Liquids

The obvious way to turn a gas into a liquid is to cool it to a temperature below its boiling point. There is another way of condensing a gas to form a liquid, however, which involves raising the pressure on the gas. Liquids boil at the temperature at which the vapor pressure is equal to the pressure on the liquid from its surroundings. Raising the pressure on a gas therefore effectively increases the boiling point of the liquid.

Suppose that we have water vapor (or steam) in a closed container at 120 o C and 1 atm. Since the temperature of the system is above the normal boiling point of water, there is no reason for the steam to condense to form a liquid. Nothing happens as we slowly compress the container thereby raising the pressure on the gas until the pressure reaches 2 atm. At this point, the system is at the boiling point of water, and some of the gas will condense to form a liquid. As soon as the pressure on the gas exceeds 2 atm, the vapor pressure of water at 120 o C is no longer large enough for the liquid to boil. The gas therefore condenses to form a liquid, as shown in the figure below.

In theory, we should be able to predict the pressure at which a gas condenses at a given temperature by consulting a plot of vapor pressure vs. temperature . In practice, every compound has a critical temperature (T_c). If the temperature of the gas is above the critical temperature, the gas can't be condensed, regardless of the pressure applied.

The existence of a critical temperature was discovered by Thomas Andrews in 1869 while studying the effect of temperature and pressure on the behavior of carbon dioxide. Andrews found that he could condense CO₂ gas into a liquid by raising the pressure on the gas, as long as he kept the temperature below 31.0 o C. At 31.0 o C, for example, it takes a pressure of 72.85 atm to liquify CO₂ gas. Andrews found that it was impossible to turn CO₂ into a liquid above this temperature, no matter how much pressure was applied.

Gases can't be liquified at temperatures above the critical temperature because at this point the properties of gases and liquids become the same, and there is no basis on which to distinguish between gases and liquids. The vapor pressure of a liquid at the critical temperature is called the critical pressure (P_c). The vapor pressure of a liquid never gets larger than this critical pressure.

The critical temperatures, critical pressures, and boiling points of a number of gases are given in the table below. There is an obvious correlation between the critical temperature and boiling point of these gases. These properties are related because they are both indirect measures of the force of attraction between particles in the gas phase.

Critical Temperatures, Critical Pressures and Boiling Points of Common Gases

There is a force of attraction between molecules in liquids, and liquids can flow until they take on the shape that maximizes this force of attraction. Below the surface of the liquid, the force of cohesion (literally, "sticking together") between molecules is the same in all directions, as shown in the figure below. Molecules on the surface of the liquid, however, feel a net force of attraction that pulls them back into the body of the liquid. As a result, the liquid tries to take on the shape that has the smallest possible surface area the shape of a sphere. The magnitude of the force that controls the shape of the liquid is called the surface tension. The stronger the bonds between the molecules in the liquid, the larger the surface tension.

There is also a force of adhesion (literally, "sticking") between a liquid and the walls of the container. When the force of adhesion is more than half as large as the force of cohesion between the liquid molecules, the liquid is said to "wet" the solid. A good example of this phenomenon is the wetting of paper by water. The force of adhesion between paper and water combined with the force of cohesion between water molecules explains why sheets of wet paper stick together.

Water wets glass because of the force of adhesion that results from interactions between the positive ends of the polar water molecules and the negatively charged oxygen atoms in glass. As a result, water forms a meniscus that curves upward in a small-diameter glass tube, as shown in the figure below. (The term meniscus comes from the Greek word for "moon" and is used to describe anything that has a crescent shape.) The meniscus that water forms in a buret results from a balance between the force of adhesion pulling up on the column of water to wet the walls of the glass tube and the force of gravity pulling down on the liquid.

The force of adhesion between water and wax is very small compared to the force of cohesion between water molecules. As a result, rain doesn't adhere to wax. It tends to form beads, or drops, with the smallest possible surface area, thereby maximizing the force of cohesion between the water molecules. The same thing happens when mercury is spilled on glass or poured into a narrow glass tube. The force of cohesion between mercury atoms is so much larger than the force of adhesion between mercury and glass that the area of contact between mercury and glass is kept to a minimum, with the net result being the meniscus shown in the above figure.

Viscosity is a measure of the resistance to flow. Motor oils are more viscous than gasoline, for example, and the maple syrup used on pancakes is more viscous than the vegetable oils used in salad dressings.

Viscosity is measured by determining the rate at which a liquid or gas flows through a small-diameter glass tube. In 1844 Jean Louis Marie Poiseuille showed that the volume of fluid (V) that flows down a small-diameter capillary tube per unit of time (t) is proportional to the radius of the rube (r), the pressure pushing the fluid down the tube (P), the length of the tube (l), and the viscosity of the fluid ().

Viscosity is reported in units called poise (pronounced "pwahz"). The viscosity of water at room temperature is roughly 1 centipoise, or 1 cP. Gasoline has a viscosity between 0.4 and 0.5 cP; the viscosity of air is 0.018 cP.

Because the molecules closest to the walls of a small-diameter tube adhere to the glass, viscosity measures the rate at which molecules in the middle of the stream of liquid or gas flow past this outer layer of more or less stationary molecules. Viscosity therefore depends on any factor that can influence the ease with which molecules slip past each other. Liquids tend to become more viscous as the molecules become larger, or as the amount of intermolecular bonding increases. They become less viscous as the temperature increases. The viscosity of water, for example, decreases from 1.77 cP at 0 o C to 0.28 cP at 100 o C.

• Most solids expand when they melt. Water expands when it freezes.
• Most solids are more dense than the corresponding liquids. Ice (0.917 g/cm 3 ) is not as dense as water.
• Water has a melting point at least 100 o C higher than expected on the basis of the melting points of H₂S, H₂Se, and H₂Te.
• Water has a boiling point almost 200 o C higher than expected from the boiling points of H₂S, H₂Se, and H₂Te.
• Water has the largest surface tension of any common liquid except liquid mercury.
• Water has an unusually large viscosity.
• Water is an excellent solvent. It can dissolve compounds, such as NaCl, that are insoluble or only slightly soluble in other liquids.
• Water has an unusually high heat capacity. It takes more heat to raise the temperature of 1 gram of water by 1 o C than any other liquid.

These anomalous properties all result from the strong intermolecular bonds in water. Water is best described as a polar molecule in which there is a partial separation of charge to give positive and negative poles. The force of attraction between a positively charged hydrogen atom on one water molecule and the negatively charged oxygen atom on another gives rise to an intermolecular bond, as shown in the figure below. This dipole-dipole interaction between water molecules is known as a hydrogen bond.

Hydrogen bonds are separated from other examples of van der Waals forces because they are unusually strong: 10-12 kJ/mol. The hydrogen bonds in water are particularly important because of the dominant role that water plays in the chemistry of living systems. Hydrogen bonds are not limited to water, however.

Hydrogen-bond donors include substances that contain relatively polar H-X bonds, such as NH₃, H₂O, and HF. Hydrogen-bond acceptors include substances that have nonbonding pairs of valence electrons. The H-X bond must be polar to create the partial positive charge on the hydrogen atom that allows dipole-dipole interactions to exist. As the X atom in the H-X bond becomes less electronegative, hydrogen bonding between molecules becomes less important. Hydrogen bonding in HF, for example, is much stronger than in either H₂O or HCl.

The hydrogen bonds between water molecules in ice produce the open structure shown in the figure below. When ice melts, some of these bonds are broken, and this structure collapses to form a liquid that is about 10% denser. This unusual property of water has several important consequences. The expansion of water when it freezes is responsible for the cracking of concrete, which forms potholes in streets and highways. But it also means that ice floats on top of rivers and streams. The ice that forms each winter therefore has a chance to melt during the summer.

The figure below shows another consequence of the strength of the hydrogen bonds in water. There is a steady increase in boiling point in the series CH₄, GeH₄, SiH₄, and SnH₄. The boiling points of H₂O and HF, however, are anomalously large because of the strong hydrogen bonds between molecules in these liquids. If this doesn't seem important, try to imagine what life would be like if water boiled at -80 o C.

The surface tension and viscosity of water are also related to the strength of the hydrogen bonds between water molecules. The surface tension of water is responsible for the capillary action that brings water up through the root systems of plants. It is also responsible for the efficiency with which the wax that coats the surface of leaves can protect plants from excessive loss of water through evaporation.

The unusually large heat capacity of water is also related to the strength of the hydrogen bonds between water molecules. Anything that increases the motion of water molecules, and therefore the temperature of water, must interfere with the hydrogen bonds between these molecules. The fact that it takes so much energy to disrupt these bonds means that water can store enormous amounts of thermal energy. Although the water in lakes and rivers gets warmer in the summer and cooler in the winter, the large heat capacity of water limits the range of temperatures that would otherwise threaten the life that flourishes in this environment. The heat capacity of water is also responsible for the ocean's ability to act as a thermal reservoir that moderates the swings in temperature that occur from winter to summer.