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  Solutions of Solids and Gases in Liquids, Chemistry tutorial

Introduction:

There is no doubt that a few variations exist among the properties of different solution types. 

Solutions of Solids in Liquids:

In the solutions of solids in liquids, the liquid is termed to as the solvent and the solid that is dissolved in it as the solute. Whenever a solid is added steadily to a particular amount of a liquid (that is, solvent) at the constant temperature, a state is reached whenever some of the solid remains un-dissolved. Then the solution is stated to be a saturated solution.

Solubility is the capability of a solid, liquid or gaseous chemical substance (termed to as the solute) to dissolve in the solvent (generally a liquid) and form a solution. The solubility of a substance basically based on the solvent used, and also pressure and temperature. The solubility of a substance in a specific solvent is measured via the concentration of the saturated solution. A solution is considered saturated whenever adding additional solute no longer raises the concentration of the solution.

The degree of solubility ranges broadly based on the substances, from infinitely soluble (completely miscible), such as ethanol in water, to poorly soluble, such as silver chloride in water. The word 'insoluble' is frequently applied to poorly soluble compounds. Under some conditions, the equilibrium solubility can be surpassed, yielding a supersaturated solution.

The solubility doesn't based on the particle size; given adequate time, even large particles will ultimately dissolve.

Whenever we try to dissolve a solid in a liquid, the attractive forces are at maximum in the solid. In order for the solid to dissolve in the liquid the solvent-solvent forces of attraction must be adequate to overcome the attractive forces that hold the solid altogether. In molecular crystals, the attractive forces are weak being of the London dispersion, dipole-dipole or hydrogen bonding kind. The solubility of molecular solids in the molecular solvents is again regulated via the like dissolves like principle.

Iodine dissolves in the carbon tetrachloride however not the water. The intermolecular attractive forces between I2 molecules are London dispersion kind as are the intermolecular attractive forces between the CCl4 molecules. Though, whenever we add the I2 to H2O the non-polar iodine molecules encompass a hard time separating the hydrogen-bonded water molecules. The water molecules don't interact as well by the I2 molecules as they do by themselves.

Whenever we add glucose, C6H12O6, to water it dissolves due to the 'like' intermolecular attractive forces (that is, hydrogen-bonding). We can write a chemical equation to explain the solution process for the molecular solids;

C6H12O6(s) + H2O → C6H12O6 (aq)

The solution comprises of glucose molecules distributed among the water molecules. This is significant to note that the glucose doesn't ionize though remains as a molecular species.

Whenever we try ionic solids we find out that some of the ionic solids are soluble in water and a few are insoluble in water. The ionic solids are insoluble in non-polar solvents. Ionic solids are held altogether via particularly strong electrostatic forces of attraction among the ions, in such a way that only the most polar solvents are capable to dissolve them.

The solubility of most of the ionic substances in water increases with temperature.  The solubility of sodium chloride rises to a very small level with the rise in temperature. The solubility of calcium acetate reduces with increase in temperature.  In most of the cases, if a solute is dissolved in a solvent, heat is absorbed that is, results in cooling. Then according to Le Chatelier's principle, if the temperature of a saturated solution in contact with the solute is increased, a change will occur in such a way that there is absorption of heat that is, all along the direction in which the cooling occurs. The solubility of the substance will thus increase with the rise in temperature.

The dissolution of several salts in water (example: calcium salts of organic acids) is accompanied through evolution of heat. Obviously, the solubility of such salts reduces with the rise in temperature.

Solutions of Gases in Liquids:

Most of the gases dissolve in water or some other liquids to a greater or less degree. In a gas, the molecules are far apart. After the dissolution in a liquid solvent, the molecules of the gas are much closer. It is just like saying that before a gas dissolves in the liquid, it should be condensed to form a liquid.  The condensation of a gas is an exothem1ic procedure. The enthalpy of condensation is bigger than the enthalpy of solution. Therefore, the dissolution of a gas is an exothermic process (that is, heat is evolved). The solubility of a gas in a liquid is computed in terms of absorption coefficient or the Bunsen coefficient. This coefficient has been named after the scientist, 'Bunsen', who introduced it.  

This is represented by 'α'. It is stated as the volume of a gas at standard temperature and pressure [273.15 K (0 °C) and 101.3 kPa] dissolved via unit volume of the solvent at the temperature of the experiment and under a pressure of 1.013 x 105 Pa. The absorption coefficients of certain gases are illustrated in the table shown below:

Absorption Coefficients at 293 K:

Solvent      Carbon (IV) Oxide      Hydrogen       Oxygen       Nitrogen

 Water                0.88                         0.018             0.028           0.015

Ethanol               3.00                         0.081             0.142           0.130

Benzene               ----                         0.060             0.165           0.105

Factors affecting the solubility of Gases:

The solubility of a gas in a liquid based on:

  • Temperature
  • Pressure and
  • Nature of the gas and the solvent.

1) Temperature:

The solubility of a particular solute in a particular solvent generally based on the temperature. For most of the solids dissolved in liquid water, solubility tends to correspond by increasing the temperature. Since water molecules heat up, they vibrate faster and are better capable to interact with and break apart the solute.

The solubility of gases exhibits the opposite relationship with temperature; that is, as the temperature rises, gas solubility tends to reduce. In a chart of solubility versus temperature, observe how solubility tends to raise with increasing temperature for the salts and decrease by rising the temperature for gases.

2) Pressure:

Pressure consists of a negligible effect on the solubility of solid and liquid solutes; though it consists of a strong effect on solutions with the gaseous solutes. This is obvious every time you open a soda can; the hissing sound from the can is due to the fact that its contents are in pressure, which makes sure that the soda stays carbonated (that is to state, that the carbon dioxide stays dissolved in the solution). The takeaway from this is that the solubility of gases tends to correlate by increasing the pressure.

3) Nature of the gas and solvent:

The crystalline substances comprise of a regular arrangement of atoms, molecules or ions; in the latter case, the forces which hold the crystal altogether are electrostatic in nature. For an ionic crystal to dissolve in water, the water molecules should be capable to shield the charges of the positive and negative ions from one other. The attractive forces among the ions in solution are less than those in the solid state as the solvent molecules; therefore, the ions behave more or less independently in solution. In common, the relative solubility of ionic substances are a measure of the magnitude of the electrostatic forces which hold the crystals altogether.

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