Physical Properties of Liquids

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The physical properties of a liquid describe how the liquid behaves under different temperatures and pressures and how it behaves when it comes into contact with other substances. Some liquids boil and evaporate at extremely low temperatures, while others boil at extremely high temperatures. For example, liquid helium boils at -269° C (-452° F), while liquid platinum boils at 3825° C (6917° F). Liquids also vary widely in freezing point and viscosity (a property that measures how a liquid flows). In addition, liquids behave differently when they come into contact with solids and when they mix with other liquids.

A. Boiling Point

B. Freezing Point

C. Viscosity

D. Surface Tension

E. Capillary Action

F. Miscibility

G. Osmosis

See also: Structure of Liquids

Physical Properties of Liquids

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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.

Physical Properties of Liquids

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B. Freezing Point

The freezing point of a substance is the temperature at which the liquid form of the substance becomes a solid. The molecules of a liquid arrange into a more ordered structure as the liquid freezes. The freezing point of a substance is essentially the same as its melting point—that is, the point at which a solid becomes a liquid.

When a liquid freezes to become a solid, its volume usually shrinks by approximately 10 percent as its molecules move closer together. In solid aluminum, for example, each atom has 12 neighboring atoms, each at a distance of 2.86 x 10-8 cm. In liquid aluminum, each atom has 10 or 11 neighboring atoms at a distance of 2.96 x 10-8 cm. Thus, the atoms are less tightly packed in the liquid, and the liquid must contract as it freezes. The exceptions to this rule are water and the liquid forms of gallium and bismuth. These substances expand upon freezing. The structure of their solid state is less dense than that of their liquid state near the freezing point. In ice, each water molecule is solidly packed into a lattice, surrounded by four molecules equally distant from each other. This structure is actually less dense than the molecular patterns that can occur in the liquid form of water, which is why ice floats on water.

Physical Properties of Liquids

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C. Viscosity

The viscosity of a liquid is a measure of how much the liquid resists flow. Flow allows a liquid to take the shape of the container that holds it. A liquid’s viscosity depends on the structure of the liquid’s molecules and on the attractive forces between the liquid’s molecules. Highly viscous liquids often contain molecules that have complicated structures. These molecules can become entangled with one another, impairing their ability to flow past one another. The viscosity of water is lower than that of heavy oils, for example, because oils contain large, convoluted molecules that catch on one another. The polarity of the molecules in water, however, causes them to attract one another, making water more viscous than a nonpolar liquid, such as propane. Viscosity decreases as temperature increases because additional heat energy enables molecules to overcome attractions to one another and move more freely.

Physical Properties of Liquids

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D. Surface Tension

Liquids behave as though they have a delicate skin on their surface. This property is called surface tension. In rain droplets, surface tension acts like a thin balloon, holding the water molecules together in each droplet. Water-strider bugs take advantage of surface tension by flitting across the surfaces of ponds without falling through the surface.

Surface tension results from the intermolecular forces of attraction in a liquid. A water molecule deep inside a droplet experiences attractive forces in all directions from other molecules in the drop. The sum of these forces is zero, leaving no net force on the molecule. A molecule that is close to the surface, however, has more neighboring molecules inside the drop than it has near the surface. The forces pulling the molecule toward the center of the drop are stronger than those at the surface, so the molecule sticks to the drop instead of falling away.

Intermolecular forces of attraction make liquids pull together and minimize their surface area. Liquids do this because, like all matter, they seek to minimize the amount of energy they require to maintain their molecular structure. A liquid requires the least amount of energy when it has the smallest possible surface area. For small amounts of liquid in air, such as raindrops, the sphere is the shape with the smallest surface area. Gravity, another force acting on raindrops, stretches the droplets so that they are not exactly round. To overcome the attraction between molecules in a liquid and increase the liquid’s surface area takes energy. For instance, a moving car can transfer energy to a droplet of rain by hitting it, making it break apart or deform against the car into a shape that has more surface area.

Physical Properties of Liquids

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E. Capillary Action

Water will climb up a paper towel if the edge of the towel touches a puddle, and it will climb up a thin glass tube if the tube is dipped in water. Water behaves this way because of an effect called capillary action. Capillary action occurs when the attraction of a liquid’s molecules for themselves differs from their attraction for a solid that the liquid contacts. The water in the paper towel example climbs the towel because the water molecules are more attracted to the paper than they are to each other.

Chemistry students demonstrate capillary action using a glass tube called a capillary tube and a beaker of water. Water climbs the glass tube when it is dipped in the beaker because the water is more attracted to the glass than it is to itself. Several forces are acting on the water: the attraction of the water molecules to the glass tube, the weight of gravity pressing down on the water in the tube, and the attraction of the water molecules for each other. The water rises in the tube until all these forces balance. For some liquids, such as mercury, the attraction between the molecules of the liquid is stronger than their attraction to the glass tube. When a glass tube is dipped in a beaker of mercury, capillary action makes the level of mercury in the tube drop below the level of mercury in the beaker.

Physical Properties of Liquids

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F. Miscibility

Miscibility is a measure of how easily different liquids will dissolve when mixed together. Miscibility depends on the polarity of a liquid’s molecules. For example, water will mix with alcohol because they are both polar liquids, so their molecules attract one another. But water will not mix well with oil, which is a nonpolar liquid. Oil floats on top of water because the polar water molecules are much more strongly attracted to each other than to the oil molecules. The rule for determining miscibility is that “like dissolves like.” Polar liquids are miscible with other polar liquids, while nonpolar liquids are miscible with other nonpolar liquids.

Physical Properties of Liquids

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G. Osmosis

When a substance dissolves in a liquid, the resulting mixture is called a solution. Osmosis occurs when molecules of the initial liquid pass through a membrane, but molecules of the dissolved substance do not. The molecules of the initial liquid can pass through the membrane because they are relatively small. Osmosis tends to equalize the concentration of the solutions on both sides of a membrane. The membrane in this case is called semipermeable, because it allows one part of the mixture to pass through but not another. Cells in living organisms consist mostly of water, and they are surrounded by a watery environment. If the concentration of a dissolved substance, such as sugar or salt, differs inside and outside a cell, osmosis causes water to pass through the cell’s membrane from the area of lower concentration to the area of higher concentration, until the concentration on each side of the membrane is equal. Osmosis makes sugar and salt good food preservatives. When harmful bacteria encounter sugary or salty foods, water flows from the area of lower concentration—the cells of the bacteria—to the area of higher concentration—the food. The flow of water out from the bacteria’s cells dehydrates the bacteria, which kills it.