Question

Water boils at lower temperature on high altitudes because (a) atmospheric pressure is low there (b) atmospheric pressure is high there (c) water is weakly hydrogen bonded there (d) water in pure form is found there

Short answer

The correct answer is (a) atmospheric pressure is low there, which is why water boils at a lower temperature at higher altitudes.

Step-by-step solution

Understanding the Concept

The boiling point of any liquid is the temperature at which its vapour pressure equals the ambient pressure. Therefore, atmospheric pressure plays a crucial role in determining the boiling point of liquid.

Analyzing the Options

Out of the given options, two refer to atmospheric pressure (options a and b) and are distinct from each other. As per the rule stated in the first step, if the atmospheric pressure decreases, the boiling point also decreases. Thus, we need to consider the option where the atmospheric pressure is low, leading to a lower boiling point.

Final Selection

Upon analyzing the options using our understanding from step 2, it becomes clear that the accurate answer is option a - atmospheric pressure is low on high altitudes and hence water boils at a lower temperature.

Key concepts

Atmospheric Pressure

The weight of air above us exerts pressure on everything it touches - this is known as atmospheric pressure. It’s a fundamental concept in understanding the environment around us, and plays a pivotal role in weather patterns, human physiology, and even the boiling points of liquids. Imagine a column of air from the top of the atmosphere to the surface of the Earth; atmospheric pressure is the force exerted by this air column on a unit area.

At sea level, atmospheric pressure is approximately 101.325 kPa (kilopascals), considered the standard. However, this pressure decreases with altitude. At higher elevations, the column of air is shorter, thus exerting less pressure. In relation to boiling point, this means that at higher altitudes, liquids boil at lower temperatures because the pressure required to equalize with the atmospheric pressure is achieved more quickly.

Taking the exercise improvement advice into account, this concept is the reason why option (a) from the textbook question is correct. The atmospheric pressure being low at high altitudes leads to a lower boiling point for water.

Vapour Pressure

Vapour pressure is an often misunderstood but essential concept tied to the physical properties of liquids. It’s the pressure created by the vapor of a liquid or solid, that is, it's the force exerted by a gas above its liquid or solid form in a closed container. Vapour pressure is determined by the substance’s temperature; as the temperature increases, the vapour pressure increases because more molecules have sufficient kinetic energy to escape the liquid’s surface and enter the gaseous phase.

This ties directly to the boiling point, which is the temperature at which the vapour pressure of the liquid equals the atmospheric pressure. With the increase in altitude, the surrounding pressure decreases, so the vapour pressure required to reach boiling point is achieved at lower temperatures. This expounds why water can boil at temperatures less than 100°C on a mountain peak, compared to at sea level.

Hydrogen Bonding

Hydrogen bonds are the magnetic-like forces of attraction between the hydrogen atom of one molecule and an electronegative atom, such as oxygen or nitrogen, of another molecule. These are relatively weak compared to covalent or ionic bonds but are essential in determining the physical properties of many substances, including water.

In water, hydrogen bonds form between the hydrogen of one water molecule and the oxygen of another. These bonds give water its high boiling point compared to other molecules of similar size because these intermolecular bonds require more energy to break.

When we talk about boiling, these hydrogen bonds must be overcome for the water molecules to transition from liquid to gas, hence the energy input (heat) is used to break these bonds before changing the state. The exercise implied that hydrogen bonding could be a factor (option c); however, the strength of these bonds does not change significantly with altitude, so it does not directly affect the boiling point in the context of the question.