Problem 54 - Chemistry: The Central Science

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 cook an egg in boiling water at high altitudes than it does at lower altitudes.

Step-by-Step Solution

Answer

On a hot, dry day, water evaporates more quickly because the air has a higher capacity to hold water vapor due to the low humidity, enhancing evaporation. In contrast, a hot, humid day has a slower evaporation rate because the air is already saturated with water vapor. For cooking an egg in boiling water, high altitudes have lower air pressure and thus a lower boiling point for water, which means the temperature may not be high enough to cook the egg efficiently, taking longer to cook compared to lower altitudes where water boils at higher temperatures.

Step 1: Understanding evaporation and humidity

Evaporation is the conversion of a liquid (like water) into vapor, which then diffuses into the surrounding air. The rate of evaporation is influenced by external factors like temperature and humidity. Higher temperature increases the rate of evaporation, while higher humidity (i.e. more water vapor in the air) decreases the rate of evaporation, as the air is already saturated with water vapor and can't hold much more.

Step 2: Observation (a) explanation: Hot, dry vs. hot, humid day

On a hot, dry day, the air temperature is high and the relative humidity is low. The high temperature provides more energy to the water molecules, which results in increased evaporation. Meanwhile, the low humidity means that the air can hold more water vapor, allowing water to evaporate more quickly. On the other hand, on a hot, humid day, the air temperature is still high, but the relative humidity is also high. The higher humidity means that the air is already saturated with water vapor, which slows down the evaporation process. Therefore, water evaporates more slowly on a hot, humid day compared to a hot, dry day.

Step 3: Understanding boiling point and air pressure

The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure. At this point, the liquid turns into a gas. The boiling point of a liquid, such as water, varies with the external pressure. Generally, when the pressure is reduced, the boiling point of water also decreases.

Step 4: Understanding the effect of altitude on air pressure and boiling point

At higher altitudes, the air pressure is lower compared to a lower altitude. This is because there is less air above pushing down on you. As the pressure decreases with altitude, the boiling point of water also decreases. This means that at high altitudes, water will boil at temperatures lower than 100°C (212°F).

Step 5: Observation (b) explanation: Cooking an egg at high vs. low altitudes

When cooking an egg in boiling water, the proteins in the egg coagulate. The coagulation temperature for egg proteins is typically around 60-70°C (140-158°F). At lower altitudes, water boils at around 100°C (212°F), so there is sufficient heat to cook the egg efficiently. However, at higher altitudes, as the boiling point of water decreases because of the lower air pressure, the boiling water temperature is also lower. The temperature may not be high enough to cook the egg as quickly or efficiently as at lower altitudes. As a result, it takes longer to cook an egg in boiling water at high altitudes than at lower altitudes.