Question

At high altitudes, water boils at a lower temperature because (a) the atmospheric pressure is high at high altitudes (b) the viscosity of water is reduced at high altitudes (c) the atmospheric pressure is low at high altitudes (d) the surface tension of water is reduced at high altitudes.

Short answer

Water boils at a lower temperature at high altitudes because the atmospheric pressure is low, which is option (c).

Step-by-step solution

Understanding the Boiling Process

The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure surrounding the liquid. At higher altitudes, the atmospheric pressure is lower than at sea level.

Identifying the Correct Option

Given that boiling occurs when the vapor pressure of the liquid equals the external pressure, and knowing this balance is achieved at lower temperatures when the external pressure is lower, we can identify the correct option.

Verifying the Option with Physical Principles

By understanding that atmospheric pressure decreases with altitude, we can conclude that water boils at a lower temperature at high altitudes due to the reduction in atmospheric pressure, which is described in option (c).

Key concepts

Vapor Pressure

When we talk about boiling water, or any liquid for that matter, we often mention the term 'vapor pressure.' So, what exactly is vapor pressure? It is the pressure exerted by a vapor in equilibrium with its liquid or solid form. Imagine you have a closed container half filled with water. The water molecules at the surface are always escaping into the air above in a process called evaporation. As some molecules break away, they form vapor. However, evaporation from the liquid also means that there's less water left behind, creating a dynamic balance point where the rate of molecules leaving the liquid equals the rate of the molecules returning. This balance is where the liquid's vapor pressure comes in.

Vapor pressure depends on the temperature; as temperature increases, more molecules have the energy needed to escape, and the vapor pressure increases. This relationship is pivotal because it explains why liquids boil at lower temperatures at high altitudes, which is directly tied to a change in atmospheric pressure.

Atmospheric Pressure

If vapor pressure tells us about the process happening inside the liquid, atmospheric pressure is the external factor that plays a crucial role in the boiling process. Atmospheric pressure is the force exerted by the weight of the air above us. At sea level, this pressure is highest because the entire atmosphere is pressing down on you. But as you climb a mountain or fly in an airplane, the amount of air above you decreases, leading to a decrease in atmospheric pressure.

Why does this matter for boiling water? Well, the boiling point of any liquid is not a fixed temperature but rather the point where its vapor pressure is equal to the external atmospheric pressure. So, when the external pressure drops—like at higher altitudes—you don't need to heat the water as much for its vapor pressure to equal the atmospheric pressure, resulting in water boiling at a lower temperature. This is critical for understanding why cooking times vary with altitude and why pressure cookers are so effective.

Boiling Process

Finally, let's delve into the boiling process itself. Boiling occurs when a liquid reaches a temperature where its vapor pressure equals the surrounding atmospheric pressure. At this temperature, bubbles of vapor form within the liquid and rise to the surface to burst, a process we see as boiling. Since vapor pressure is determined by temperature but the boiling point is also influenced by atmospheric pressure, any change in the latter affects the boiling point.

At high altitudes, because the atmospheric pressure is lower, water reaches that pivotal vapor pressure at a lower temperature, and thus, it starts boiling. This is why mountaineers and hikers must adjust their cooking times when preparing food. The decreased boiling temperature can also affect the taste and texture of foods since the lower temperature might not be sufficient to cook food properly, necessitating adjustments or special equipment for proper food preparation.