boiling

all steamed up

Steam actually consists of tiny droplets of liquid water. What you cant see in the picture is the water vapor that is also present in the air above the spring. Water vapor is water in the gaseous state. It constantly rises up from the surface of boiling hot water. Why? At high temperatures, particles of a liquid gain enough energy to completely overcome the force of attraction between them, so they change to a gas. The gas forms bubbles that rise to the surface of the liquid because gas is less dense than liquid. The bubbling up of the liquid is called boiling. When the bubbles reach the surface, the gas escapes into the air. The entire process in which a liquid boils and changes to a gas that escapes into the air is called vaporization. Q: Why does steam form over the hot spring pictured above? A: Steam forms when some of the water vapor from the boiling water cools in the air and condenses to form droplets of liquid water.

boiling point

The temperature at which a substance boils and changes to a gas is called its boiling point. Boiling point is a physical property of matter. The boiling point of pure water is 100 C. Other substances may have higher or lower boiling points. Several examples are listed in the Table 1.1. Pure water is included in the table for comparison. Substance Hydrogen Nitrogen Carbon dioxide Ammonia Pure water Salty ocean water Petroleum Olive oil Sodium chloride Boiling Point ( C) -253 -196 -79 -36 100 101 210 300 1413 Q: Assume you want to get the salt (sodium chloride) out of salt water. Based on information in the table, how could you do it? A: You could heat the salt water to 101 C. The water would boil and vaporize but the salt would not. Instead, the salt would be left behind as solid particles. Q: Oxygen is a gas at room temperature (20 C). What does this tell you about its boiling point? A: The boiling point of oxygen must be lower than 20 C. Otherwise, it would be a liquid at room temperature.

vaporization vs. evaporation

Vaporization is easily confused with evaporation, but the two processes are not the same. Evaporation also changes a liquid to a gas, but it doesnt involve boiling. Instead, evaporation occurs when particles at the surface of a liquid gain enough energy to escape into the air. This happens without the liquid becoming hot enough to boil.

instructional diagrams

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questions

a big pot of water is boiling on a stove. the air in the kitchen is steamy. what process is the direct cause of steam forming in the air?

a) vaporization

b) evaporation

-->  c) condensation

d) melting

vaporization is the same process as evaporation.

a. true

-->  b. false

a liquid must reach the boiling point to evaporate.

a. true

-->  b. false

gas bubbles rise to the surface of a boiling liquid because they are hotter than the liquid.

a. true

-->  b. false

which of the following substances has the highest boiling point?

a) hydrogen

b) nitrogen

c) water

-->  d) aluminum

diagram questions

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