Heat is added to boiling water. Explain why the temperature of the boiling water does not change. What does change?

When you boil water and notice the bubbling and steam, you're witnessing the latent heat of vaporization in action. This concept refers to the amount of energy needed to convert a liquid into a gas without changing its temperature. In simpler terms, it's the energy used to break the molecular attractions in the liquid phase so that the molecules can move freely and form vapor.

For water, this energy is quite significant and that's precisely why the temperature remains constant when you boil water; because all that added heat is being used up in converting water to steam. Imagine pouring your effort into lifting a rock – while you're pulling up, there's no outward result, similar to how the added heat is consumed in the phase change. The value for water at atmospheric pressure is around 2260 kJ/kg, which is substantial, and shows just why we don't see temperature changes during boiling.

A phase change is like a behind-the-scenes costume change in a play, where the actors (molecules in this case) remain the same but appear different to the audience. When heat is added to boiling water, a phase change occurs from liquid to gas. Think of it as a busy highway – as more cars (heat) enter, they don't make the highway (the water's temperature) longer but instead, they enable more cars (water molecules) to exit onto the off-ramp (the gas phase).

A phase change requires that all the energy supplied goes into rearranging the molecular structure, like when solid ice melts to become liquid water, or in our case, when boiling water becomes steam. This process absorbs heat energy consistently and is why we don't record a thermometer spike, but rather, see a steady stream of steam as our visual cue of the ongoing change.

Vapor pressure is an often misunderstood character in the story of boiling water. This pressure is exerted by a vapor in equilibrium with its liquid or solid form. As you heat water, the molecules jostle around and some gain enough energy to escape into the air; this is the source of vapor pressure. When you reach boiling point, the vapor pressure of the water matches the surrounding atmospheric pressure, and this is the grand stage where boiling occurs.

Picturing a pot of water on the stove, as the temperature rises, the excitement within the water increases – molecules move faster, collide more and eject some of their own into the air as vapor. When the vapor pressure is as pushy as the air pressure above, we've hit boiling point, and now, instead of getting hotter, water gets busier turning into steam. If you were to hike up a mountain and boil water, you'd find it boils at a lower temperature because there's less air pressure pushing back, underscoring the intricate balance between vapor pressure and environmental conditions.