Physical Properties of Liquids
.
A. Boiling Point
The boiling point of a liquid is the temperature at which molecules escape from the liquid and enter the gaseous state. Heat causes a liquid to boil by adding energy to the liquid’s molecules. As the molecules gain energy, they move about more quickly and range farther from each other. When the molecules are far enough apart, intermolecular forces are too weak to pull them back together, so the molecules form a vapor. Boiling starts when bubbles of vapor form within the liquid. These bubbles rise to the top of the liquid and release the gaseous molecules to the atmosphere above the liquid’s surface. It takes 2,260 Joules (540 calories) of heat energy to evaporate 1 gram of water at 100° C (212° F) at sea level.
At the boiling point, the vapor pressure of a liquid must equal the pressure of the atmosphere above the liquid. For a liquid boiling in an open container, the atmosphere above the liquid is simply Earth’s atmosphere. The pressure in the bubbles of vapor must equal the pressure of Earth’s atmosphere pressing down on the liquid. If this were not true, the air pressing down would squeeze and collapse the bubbles before they could form and rise to the surface. The boiling point of a liquid is lower at higher elevations because atmospheric pressure decreases as altitude increases. For example, the boiling point of water is 100° C (212° F) at sea level, where the air pressure measures one atmosphere (atm). On top of Mount Everest, which is 8,850 m (29,035 ft) above sea level, water boils at only 70° C (158° F) because the air pressure at this height is only 1/3 atm.
Different materials have different boiling points because the forces of attraction between their molecules differ. For example, water molecules strongly attract each other because of their structure. A water molecule consists of one oxygen atom and two hydrogen atoms. The oxygen atom attracts the electrons it shares with the hydrogen atoms more strongly than the two hydrogen atoms do. Electrons have a negative electric charge and thus make the oxygen end of the water molecule more negatively charged, while the hydrogen end of the molecule has a positive charge. This separation of charge makes the water molecule strongly polar. The negative charge on the oxygen atom attracts positive hydrogen atoms from other water molecules, causing the water molecules to bond tightly to each other. Breaking this bond requires considerable heat, which is why the boiling point for water, 100° C (212° F), is relatively high . Without this bonding, water would boil near -80° C (-112° F). Ethyl alcohol is also a polar liquid, and its boiling point is 78.5° C (173.3° F).
Nonpolar liquids have lower boiling points than polar liquids because electric charge is evenly distributed around their molecules. This even distribution makes the molecule-to-molecule attractions in nonpolar liquids relatively weak. Examples of nonpolar liquids are the hydrocarbons, substances that consist entirely of hydrogen and carbon molecules. Many common fuels, such as gasoline and methane, are hydrocarbons. In the molecules of these substances, the carbon and hydrogen atoms share their electrons more equally than do the hydrogen and oxygen atoms of water. As a result, the bonds between the molecules are relatively weak, and the liquids boil at lower temperatures. The hydrocarbon propane boils at –42.1° C (-43.8° F), and butane boils at –0.5° C (31.1° F). These substances exist as gases at room temperature.
Sometimes a liquid can be superheated—that is, heated above its usual boiling point without changing into vapor. Superheating occurs when vapor bubbles inside a liquid don’t have an appropriate surface on which to form. For example, when water in a smooth-walled container is heated in a microwave oven, it can reach a higher temperature than its boiling point and remain a liquid. If a rough surface enters the liquid, such as a teabag, vapor bubbles can form and the liquid will begin to boil rapidly.
At the boiling point, the vapor pressure of a liquid must equal the pressure of the atmosphere above the liquid. For a liquid boiling in an open container, the atmosphere above the liquid is simply Earth’s atmosphere. The pressure in the bubbles of vapor must equal the pressure of Earth’s atmosphere pressing down on the liquid. If this were not true, the air pressing down would squeeze and collapse the bubbles before they could form and rise to the surface. The boiling point of a liquid is lower at higher elevations because atmospheric pressure decreases as altitude increases. For example, the boiling point of water is 100° C (212° F) at sea level, where the air pressure measures one atmosphere (atm). On top of Mount Everest, which is 8,850 m (29,035 ft) above sea level, water boils at only 70° C (158° F) because the air pressure at this height is only 1/3 atm.
Different materials have different boiling points because the forces of attraction between their molecules differ. For example, water molecules strongly attract each other because of their structure. A water molecule consists of one oxygen atom and two hydrogen atoms. The oxygen atom attracts the electrons it shares with the hydrogen atoms more strongly than the two hydrogen atoms do. Electrons have a negative electric charge and thus make the oxygen end of the water molecule more negatively charged, while the hydrogen end of the molecule has a positive charge. This separation of charge makes the water molecule strongly polar. The negative charge on the oxygen atom attracts positive hydrogen atoms from other water molecules, causing the water molecules to bond tightly to each other. Breaking this bond requires considerable heat, which is why the boiling point for water, 100° C (212° F), is relatively high . Without this bonding, water would boil near -80° C (-112° F). Ethyl alcohol is also a polar liquid, and its boiling point is 78.5° C (173.3° F).
Nonpolar liquids have lower boiling points than polar liquids because electric charge is evenly distributed around their molecules. This even distribution makes the molecule-to-molecule attractions in nonpolar liquids relatively weak. Examples of nonpolar liquids are the hydrocarbons, substances that consist entirely of hydrogen and carbon molecules. Many common fuels, such as gasoline and methane, are hydrocarbons. In the molecules of these substances, the carbon and hydrogen atoms share their electrons more equally than do the hydrogen and oxygen atoms of water. As a result, the bonds between the molecules are relatively weak, and the liquids boil at lower temperatures. The hydrocarbon propane boils at –42.1° C (-43.8° F), and butane boils at –0.5° C (31.1° F). These substances exist as gases at room temperature.
Sometimes a liquid can be superheated—that is, heated above its usual boiling point without changing into vapor. Superheating occurs when vapor bubbles inside a liquid don’t have an appropriate surface on which to form. For example, when water in a smooth-walled container is heated in a microwave oven, it can reach a higher temperature than its boiling point and remain a liquid. If a rough surface enters the liquid, such as a teabag, vapor bubbles can form and the liquid will begin to boil rapidly.
Comments