window is not due to the presence of solid CO2, since temperatures on Earth rarely reach -78°C. Instead, the fog is primarily formed by the condensation of water vapor (H2O) present in the exhaled breath. This tells us that the fog on the window is mainly due to the physical change of water vapor into liquid droplets, not the sublimation of CO2.

Key Concepts

Physical Change

A physical change is a transformation that alters the appearance or physical state of a substance without changing its chemical identity. It is like reshaping clay; the clay's form changes, but it remains the same material. One common example of a physical change is when you exhale warm, moist air onto a cold window, causing the window to fog up. In this process, the water vapor in the breath cools down and changes to tiny droplets.
This process is called condensation, where gas (water vapor) changes to liquid (water droplets).

• Condensation is a good example of a physical change because only the state of water changes, not its chemical composition.
• Physical changes are usually reversible, meaning they can often be undone by reversing the conditions that caused them. For instance, warming the window can lead the liquid droplets to evaporate back into vapor.

Metabolism

Metabolism is a chemical process occurring within our bodies that converts what we eat and drink into energy. It's a bit like a complex factory where raw materials—like sugar—are broken down and used for energy, growth, and repair. When you consume sugar, it undergoes metabolism.
This process involves breaking down the sugar to release energy and produce carbon dioxide ( CO ₂ ) and water ( H ₂ O ) as by-products. Some key points about metabolism:

• It is vital for maintaining life, supporting growth and bodily functions.
• Metabolic reactions include both breaking down food molecules to produce energy and building up new molecules for body repairs.

Without metabolic processes, the nutrients from our diet would not be converted into the energy and materials needed for survival.

Condensation

Condensation is a process where a gas changes into a liquid. It often occurs when warm, moist air hits a cooler surface. When you exhale, your breath is warm and full of moisture. When this breath meets a cold window, the water vapor cools, transforming into tiny droplets, forming a misty layer.
Condensation is important in many everyday phenomena, such as:

• The formation of dew on grass in the morning.
• Fogging up of eyeglasses when entering a warm room from the cold outside.

The key aspect of condensation is that it is a physical change, not altering the chemical properties of water.

This process underlines why the fog on a window formed when exhaled breath hits it, as it results from the condensation of water vapor, not any other factors related to CO ₂ .

Sublimation

Sublimation is an intriguing process where a solid transforms directly into a gas, skipping the liquid phase entirely. This is unusual as most substances typically go through a liquid stage before becoming gas. An everyday example of sublimation is dry ice, solid carbon dioxide ( CO ₂ ), which sublimates at -78^{ ext{°C}} .
On Earth, temperatures rarely drop to such extremes, hence we usually encounter CO ₂ as a gas, even when it's cold outside. Key points about sublimation include:

• It is not a typical process in everyday life because it requires specific conditions, like very low temperatures.
• Sublimation is a physical change, where the substance's state changes without altering its chemical structure.

Understanding sublimation helps explain why cold windows fog up; it's due to water vapor condensation, not the sublimation of solid CO ₂ .