Question

Explain the following observations: (a) Water evaporates more quickly on a hot, dry day than on a hot, humid day. (b) It takes longer to boil water for tea at high altitudes than at lower altitudes.

Short answer

Water evaporates more quickly on a hot, dry day compared to a hot, humid day due to the difference in humidity. Higher temperature accelerates evaporation, but high humidity hinders the process, as saturated air has a reduced ability to absorb more water vapor. Boiling water takes longer at high altitudes because the boiling point of water is lower at higher altitudes due to decreased atmospheric pressure. Consequently, it takes more time for the water to reach the desired temperature for making tea compared to lower altitudes where water boils at a higher temperature.

Step-by-step solution

(Observation 1: Evaporation on hot, dry, and humid days)

To understand why the evaporation is different on hot, dry days compared to hot, humid days, we first need to take a look at the factors that influence evaporation. Evaporation is directly related to temperature and humidity.

(Step 1: Temperature and evaporation)

As temperature increases, the kinetic energy of water molecules also increases. When water molecules have enough energy, they escape from the surface of the liquid, turning into water vapor, and this is the process of evaporation. Therefore, a higher temperature accelerates the evaporation process.

(Step 2: Humidity and evaporation)

Humidity measures the amount of water vapor currently in the air, which can either absorb more water vapor or is already saturated. High humidity implies the air is nearly saturated with water vapor and has little capacity to hold additional moisture. Evaporation slows down under high humidity conditions because the air has a reduced ability to absorb more water vapor.

(Step 3: Comparing evaporation on hot, dry, and humid days)

On a hot day, the temperature is high, which favors an increased evaporation rate. However, on a hot, humid day, the high humidity hinders the evaporation process due to the saturated air, thus making the evaporation slower compared to a hot, dry day.

(Observation 2: Boiling water at high and low altitudes)

To understand why boiling water takes longer at high altitudes, we have to look at the relationship between boiling point and atmospheric pressure.

(Step 1: Pressure and boiling point)

The boiling point of water depends on the atmospheric pressure acting on its surface. At sea level, the atmospheric pressure is sufficiently high enough to make the water boil at 100°C (212°F). As altitude increases, atmospheric pressure decreases, which means the pressure acting on the water surface is lower at higher altitudes.

(Step 2: Boiling process at high altitudes)

At high altitudes, where the atmospheric pressure is lower, water starts to boil at a lower temperature than at sea level. However, since the boiling temperature is lower, it takes longer for the water to absorb enough heat energy to reach the desired temperature for making tea, which is generally around 100°C (212°F).

(Step 3: Comparing boiling times at high and low altitudes)

It takes longer to boil water for tea at high altitudes because the boiling point of water is lower due to the decreased atmospheric pressure. As a result, it takes more time for the water to reach the desired temperature for making tea, compared to boiling water at lower altitudes where the atmospheric pressure is higher and water boils at a higher temperature.

Key concepts

Humidity and Evaporation

The process of evaporation is greatly influenced by humidity. Humidity refers to the amount of water vapor present in the air. When the air has high humidity, it means it is already filled with moisture and cannot absorb much more.

On a hot, humid day, the air is almost saturated with water vapor, so the evaporation from water bodies slows down. The air has limited capacity to hold additional water vapor, making it harder for water molecules to escape into the air.

In contrast, on a hot, dry day, the air has low humidity, meaning it can absorb more water vapor freely. With less water vapor present, evaporation speeds up because there is ample capacity in the surrounding air to accommodate more water vapor. Therefore, water evaporates more quickly in dry conditions compared to humid ones.

• High Humidity = Slower Evaporation
• Low Humidity = Faster Evaporation

Boiling Point and Altitude

Boiling water at different altitudes reveals how atmospheric pressure affects the boiling point. Atmospheric pressure decreases as altitude increases. This change impacts the boiling point of water significantly.

At sea level, atmospheric pressure is relatively higher, allowing water to reach its boiling point at 100°C (212°F). At this pressure, the water molecules have enough energy to escape and turn into gas.

When you are at higher altitudes, atmospheric pressure is lower. This reduced pressure means water doesn't need to reach such a high temperature to overcome the atmosphere's push against it, causing water to boil at a lower temperature than at sea level.

For instance, water might start boiling at 95°C (203°F) instead of 100°C (212°F). However, even though water boils at a lower temperature, it takes longer to reach a suitable temperature for cooking or making tea.

• Higher Altitude = Lower Boiling Point
• Lower Altitude = Higher Boiling Point

Temperature and Evaporation

Temperature plays a crucial role in the evaporation process. When the temperature rises, the kinetic energy of water molecules increases. Higher kinetic energy means the molecules are moving faster and are more likely to escape from the surface of the liquid, converting into water vapor.

So, on a hot day, the increased temperature speeds up evaporation because more water molecules gain enough energy to break free from the liquid's surface and become vapor.

Conversely, cooler temperatures slow down this process since the water molecules have less energy and are less likely to leave the liquid form.

• Higher Temperature = Faster Evaporation
• Lower Temperature = Slower Evaporation

This relationship between temperature and evaporation explains why water evaporates quicker on a warm sunny day compared to a cooler cloudy day.