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.

Short answer

On a hot, dry day, water evaporates more quickly due to higher temperature and low humidity, both of which contribute to higher evaporation rates. Conversely, on a hot, humid day, the high humidity slows down the evaporation rate. Cooking an egg takes longer at high altitudes because the boiling point of water decreases as atmospheric pressure decreases, leading to a lower cooking temperature and increased cooking time compared to lower altitudes.

Step-by-step solution

(a) Water evaporation on hot, dry days and hot, humid days

To understand the difference between water evaporation on hot, dry days and hot, humid days, we need to consider how both temperature and humidity affect the rate of evaporation. 1. Effect of temperature: As the temperature increases, the water molecules gain more energy and can escape from the surface more easily, increasing the rate of evaporation. 2. Effect of humidity: Humidity is the amount of moisture present in the air. When the air has a high humidity, it cannot absorb as much water vapor as dry air. As a result, the water molecules in the air have less chance of escaping, and the evaporation rate decreases. So, on a hot, dry day, both the temperature and the low humidity contribute to a higher evaporation rate. In contrast, on a hot, humid day, even though the temperature is high, the high humidity slows down the evaporation rate.

(b) Cooking an egg in boiling water at high altitudes vs lower altitudes

This observation can be explained by understanding the effect of atmospheric pressure on the boiling point of water and the subsequent effect on cooking time. 1. Atmospheric pressure: As the altitude increases, the atmospheric pressure decreases. Atmospheric pressure is essentially the weight of the air above a location. At higher altitudes, there is less air above a location, and the pressure exerted by the atmosphere decreases. 2. Boiling point of water: The boiling point of water is the temperature at which it turns from a liquid to a gas. This phase transition happens when the vapor pressure of water equals the surrounding atmospheric pressure. As the atmospheric pressure decreases at higher altitudes, the boiling point of water also decreases. 3. Cooking time: Cooking an egg involves denaturing its proteins and solidifying them. This process is temperature-dependent, with higher temperatures leading to faster cooking. Since the boiling point of water is lower at higher altitudes, it takes longer to cook an egg in the reduced temperature boiling water at high altitudes compared to lower altitudes, where the boiling point is higher and the egg is cooked at a higher temperature.

Key concepts

Rate of Evaporation

Understanding the rate of evaporation is essential when examining the behavior of liquids in different conditions. Evaporation is the process by which water transitions from a liquid to a gaseous state. Factors influencing this rate include temperature, surface area, air movement, and the presence of other substances in the liquid.

The key to grasping this concept is to realize that for evaporation to occur, molecules at the surface of the liquid need sufficient energy to break free from the liquid’s surface tension. Higher temperatures provide this energy, increasing the rate at which molecules transition to the gas phase. Conversely, the presence of other substances like salts or sugars in the water can reduce the rate of evaporation by increasing the boiling point and decreasing the vapor pressure of the liquid.

Wind or air movement accelerates evaporation by moving the air that has already absorbed moisture away from the liquid surface, allowing more water vapor to enter the atmosphere. Lastly, an increased surface area also speeds up evaporation since more molecules are exposed to potential escape routes.

Humidity and Evaporation

Humidity plays a pivotal role in the evaporation process. It refers to the amount of water vapor present in the air. When we talk about high humidity, we're referring to air that has absorbed a lot of moisture; this air is less willing to take on additional water vapor.

To visualize this, imagine a sponge soaked with water—just as the sponge cannot absorb more liquid, air with high humidity cannot effectively accept more water vapor. As a result, on hot, humid days, when the air is already loaded with moisture, the rate of evaporation decreases because the water vapor has nowhere to go. This explains why sweat does not evaporate as quickly on a humid day, making us feel warmer. On the other hand, on hot, dry days, the air has a lower moisture content, meaning it can absorb more water vapor, leading to a higher evaporation rate and making us feel cooler as sweat evaporates more efficiently.

Atmospheric Pressure and Boiling Point

Atmospheric pressure is a key factor in determining the boiling point of water. Boiling occurs when the vapor pressure of the liquid matches the pressure of the surrounding environment, allowing bubbles of vapor to form within the liquid. At sea level, this happens at 100 degrees Celsius as the atmospheric pressure is about 101.3 kilopascals (kPa).

However, as you ascend in altitude, the pressure of the overlying atmosphere decreases. This means that at higher altitudes, water can reach that pivotal vapor pressure threshold at a lower temperature. That's why water boils at temperatures lower than 100 degrees Celsius on mountains and in high-altitude regions. This phenomenon has practical implications in cooking and industrial processes, as changes in boiling point can affect cooking times and the efficiency of heat-dependent procedures.

Cooking at High Altitudes

If you've ever tried to cook or bake at high altitudes, you might have noticed recipes don't always turn out as expected. This is not just because of drier air but also because the decrease in atmospheric pressure affects cooking processes. As mentioned earlier, water boils at a lower temperature at higher altitudes due to the reduced atmospheric pressure.

This means that foods that are boiled or steamed, like eggs or pasta, will cook at a lower temperature than they would at sea level, resulting in longer cooking times. To compensate for this, you might need to adjust cooking times or temperatures when preparing certain dishes. In some cases, adding pressure with a pressure cooker can help reach higher temperatures than the reduced boiling point, thus restoring typical cooking conditions. These adjustments ensure that foods are cooked thoroughly and reach the desired texture and flavor, even amidst the challenges of a high-altitude environment.