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When we start to heat water, the molecules gain energy and move faster. Eventually, some of these molecules have enough energy to escape the liquid phase and become gas. The pressure exerted by this gas just above the surface of the water is what we call the water vapor pressure. This pressure increases with temperature because more water molecules have the necessary energy to transition into the vapor phase.

At a certain temperature, known as the boiling point, the vapor pressure becomes equal to the atmospheric pressure surrounding the liquid. When this happens, bubbles of water vapor can form within the water, rise and escape into the air. In the context of boiling water, the vapor pressure is crucial in understanding when and how these bubbles form, as described in our exercise where the bubble's formation signifies the water has reached its boiling point.

Air is a mixture of gases, primarily nitrogen and oxygen, that can dissolve in water. The solubility of air in water is influenced by several factors, with temperature being one of the most significant. As water heats up, the kinetic energy of the water molecules increases, leading to the decreased ability of water to keep gases dissolved. This is due to the increased movement of water molecules, which disturbs the air molecules and makes it easier for them to escape.

As we see in the step-by-step solution provided, the solubility of air decreases as water reaches its boiling point. This reduction in solubility contributes to the formation of bubbles primarily made of water vapor, rather than air, since the dissolved air is released from the water before it even starts to boil.

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure. For water at sea level, this is typically 100 degrees Celsius (212 degrees Fahrenheit). When water reaches this temperature, it begins to convert into steam, which is observed as boiling.

In our exercise, the boiling point marks the moment when water vapor bubbles start to form, displacing any air previously dissolved in the water. It's important to comprehend that the boiling point can change with the ambient pressure; for example, at higher altitudes, the atmospheric pressure is lower, which means water will boil at a lower temperature. Understanding this concept helps explain why cooking time adjustments are often required when preparing food in high-altitude locations.