What Is Vapor Pressure? — Reading Comprehension

Have you ever seen water slowly disappear from a cup left out for days? That’s not magic—it’s science! Even without boiling, water can turn into vapor. This process is called evaporation, and it's closely linked to something called vapor pressure.

Vapor pressure is the pressure caused by a liquid's vapor when the liquid and gas are in balance. When a liquid evaporates, its molecules escape into the air. These gas molecules bump into things around them, creating pressure. The more molecules that evaporate, the higher the vapor pressure becomes. If a liquid is in a closed container, the vapor collects above the liquid, and the pressure it causes is the vapor pressure.

Different liquids have different vapor pressures. For example, alcohol has a higher vapor pressure than water at the same temperature. That means alcohol evaporates faster. Liquids with high vapor pressure are usually more "volatile," meaning they change into gas easily. Temperature affects vapor pressure too. As heat increases, molecules move faster and more of them escape into gas form—so vapor pressure rises.

Vapor pressure also helps explain how boiling works. A liquid boils when its vapor pressure equals the air pressure around it. At sea level, water boils at 100°C (212°F) because its vapor pressure matches the outside air pressure at that temperature. But on a mountain, where the air pressure is lower, water boils at a lower temperature.

Fun Fact: In space, where there’s almost no air pressure, your sweat would boil right off your skin—even at body temperature!

What is vapor pressure?

What happens when molecules in a liquid evaporate?

What liquid has a higher vapor pressure than water at the same temperature?

What happens to vapor pressure as temperature increases?

What causes a liquid to boil?

Why does water boil at a lower temperature on a mountain?

What is the main idea of the passage?

A student seals a liquid in a jar and heats it. What will likely happen?