Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

Pick out the correct statements of the following about liquids? (a) The intermolecular forces of attraction in a liquid are high. (b) All liquids suffer cooling on evaporation. (c) Lower the boiling point of a liquid, greater the vapour pressure at room temperature. (d) At higher altitudes water boils at a higher temperature than at the sea level.

Step-by-Step Solution

Answer

Statements (a), (b), and (c) are correct.

Step 1: Evaluate Statement (a)

For liquids, the intermolecular forces are moderate, which are neither as strong as in solids nor as weak as in gases. Therefore, statement (a) is correct in the context of comparing it to gases.

Step 2: Evaluate Statement (b)

Evaporation is a process where molecules with higher kinetic energy escape the liquid surface, leading to a decrease in average kinetic energy, and causing cooling. Hence, statement (b) is correct.

Step 3: Evaluate Statement (c)

Liquids with lower boiling points usually have higher vapor pressures at room temperature because they require less energy to vaporize. Therefore, statement (c) is correct.

Step 4: Evaluate Statement (d)

At higher altitudes, atmospheric pressure is lower, which means liquids boil at lower temperatures. Thus, statement (d) is incorrect, as it is the opposite.

Key Concepts

Evaporation and Cooling

Evaporation is a fascinating process that occurs when molecules in a liquid gain enough energy to escape into the air as gas. This is not just a simple transition of state; it's also a mechanism that results in cooling. When a molecule at the surface of a liquid escapes, it takes away some thermal energy with it. This reduction in energy decreases the average kinetic energy of the remaining molecules, which effectively lowers the temperature of the liquid.
Evaporation is a key part of how sweat cools our bodies. When you sweat, the liquid on your skin evaporates, leaving your skin feeling cooler. This principle is essential for regulating body temperature and is an everyday example of this process at work.

• Evaporation leads to a reduction in temperature because of the escape of higher-energy molecules.
• This process causes cooling as it lowers the kinetic energy of the remaining liquid molecules.

Understanding evaporation and cooling helps you appreciate why, on a hot day, a splash of water can keep you feeling refreshed.

Boiling Point and Vapor Pressure

The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding atmospheric pressure, causing the liquid to turn into vapor or gas. Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid state. When you heat a liquid, its molecules move faster, increasing the vapor pressure.
Liquids with lower boiling points often have a high vapor pressure at room temperature since they require less energy to disrupt the intermolecular forces holding them in the liquid state. This means such liquids can easily transition to a gaseous state even without applying significant heat.

• Low boiling point indicates a high vapor pressure as less energy is needed to transition to gas.
• Boiling happens when vapor pressure matches atmospheric pressure.

Understanding the relationship between boiling point and vapor pressure is crucial for predicting how a liquid will behave under different temperatures.

Boiling Point and Altitude Effect

Altitude has a fascinating effect on boiling points due to changes in atmospheric pressure. At sea level, where atmospheric pressure is higher, the boiling point of water is 100°C. However, at higher altitudes, atmospheric pressure decreases, causing water to boil at a lower temperature. This is because the vapor pressure required to initiate boiling is reached at a lower temperature when the pressure is lower.
For instance, atop a mountain, you might notice water boiling at around 90°C or even lower. This can affect cooking times and processes, as water won't get as hot as it does at sea level. Thus, food might take longer to cook because the boiling water is at a lower temperature.

• Higher altitudes result in lower atmospheric pressure and lower boiling points for liquids.
• Cooking at high altitudes requires adjustments due to lower boiling points leading to longer cooking times.

Recognizing the impact of altitude on boiling points helps in adjusting recipes and understanding cooking processes at different elevations.