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The boiling point of a substance is the temperature at which its liquid form transitions into a gas. This typically occurs when the vapor pressure of the liquid equals the surrounding external pressure. The normal boiling point is a specific boiling point that happens when the pressure is exactly 1 atmosphere (atm).
Understanding the boiling point is crucial because it provides insight into both the physical properties of a substance and the external conditions affecting it.
For any liquid:

• If the external pressure is lower than 1 atm, the boiling point drops below the normal boiling point.
• If the external pressure is higher, the boiling point rises.

This means that the boiling point is directly influenced by changes in pressure.

Pressure is the force exerted by the molecules of a gas or liquid per unit area. It plays a crucial role in determining the boiling point of a substance.
As mentioned earlier, a change in pressure can alter the boiling point:

• Higher pressure usually results in a higher boiling point.
• Lower pressure can make liquids boil at lower temperatures.

This is why water boils at a lower temperature at high altitudes (where atmospheric pressure is lower) than at sea level. Having a clear understanding of pressure is essential when discussing the physical behaviors of substances.

Intermolecular forces are the forces that occur between molecules. They are vital in understanding boiling points because they determine how much energy is needed for a liquid to become a gas.
Stronger intermolecular forces mean more energy (in the form of heat) is required to break these forces. Consequently, the boiling point is higher. Types of intermolecular forces include:

• Dipole-Dipole Attraction — present in polar molecules.
• London Dispersion Forces — momentary attractions present in all molecules, particularly significant in non-polar compounds.
• Hydrogen Bonds — a strong type of dipole-dipole attraction commonly found in molecules like water.

Understanding these forces helps explain why different substances have different boiling points based on their molecular makeup.

Atmospheric pressure is the force exerted by the weight of the air above us. It affects many physical phenomena including the boiling point of liquids.
As atmospheric pressure changes, so does the boiling point. That's why at higher altitudes, where atmospheric pressure is lower, water boils at temperatures less than 100 °C.
The normal boiling point refers specifically to the boiling point at an atmospheric pressure of 1 atm (which is the average sea level pressure). Knowing this stabilizes many calculations and experiments, giving scientists a standard reference point when dealing with boiling points. Consequently, atmospheric pressure is a key factor in understanding how and why boiling points aren't constant in everyday scenarios.