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

(a) On hot, dry days, the air has lower humidity, allowing it to absorb more water molecules from the surface and thus increasing the rate of evaporation compared to hot, humid days where the air already has a high level of water vapor, slowing down evaporation. (b) Water boils at lower temperatures at higher altitudes due to decreased air pressure. To achieve the desired flavor extraction from tea leaves, boiling water at a lower temperature takes longer, resulting in longer boiling times at high altitudes compared to lower altitudes.

Step-by-step solution

(a) Explanation of evaporation on hot, dry and hot, humid days

Evaporation is the process where water molecules change their state from liquid to gas. This occurs when the water gains enough energy to overcome molecular bonds holding them together. It depends on multiple factors like temperature, humidity, and air pressure. On a hot day, the temperature is high, providing more energy to the water molecules to overcome molecular bonds, thus evaporating faster. The key difference between a hot, dry day and a hot, humid day is the degree of humidity. Humidity is the measure of moisture in the air. On hot, dry days, the air has lower humidity, meaning it has less water vapor in it. When there's less water vapor in the air, the air can absorb more water molecules from the surface, and the rate of evaporation increases. On hot, humid days, the air already has a high level of water vapor, which means it can absorb less water from the surface. This results in a slower rate of evaporation in comparison to a hot, dry day.

(b) Explanation of boiling water at high altitudes compared to lower altitudes

Boiling is the process where water reaches a temperature at which it starts converting from liquid to vapor. The boiling point of water depends on the surrounding air pressure. As altitude increases, air pressure decreases because there is less atmospheric pressure on the water from the weight of the air above it. At lower altitudes, air pressure is higher due to greater atmospheric pressure. This means that more energy is required to break the surface tension and allow water to evaporate. At higher altitudes, there is less air pressure, and therefore less energy is needed to break the surface tension. Due to this, water boils at lower temperatures at higher altitudes compared to lower altitudes. So, in order to make tea, the water needs to reach the desired boiling temperature for extraction of tea leaves' flavor and benefits. Boiling water at a lower temperature will take longer to extract the flavors, which means that it takes more time to reach the desired taste when boiling water at high altitudes compared to lower altitudes.