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4. Properties of Solutions
If a lump of sugar is dropped into a glass of water it gradually disappears. The sugar dissolves in the water. A microscopic examination of the water would not show the sugar. Only examination at the molecular level, which is not easily done, would show sugar and water molecules intimately mixed. If you taste the liquid, of course, you would know that the sugar is there. And you could recover the sugar by evaporating the water. In a solution, the molecules of the sugar, the solute, are homogeneously dispersed among the molecules of water, the solvent. This mixture of sugar and water is homogenous on a molecular level. Such a homogenous mixture is called a solution. The composition of a solution can be varied within certain limits.
4.1 Solute and Solvent
The dissolving medium is called the solvent and the dissolved substance is called the solute. In a solution of sugar in water, the solute and solvent are obvious. However, if a solution is prepared from two liquids, say alcohol and water, the solute and solvent are more difficult to discern. The solute is usually considered to be the component present in lower quantity, or the mixture is just called a alcohol/water solution, without trying to define solute and solvent.

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4.2 Types of Solutions
There are three common states of matter, and seven different types of solutions are possible. These are:
Table: 4.2.1 Different types of solutions
Solute
Solvent
Example
gas
gas
air
gas
liquid
soda
gas
solid
Hydrogen on Platinum
liquid
gas
Water vapor in air
liquid
liquid
Alcohol in Water
liquid
solid
Dental fillings (Mercury in Silver)
solid
gas
Sulfur vapor in air
solid
liquid
sugar solution in water
solid
solid
Solder (tin in lead)

 

A gas cannot act as a solvent for a liquid or a solid. If the solute is a solid or a liquid, it must be converted to a gas to form a gaseous solution. Why is fog, for instance, not a solution of liquid water in air? The water droplets are not mixed on a molecular level--they are clusters of many many molecules suspended in air.

Fig: 4.1 Different types of Solutions 
Source: Basic Chemistry, 3rd Edition, William S. Seese/Guido H. Daub. Prentice Hall Inc.
 
All mixtures of gases are solutions, since they consist of homogenous systems of different kinds of molecules. Solutions of solids in liquids are very common. Since water is a liquid at ordinary temperatures one may consider water vapor in air to be a solution of a liquid in a gas. Solutions of gases in solids are rare, one example is the condensation of hydrogen on the surface of palladium and platinum. This phenomenon, called adsorption, approaches the nature of a solution. 
In general substances that are alike in their chemical makeup are more likely to form solutions.
 
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4.3 Solvents are Selective

A phrase commonly used is like dissolves like. The more the molecules of a solute and solvent are, the more likely they are to form a homogenous solution. Because of molecular differences, solvents are selective, that is, they dissolve some substances readily and others only to an insignificant extent.

The water molecule is polar, because it has a separation between its center of positive charge and the center of negative charge. The water molecule is bent, and the oxygen atom is more negative than the hydrogen atoms. Therefore, a plane separating the positive side, the O atom, from the negative side, the H atoms, can be imagined. On the other hand, the carbon tetrachloride (CCl4) molecule has one carbon atom surrounded by four equally spaced chlorine atoms. The molecule has the shape of a triangular pyramid, or a regular tetrahedron. The chlorines are more negative and the carbon is more positive, but because of the symmetrical shape, the molecule as a whole is non-polar. According to the rule "like dissolves like" we would expect water to dissolve polar substances and carbon tetrachloride to dissolve non polar substances. We would also expect that water would not be very soluble in carbon tetrachloride or vice versa, and experiments show this to be true.

Solute crystals composed of polar molecules or ions and are held together by strong attractive forces. They are more likely to be attracted away from their solid structures by polar water molecules than by non-polar solvents. Thus many crystalline ionic salts like NaCl (table salt) or polar molecular solids like sugar dissolve readily in water. Non-polar compounds which are insoluble in water, oils and greases for example, readily dissolve in non-polar solvents like CCl4.

 

 
Fig:4.2 Polarity in solvents
Source: Basic Chemistry, 3rd Edition, William S. Seese/Guido H. Daub, Prentice Hall Inc.
 
 
Ethanol (C2H5OH)on the other hand is typical of a group of substances that dissolve in both polar and non-polar solvents. In ethanol there are 5 non-polar carbon hydrogen bonds. Also there is one non-polar carbon-carbon bond. However the carbon oxygen and oxygen hydrogen bonds are polar. Thus an ethanol molecule has some polar character. This may account for the fact that ethanol is a good solvent for some polar and some non-polar substances, as a solvent it is intermediate between the strongly polar water molecule and the non-polar CCl4 molecule.

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4.4 Hydrogen Bonds and properties of Solvents
Electronegativity is the measure of the tendency of an atom in a molecule to attract shared electrons. Hydrogen atoms form distinctly polar covalent bonds with atoms of such highly electronegative elements as fluorine, oxygen, chlorine, and nitrogen. The hydrogen end of these polar bonds is unique. It consists of an essentially exposed proton, shared electrons are more strongly attracted by the highly electronegative atom than by hydrogen. The effect of this unequal sharing is to leave the hydrogen nucleus (a proton) without an electron shield to isolate its positive charge.

Because of this unique character of the hydrogen atoms in these very polarized bonds, the molecules formed show relatively strong intermolecular forces. The positive hydrogen atom in one molecule will attract the highly electronegative atom of another molecule much more strongly than the ordinary intermolecular forces found between polar molecules. This stronger intermolecular force is called the hydrogen bond. The hydrogen bond energy, a measure of the bond strength, is 4 to 30 kJ/mole, while ordinary intermolecular attractions are on the order of 1 kJ/mol. However, covalent bonds have bond energies of 140 to 600 kJ/mol. The formation of hydrogen bonds between a solvent and a solute increases the solubility of the solute.

 
Fig: 4.3 Hydrogen bond formation in an ice crystal
Source: Basic Chemistry, 3rd Edition, William S. Seese/Guido H. Daub, Prentice Hall Inc.
 
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4.5 Solution equilibrium

The formation of a solution is reversible. When a solid dissolves, molecules leave the lattice randomly. Dissolved molecules of solute which strike the solid may also be recaptured into the lattice. In other words, there is an equilibrium between dissolution and crystallization.

solute in crystal <> solute in solution

Initially when sugar is added to water, there are no sugar molecules in solution. Therefore, dissolution is the only process which can occur. As the process continues, the concentration of sugar in solution increases, and the rate of sugar molecules falling back into the crystal lattice increases, since more molecules are available. Eventually, the forward and reverse reactions reach the same rate, and the reaction has reached equilibrium. It appears that no more sugar is dissolving, but in reality a dynamic equilibrium has been established, and both dissolution and crystallization are taking place simultaneously.

At this point, the concentration of the solute in solution is the maximum possible under the existing conditions. The solution is saturated. An equilibrium is reached between undissolved sugar and sugar dissolved in water. A saturated solution is one in which the dissolved and undissolved solutes are in equilibrium.

If water is added to the sugar solution, it is no longer saturated; the concentration of solute molecules has been decreased. According to Le Chatelier's Principle, the decrease in concentration of the solute molecules places a stress on the equilibrium, this stress is relieved by dissolving more sugar to reestablish the equilibrium. More sugar dissolves and restores the same equilibrium concentration of solute molecules. Solution equilibrium exists when no more solute can dissolve in a given quantity of solvent. The solubility of a substance is defined as the maximum amount of that substance that can dissolve in a given amount of a certain solvent. As with all chemical equilibria, the position of equilibrium is governed by an equilibrium constant which changes only with temperature.

The solubility of a substance is defined as the maximum amount of that substance that can dissolve in a given amount of a certain solvent under specified conditions.

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4.6 Solubility and Pressure

Ordinary changes in pressure affect the solubility of solids and liquids so slightly that we may ignore them altogether. Because all gas mixtures are homogenous, the solubility of one gas in another is independent of pressure. The solubility of gases in solids and liquids is markedly affected by pressure. Soda water fizzes when the cap is removed, although no bubbles are evident in the sealed container. At the bottling plant, CO2 gas is forced into solution in the flavored water under a pressure of 5-10 atmospheres and the bottles are sealed. When the bottle is opened, the pressure is reduced to atmospheric pressure and much of the CO2 eventually escapes from solution as gas bubbles. Solutions of gases in liquids reach equilibrium just as solids in liquids do. The attractive forces between gas molecules are insignificant on the average and their motions are relatively free. If a gas is in contact with the surface of a liquid, gas molecules can easily dissolve. As the concentration of dissolved gas molecules increases, some begin to escape from the liquid. Eventually an equilibrium is reached between the rates at which gas molecules enter the liquid and the rate at which they leave it.. At equilibrium, there is no increase in the concentration of the dissolved gaseous solute. The solubility of the gas is limited to its equilibrium concentration in the liquid. If the pressure of the gas above the liquid is increased, the equilibrium is disturbed. The increase in gas pressure increases the concentration of undissolved gas in contact with the solvent. According to Le Chatelier's principle, more gas will dissolve. This increases the concentration of the dissolved gas, and gas molecules will escape from the liquid surface at a faster rate. When equilibrium is restored, there is a higher concentration of the gas in the solution at the higher external pressure. The solubility of a gas in a liquid is dependent on the pressure of the gas over the liquid. . The solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid. This statement is known as Henry's law, after William Henry the English chemist).

Gases that react chemically with their liquid solvents are in general more soluble than that do not. O2, H2, N2 are only slightly soluble in water, on the other hand ammonia, CO2 and sulfur dioxide are more soluble probably because they from weak monohydrates in water ( NH3 . H2O ). These gases do not follow Henry's Law.

If a mixture of gases is placed contact with a liquid, the solubility of each gas is proportional to its partial pressures. If the gases do not react with the liquid, each gas will dissolve to the same extent as it would if there were no other gases present.

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4.7 Temperature and Solubility

Solubility is an equilibrium and is governed by equilibrium constants. These change with temperature.

Gases in liquids : A glass of water from a hot water tap appears milky. Tiny bubbles of air suspended throughout the water causes this cloudiness. The suspended air was originally dissolved in cold water. It was driven out of solution as the temperature increased.

Raising the temperature of a solution increases the average speed of its molecules. Molecules of dissolved gas leave the solvent at a faster rate than the gas molecules that enter it. This lowers the equilibrium concentration of the solute. Thus the solubility of a gas decreases as the temperature of the solvent is increased.

Solids in liquids : The solubility of solids in liquids changes with temperature, and the solubility often increases with an increase in temperature. However, even when ionic solutes dissolving in water are compared, it is found that some change little with temperature, some are much more soluble at higher temperatures and a few even decrease in solubility at higher temperature. Chemists often use these solubility differences to perform separation of compounds

Fig: 4.4 Sugar Crystals growing on a string from a supersaturated solution of sugar
Source: Modern Chemistry, H Clark Metcalfe, John E. Williams, Joseph F. Castka, Holt Rinehart and Winston, Publishers
 
A solution is saturated when no more solute can be dissolved at the current temperature. If the temperature of a saturated solution is lowered, and the solute is more soluble at a high temperature, we expect that the equilibrium will be reestablished by the crystallization of the excess solute. However, to start crystals growing, an initial lattice must be established. If there are no solid crystals in the solution, and the container walls are smooth and unscratched, it may be difficult for such crystal nuclei to start to form. This can lead to solutions which are super saturated. Super saturated solutions contain a higher concentration of solute than does the saturated solution at the same temperature. This solution is not at equilibrium, and is considered to be in a metastable state. If a seed of crystal is added, or sometimes if the side of the beaker is just scratched with a stirring rod, crystals will start to form. The process may be complete in a few seconds or take days. In the picture, crystals of sugar are growing slowly from a supersaturated solution of sugar in water. A string provides a surface for the crystals to nucleate, and the crystallization will occur slowly over a period of several days.

Liquids in liquids: A similar logic applies to solutions of liquids in liquids. As no change of phase occurs we expect little change in temperature. A great change in temperature suggests some type of a chemical reaction between solute and solvent, as in the case of water and sulfuric acid. When water is he solvent, this reaction usually involves hydration, a clustering of water dipoles about the solute particles.

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4.8 Increasing the rate of dissolving
The rate at which a solid dissolves in a liquid depends primarily on the nature of the solid and the liquid but also depends on three other factors:
4.9 Concentration of Solutions
The terms "concentrated" (conc.) and "dilute" (dil) are sometimes used to express concentration, but these are very qualitative. Dilute solutions are less concentrated, but beyond this, little can be said about them. There are various quantitative measures of the concentration of solutions. Some of these are:
  • Percent by mass
  • Parts per million (ppm)
  • Molality
  • Molarity
  • Normality
  • Each method of expressing concentration is useful in the right situation. For example, if you want to know the number of pounds of salt in a given mass of ocean water, it would be more convenient to express the concentration in percent by mass.

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    4.10 Percent by Mass
    The percent by mass of a solute in a solution is the same as parts by mass of solute per 100 parts by mass of solution:

    % by mass = (mass of solute / mass of solution ) x 100

    The mass of solution is equal to the mass of the solute plus the mass of the solvent. For example, a 20.0 percent solution of sodium sulfate would contain 20.0 g of sodium sulfate in 100 g of solution (i.e. 80.0 g of water)

    In addition to percent by mass, it is occasionally convenient to express concentration as percent by volume. Percent by volume expresses concentration as parts by volume of the solute per 100 parts by volume of solution. This method is generally used to express the concentration of alcohol in alcoholic beverages in fact most solutions of liquids in liquids are expressed in percent by volume.

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    4.11 Parts per million (ppm)
    In percent by mass the concentration was expressed as parts by mass of solute per 100 parts by mass of solution. If the concentration is expressed as parts by mass of solute per 1,000,000 parts by mass of solution. (The units of mass for the solute and solvent must be the same). Then this concentration is termed as parts per million (or more conveniently ppm). This concentration is used for very dilute solutions such as water analysis or in biological preparations. (In these very very dilute solutions, the density of the solution is very near to that of water, i.e. 1.00 gm/ml ).

    parts per million (ppm) = mass of solute x 1,000,000/ mass of solution

    (The units used to express the mass of solute and mass of solution must be the same)

    Consider this example

    A water sample contains 3.5 mg of Arsenic (As ) salts in 825 ml of the water sample. What would be the parts per million (ppm) of the Arsenic salts in the sample. (Assuming that the density of the very dilute water sample is 1.00 gm/ml)

    SOLUTION

    825 ml of the sample weighs 825 gms or 825,000 mgs. Thus the ppm of As salt in the solution would be:

    ppm = 3.5 X 1,000,000 / 825,000 = 4.24

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    4.12 Molality
    Is defined as the number of moles of solute per kilogram of solvent. This method of expressing concentration is based on the mass of solute (expressed as moles) per unit mass (i.e. 1.00 kg) of solvent:

    molality = moles of solute /kilogram of solvent

    (Notice that solvent is in bold, it is a common error to take kilograms of solution instead)

    To prepare one molal aqueous solution of sodium sulfate, 1 mole of sodium sulfate (142.1 g) has to be dissolved in 1.000 kilogram (1000 g) of water, The total volume of the solution is not known. However, the mass of the solution can be found out by adding the mass of the solute and the mass of the solvent. (If the density of the solution is known then the total volume can be calculated). To express the concentration in terms of molality, the masses of solute and solvent only are required and their volumes are not involved.

    Consider this:

    What is the molality of a phosphoric acid solution containing 32.7 g of H3PO4 in 100 g of water.

    SOLUTION:

    The molality of the solution must express the concentration of H3PO4 as moles per kg of water. The molecular weight of H3PO4 is 98.0 gms/mole hence, the molality is calculated as:

    32.7 g of H3PO4 is equivalent to 32.7/98.0 moles of H3PO4 i.e 0.336 moles

    Thus there are 0.336 moles of H3PO4 in 100 g (or 0.1 kg)of water (the solvent here)

    Therefore molality is = 0.336/0.1 = 3.36 Molal solution

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    4.13 Molarity (abbreviated as M)
    Is defined as the number of moles of solute per liter of solution:

    molarity = moles of solute/liter of solution

    This method of expressing concentration is very useful when volumetric equipment ( i.e. graduated cylinders, burettes, etc.) is used to measure a quantity of the solution. From the volume measured, a simple calculation gives the mass of solute used.

    To prepare one liter of a one-molar aqueous solution of sodium sulfate, one mole of sodium sulfate (142.1 g) is dissolved in water. Enough water is then added to bring the volume of the solution to one liter in a volumetric flask, An important point to note here is that no information is stated as to the amount of solvent added, only that the solution is made to bring the total volume to one liter. The amount of water used can be calculated if the density of the solution is known.

    Consider this

    How many grams of sodium chloride (NaCl) is necessary to prepare 236 ml of 2.00 M sodium chloride solution.

    SOLUTION

    The molecular weight of NaCl is 58.5 gms/mole. In a 2.00 M NaCl solution, there are 2.00 moles NaCl per 1.00 liter of solution. The number of moles of NaCl necessary for preparing 236 ml of a 2.00 M solution is as:

    (236/1000) X 2 = 0.472 moles in 236 ml

    In gms 0.472 X 58.5 gms = 27.612 gms. are required

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    4.14 Normality (abbreviated as N)
    Is defined as the number of equivalents of solute per liter of solution:

    N normality = equivalents of solute/liter of solution

    The equivalent mass in grams (or one equivalent) of the solute is based on the reaction involved and is defined by either the acid-base concept or the oxidation-reduction concept, depending upon the ultimate use of the solution. However, in this text we shall limit our discussion of equivalents and normality to applications using the acid-base concept of equivalence.

    One equivalent of any acid is equal to the mass in grams of that acid capable of supplying 6.02 x 1023 (Avogadro's number) of hydrogen ions (i.e. 1 mole). One equivalent of any base is equal to the mass in grams of that base that will combine with 6.02 X 1023 hydrogen ions (1 mole) or supply 6.02 x 1023 hydroxide ions (1 mole). Thus, one equivalent of any acid will exactly combine with one equivalent of any base. One equivalent of any salt is defined by the reaction the salt undergoes and is equal to the mass in grams of the salt capable of supplying 6.02 x 1023 positive charges or 6.02 x 1023negative charges.

  • The equivalent mass in grams (one equivalent) of an acid is determined by dividing the molecular weight of the acid by the number of moles of hydrogen ion per mole of acid used in the reaction.
  • The equivalent mass in grams (one equivalent) of a base is determined by dividing the molecular weight of the base by the number of moles of hydrogen ions combining with 1 mole of the base used in the reaction.
  • The equivalent mass in grams (one equivalent) of a salt is determined by dividing the molecular weight of the salt by the number of moles of positive or negative charges per mole of the salt used in the reaction.
  • In all cases, the reaction must be considered.

    Since we are dividing the formula mass by whole numbers, a one normal solution (1.00 N) of a compound will then bear a certain whole number ratio to a one molar solution (1.00 M) of the same compound, One normal sodium chloride (NaCl) solution converted to molarity would be one molar, since there is only one equivalent in one mole of sodium chloride. However one normal sodium sulfate (Na2SO4) solution replacing both sodium ions converted to molarity would be 0.500 M, because there are two equivalents of sodium sulfate in one mole of sodium sulfate.

    To prepare a one normal aqueous solution of sodium sulfate where both sodium ions are replaced, dissolve one equivalent (142.1 g/2 = 71.0 g) in water. Add enough water to bring the volume of the solution to one liter

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