Question

You are high up in the mountains and boil water to make some tea. However, when you drink your tea, it is not as hot as it should be. You try again and again, but the water is just not hot enough to make a hot cup of tea. Which is the best explanation for this result? (a) High in the mountains, it is probably very dry, and so the water is rapidly evaporating from your cup and cooling it. (b) High in the mountains, it is probably very windy, and so the water is rapidly evaporating from your cup and cooling it. (c) High in the mountains, the air pressure is significantly less than \(101,3 \mathrm{kPa}\), so the boiling point of water is much lower than at sea level. (d) High in the mountains, the air pressure is significantly less than \(101.3 \mathrm{kPa}\), so the boiling point of water is much higher than at sea level.

Short answer

The correct explanation is (c): lower air pressure at high altitudes decreases the boiling point of water.

Step-by-step solution

Understand the Effect of Altitude on Air Pressure

At high altitudes, the atmospheric pressure is lower than at sea level. This reduced pressure affects various physical properties and reactions, including the boiling point of water.

Analyze Boiling Point Change with Pressure

Under lower atmospheric pressure, the boiling point of liquids decreases. This means that water will boil at a temperature lower than 100°C when at high altitudes.

Apply the Effect on Boiling Water to the Scenario

Since the boiling point of water is lower due to the lower pressure at high altitudes, water will turn into steam at a lower temperature than at sea level. Consequently, when water boils, it does not reach the usual high temperature, resulting in less hot tea.

Review the Answer Choices

Examining the options, choices (a) and (b) refer to water cooling due to evaporation, which is a factor but not the primary reason related to altitude. Choices (c) and (d) directly involve air pressure: (c) correctly states that lower air pressure decreases the boiling point, whereas (d) incorrectly states that it increases it.

Choose the Correct Explanation

The best explanation for the water not boiling hot enough is choice (c): at high altitudes, the air pressure is significantly less than 101.3 kPa, which lowers the boiling point of water, preventing it from reaching the usual high temperature.

Key concepts

Altitude and Its Effect on Boiling Water

When you are high up in the mountains, or at high altitude, you may notice that things behave a bit differently than at sea level. One key difference involves boiling water. The reason for this lies in how altitude affects atmospheric pressure. As you go higher above sea level, the weight of the air above you decreases. This means there is less air pressure pressing down on your pot or kettle. Lower air pressure leads to changes in boiling points. Since the pressure doesn't push down as much, it becomes easier for water molecules to escape into the air as they turn into steam. This means water boils at a lower temperature than the usual 100°C (212°F). So, while you may see bubbles and steam, the water isn’t actually as hot as it would be at lower altitudes.

How Air Pressure Influences Boiling Points

Air pressure is the force exerted by the weight of air in the atmosphere. This pressure affects many physical reactions, including how temperatures influence changes in state, like boiling. At sea level, air pressure is approximately 101,3 kPa. There, water boils at 100°C because the pressure is just enough to allow water molecules to escape into the air as steam. However, when you move to high altitudes, the atmospheric pressure drops, and this directly impacts the boiling point:

• The air pressure is lower, therefore, water molecules need less energy to escape as steam.
• Water boils at a temperature below 100°C, which can be surprising if unprepared.
• This can result in cooking times being longer or drinks not being as hot as expected.

This is why you might find your tea not hot enough if you've only let it come to a rolling boil without accounting for these changes.

Understanding Evaporation and Its Role in Cooling

While evaporation is not the main reason why water boils at lower temperatures at high altitudes, it's still an important concept to understand in everyday life. Evaporation is the process where liquid water changes into a gas or vapor. This can occur at any temperature, unlike boiling which requires a specific temperature. Some factors that influence evaporation include:

• Temperature: Higher temperatures increase evaporation rates.
• Surface Area: Greater surface area speeds up evaporation.
• Air Movement: Wind or moving air removes vapor, allowing more water to evaporate.

In the mountains, because the air pressure is lower, water can evaporate more quickly. While this contributes to cooling, it mainly affects the process happening after the water has been boiled, not the boiling temperature itself. Therefore, while evaporation can make your tea feel cooler due to faster cooling post-boil, the primary reason for the temperature being lower when it 'boils' is the reduced boiling point.