The Liquid Phase Help

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By — McGraw-Hill Professional
Updated on Sep 5, 2011


In the liquid state or phase, a substance has two properties that distinguish it from the solid phase. First, a liquid changes shape so that it conforms to the inside boundaries of any container in which it is placed. Second, a liquid placed in an open container (such as a jar or bucket) flows to the bottom of the container and develops a defined, flat surface. At least this is the way a sample of liquid behaves in an environment where there is gravity.

Diffusion Of Liquids

Imagine a jar on board a space ship in which the environment is weightless (there is no acceleration force). Suppose that the jar is filled with liquid, and then another liquid that does not react chemically with the first liquid is introduced into the jar. Gradually, the two liquids blend together until the mixture is uniform throughout the jar. This blending process is called diffusion .

In a liquid, diffusion takes place rather slowly; some liquids undergo the process faster than others. Alcohol diffuses into water at room temperature much more quickly than heavy motor oil into light motor oil. Eventually, however, when any two liquids are mixed (as long as they don’t react chemically, as do an acid and a base), the mixture will become uniform throughout any container of finite size. This happens without the need for shaking the container because the molecules of a liquid are always in motion, and this motion literally causes them to push and jostle each other until they become uniformly mixed.

If the same experiment is conducted in a bucket on Earth where there is acceleration force produced by gravity, diffusion will occur, but “heavier” liquids will sink toward the bottom and “lighter” liquids will rise toward the surface. Alcohol, for example, will float on water. However, the “surface” between the alcohol and water will not be sharply defined, as is the surface between the water and the air. The motion of the molecules constantly tries to mix the two liquids. However, gravitation prevents the mixture from becoming uniform throughout the bucket unless the two liquids are of exactly the same density. We’ll talk about the meaning of density for liquids shortly.

Viscosity Of Liquids

Some liquids are “runnier” than others. You know there is a difference at room temperature between, say, water and thick molasses. If you fill a glass with water and another glass with an equal amount of molasses and then pour the contents of both glasses into the sink, the glass containing the water will empty much faster. The molasses is said to have higher viscosity than the water at room temperature. On an extremely hot day, the difference is less obvious than it is on a cold day, unless, of course, you have air conditioning that keeps the air in your house at the same temperature all the time.

Some liquids are far more viscous even than thick molasses. An example of a liquid with extremely high viscosity is hot tar as it is poured to make the surface of a new highway. Another example is warm petroleum jelly. These substances meet the criteria as defined above to qualify as liquids, but they are thick indeed. As the temperature goes down, these substances become less and less liquid-like and more solid-like. In fact, it’s impossible to draw an exact line between the liquid and the solid phases for either of these two substances. They aren’t like water; they don’t freeze into ice and change state in an obvious way. As hot tar cools, where do we draw the line? How can we say, “Now, this stuff is liquid,” and then 1 second later say, “Now, this stuff is solid,” and be sure of the exact point of transition?

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