Kids Research Express

. 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

. 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 ...

. 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 fr...

. 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.

. 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 togethe...

. 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 w...

. 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.

. 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 ha...