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

(a), (b), and (c) are correct; (d) is incorrect.

Step 1: Understanding (a)

Intermolecular forces in liquids are generally weaker than in solids but stronger than in gases. Liquids have high enough forces to maintain volume but allow particles to move freely.

Step 2: Analyzing (b)

Evaporation is a cooling process because molecules with higher kinetic energy escape, lowering the energy of the remaining liquid and resulting in cooling.

Step 3: Examining (c)

A lower boiling point implies a liquid has weaker intermolecular forces, and typically a greater vapor pressure at room temperature as the molecules can escape more easily into the gaseous state.

Step 4: Evaluating (d)

At higher altitudes, atmospheric pressure is lower, which decreases the boiling point, causing water to boil at lower temperatures, not higher.

Key Concepts

Intermolecular Forces in Liquids

Intermolecular forces are the interactions that occur between molecules. In liquids, these forces are stronger than in gases but generally weaker than in solids. This is because, in solids, molecules are tightly packed in a fixed structure, while in gases, they are far apart and moving rapidly. Liquids exist in the middle ground; they have enough intermolecular forces to maintain a definite volume but not a definite shape.

• Van der Waals Forces: These are the weakest intermolecular forces and include London dispersion forces. These play a significant role in non-polar molecules.
• Dipole-Dipole Interactions: These occur in polar molecules where positive and negative ends attract each other.
• Hydrogen Bonds: A special type of dipole-dipole interaction that occurs between hydrogen and electronegative atoms like oxygen, nitrogen, or fluorine. It is much stronger than other types of dipole-dipole interactions.

These forces dictate many of the liquid's properties, such as surface tension, viscosity, and how they mix or interact with other substances.

Evaporation and Cooling

Evaporation is an important process where molecules at the surface of a liquid gain enough energy to escape into a gaseous state. This occurs at any temperature, not just at the boiling point.

• Energy Escape: The molecules with the highest kinetic energy are the ones that escape first. This leaves behind molecules with lower average energy.
• Cooling Effect: Since the most energetic molecules leave, the average energy of the remaining liquid decreases, effectively lowering its temperature. This is why evaporation has a cooling effect.
• Surrounding Environment: The rate of evaporation can vary with environmental conditions like humidity and temperature. Low humidity and high temperatures generally increase evaporation rates.

Cooling on evaporation is a common observation, such as when sweating cools down your body.

Boiling Point and Vapor Pressure

Boiling point and vapor pressure are two related concepts crucial for understanding liquids. When a liquid is heated, its molecules gain kinetic energy.

• Boiling Point: This is the temperature at which the vapor pressure of the liquid equals the surrounding pressure, causing the liquid to turn into vapor.
• Vapor Pressure: Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase. A liquid with a high vapor pressure at room temperature will generally have a lower boiling point.
• Intermolecular Forces: Liquids with strong intermolecular forces have lower vapor pressures and higher boiling points due to the difficulty of molecules escaping the liquid form. Conversely, weaker intermolecular forces contribute to higher vapor pressures and lower boiling points.

Understanding these concepts helps explain why cooking involves different times and techniques based on conditions and types of liquids.

Altitude and Boiling Point Changes

Altitude plays a significant role in the boiling point of liquids, especially water. At higher altitudes, atmospheric pressure is decreased.

• Atmospheric Pressure: This is the weight of the air above a given point. It decreases as altitude increases.
• Boiling Point Decrease: At lower atmospheric pressures found at higher altitudes, liquids boil at lower temperatures. This is because vapor pressure can more easily match the lower atmospheric pressure.
• Practical Implications: Cooking and baking time may need adjustment in high-altitude areas. Liquids boil at lower temperatures, potentially affecting the cooking process.

These changes mean that living in or visiting high-altitude areas involves adaptations, particularly in the kitchen.