10.6: Properties of Solutions

Colligative Properties

Solutes affect some properties of solutions that depend only on the concentration of the dissolved particles. These properties are called colligative properties. Four important colligative properties that we will examine here are vapor pressure depression, boiling point elevation, freezing point depression, and osmotic pressure.

Molecular compounds separate into individual molecules when they are dissolved, so for every 1 mol of molecules dissolved, we get 1 mol of particles. In contrast, ionic compounds separate into their constituent ions when they dissolve, so 1 mol of an ionic compound will produce more than 1 mol of dissolved particles. For example, every mole of NaCl that dissolves yields 1 mol of Na⁺ ions and 1 mol of Cl⁻ ions, for a total of 2 mol of particles in solution. Thus, the effect on a solution’s properties by dissolving NaCl may be twice as large as the effect of dissolving the same amount of moles of glucose (C₆H₁₂O₆).

Vapor Pressure Depression

All liquids evaporate. In fact, given enough volume, a liquid will turn completely into a vapor. If enough volume is not present, a liquid will evaporate only to the point where the rate of evaporation equals the rate of vapor condensing back into a liquid. The pressure of the vapor at this point is called the vapor pressure of the liquid.

The presence of a dissolved solid lowers the characteristic vapor pressure of a liquid so that it evaporates more slowly. (The exceptions to this statement are if the solute itself is a liquid or a gas, in which case the solute will also contribute something to the evaporation process. We will not discuss such solutions here.) This property is called vapor pressure depression and is depicted in Figure \(\PageIndex{1}\).

Boiling Point and Freezing Point Effects 

A related property of solutions is that their boiling points are higher than the boiling point of the pure solvent. Because the presence of solute particles decreases the vapor pressure of the liquid solvent, a higher temperature is needed to reach the boiling point. This phenomenon is called boiling point elevation. For every mole of particles dissolved in a liter of water, the boiling point of water increases by about 0.5°C. The addition of one mole of sucrose (molecular compound) in one liter of water will raise the boiling point from 100⁰C to 100.5⁰C but the addition of one mole of NaCl in one liter of water will raise the boiling point by 2 x 0.5⁰C = 1⁰C. Furthermore, the addition of one mole of CaCl2CaCl2 in one liter of water will raise the boiling point by 3 x 0.5⁰C = 1.5⁰C.

The presence of solute particles has the opposite effect on the freezing point of a solution. When a solution freezes, only the solvent particles come together to form a solid phase, and the presence of solute particles interferes with that process. Therefore, for the liquid solvent to freeze, more energy must be removed from the solution, which lowers the temperature. Thus, solutions have lower freezing points than pure solvents do. This phenomenon is called freezing point depression. For every mole of particles in a liter of water, the freezing point decreases by about 1.9°C (Figure \(\PageIndex{2}\)).

Both boiling point elevation and freezing point depression have practical uses. For example, solutions of water and ethylene glycol (C₂H₆O₂) are used as coolants in automobile engines because the boiling point of such a solution is greater than 100°C, the normal boiling point of water. In winter, salts like NaCl and \(\ce{CaCl2}\) are sprinkled on the ground to melt ice or keep ice from forming on roads and sidewalks (Figure \(\PageIndex{3}\)). This is because the solution made by dissolving sodium chloride or calcium chloride in water has a lower freezing point than pure water, so the formation of ice is inhibited.

Osmotic Pressure

Certain natural and synthetic materials exhibit selective permeation, allowing only specific molecules or ions—based on size, shape, polarity, or charge—to pass through. Biological cell membranes are excellent natural examples of selective permeation, while dialysis tubing used in medical treatments serves as a simplified synthetic counterpart. Such materials are generally known as semipermeable membranes.

Osmosis is the process by which solvent molecules move through a semipermeable membrane, while solute particles are unable to pass. When solutions of different concentrations are separated by such a membrane, solvent molecules move from the side with lower solute concentration (more dilute) to the side with higher solute concentration (more concentrated). This movement continues until equilibrium is reached. The driving force behind this movement is known as osmotic pressure, and it can be counteracted by applying external pressure to the more concentrated solution. The pressure required to stop the net movement of solvent is equal to the solution’s osmotic pressure.

In a typical setup, such as the one shown in Figure \(\PageIndex{4}\), a semipermeable membrane separates a pure solvent from a solution. Although solvent molecules move in both directions across the membrane, the net flow is from the pure solvent into the solution, due to the higher solvent concentration on the pure side. This directional movement of solvent molecules, driven by concentration differences, defines the process of osmosis.

A solution composed of 100 mL of water and 15 g of table sugar consists of a solute (sugar) dissolved in a solvent (water). A solution is defined as a homogeneous mixture in which one or more solutes are uniformly distributed within a solvent.

A second solution prepared with 100 mL of water and 45 g of table sugar also consists of sugar as the solute and water as the solvent, but has a higher solute concentration than the first. When two solutions have unequal solute concentrations, the one with the higher solute concentration is termed hypertonic, while the one with the lower solute concentration is termed hypotonic. If two solutions contain equal solute concentrations, they are described as isotonic. In this case, the first solution is hypotonic relative to the second, and the second solution is hypertonic relative to the first.

When these two solutions are placed on opposite sides of a semipermeable membrane that allows the passage of water molecules but not sugar molecules, a concentration gradient of water is established. Water moves from the hypotonic solution (higher water concentration) to the hypertonic solution (lower water concentration). This net movement of water continues until equilibrium is reached and both solutions become isotonic.

Red blood cells behave the same way (see figure below). When red blood cells are in a hypertonic (higher concentration) solution, water flows out of the cell faster than it comes in. This results in crenation (shriveling) of the blood cell. On the other extreme, a red blood cell that is hypotonic (lower concentration outside the cell) will result in more water flowing into the cell than out. This results in swelling of the cell and potential hemolysis (bursting) of the cell. In an isotonic solution, the flow of water in and out of the cell is happening at the same rate.

Osmosis is the diffusion of water molecules across a semipermeable membrane from an area of lower concentration solution (i.e., higher concentration of water) to an area of higher concentration solution (i.e., lower concentration of water). Water moves into and out of cells by osmosis.

• If a cell is in a hypertonic solution, the solution has a lower water concentration than the cell cytosol, and water moves out of the cell until both solutions are isotonic.
• Cells placed in a hypotonic solution will take in water across their membranes until both the external solution and the cytosol are isotonic.

A red blood cell will swell and undergo hemolysis (burst) when placed in a hypotonic solution. When placed in a hypertonic solution, a red blood cell will lose water and undergo crenation (shrivel). Animal cells tend to do best in an isotonic environment, where the flow of water in and out of the cell is occurring at equal rates.

Osmolarity (osmol) is a way of reporting the total number of particles in a solution to determine osmotic pressure. It is defined as the molarity of a solute times the number of particles a formula unit of the solute makes when it dissolves (represented by \(i\)):

\[osmol = M \times i\label{Eq1} \]

If more than one solute is present in a solution, the individual osmolarities are additive to get the total osmolarity of the solution. Solutions that have the same osmolarity have the same osmotic pressure. If solutions of differing osmolarities are present on opposite sides of a semipermeable membrane, solvent will transfer from the lower-osmolarity solution to the higher-osmolarity solution. Counterpressure exerted on the high-osmolarity solution will reduce or halt the solvent transfer. An even higher pressure can be exerted to force solvent from the high-osmolarity solution to the low-osmolarity solution, a process called reverse osmosis. Reverse osmosis is used to make potable water from saltwater where sources of fresh water are scarce.

Link to Learning: Chemistry in Everyday Life, Dialysis 

The main function of the kidneys is to filter the blood to remove wastes and extra water, which are then expelled from the body as urine. Some diseases rob the kidneys of their ability to perform this function, causing a buildup of waste materials in the bloodstream. If a kidney transplant is not available or desirable, a procedure called dialysis can be used to remove waste materials and excess water from the blood. In one form of dialysis, called hemodialysis, a patient’s blood is passed though a length of tubing that travels through an artificial kidney machine (also called a dialysis machine). A section of tubing composed of a semipermeable membrane is immersed in a solution of sterile water, glucose, amino acids, and certain electrolytes. The osmotic pressure of the blood forces waste molecules and excess water through the membrane into the sterile solution. Red and white blood cells are too large to pass through the membrane, so they remain in the blood. After being cleansed in this way, the blood is returned to the body.

Dialysis is a continuous process, as the osmosis of waste materials and excess water takes time. Typically, 5–10 lb of waste-containing fluid is removed in each dialysis session, which can last 2–8 hours and must be performed several times a week. Although some patients have been on dialysis for 30 or more years, dialysis is always a temporary solution because waste materials are constantly building up in the bloodstream. A more permanent solution is a kidney transplant (Figure \(\PageIndex{6}\)).

Cell walls are semipermeable membranes, so the osmotic pressures of the body’s fluids have important biological consequences. If solutions of different osmolarity exist on either side of the cells, solvent (water) may pass into or out of the cells, sometimes with disastrous results. Consider what happens if red blood cells are placed in a hypotonic solution, meaning a solution of lower osmolarity than the liquid inside the cells. The cells swell up as water enters them, disrupting cellular activity and eventually causing the cells to burst. This process is called hemolysis. If red blood cells are placed in a hypertonic solution, meaning one having a higher osmolarity than exists inside the cells, water leaves the cells to dilute the external solution, and the red blood cells shrivel and die. This process is called crenation. Only if red blood cells are placed in isotonic solutions that have the same osmolarity as exists inside the cells are they unaffected by negative effects of osmotic pressure. Glucose solutions of about 0.31 M, or sodium chloride solutions of about 0.16 M, are isotonic with blood plasma. The concentration of an isotonic sodium chloride (NaCl) solution is only half that of an isotonic glucose (C₆H₁₂O₆) solution because NaCl produces two ions when a formula unit dissolves, while molecular glucose, C₆H₁₂O₆ produces only one particle when a formula unit dissolves. The osmolarities are therefore the same even though the concentrations of the two solutions are different. 

Osmotic pressure explains why you should not drink seawater if you are abandoned in a life raft in the middle of the ocean. Its osmolarity is about three times higher than most bodily fluids. You would actually become thirstier as water from your cells was drawn out to dilute the salty ocean water you ingested. Our bodies do a better job coping with hypotonic solutions than with hypertonic ones. The excess water is collected by our kidneys and excreted. Osmotic pressure effects are used in the food industry to make pickles from cucumbers and other vegetables and in brining meat to make corned beef. It is also a factor in the mechanism of getting water from the roots to the tops of trees!