Properties of Solution

Let us first know what is a solution. Solutions are homogeneous mixtures of two or more substances, containing very small sized solute particles. They do not scatter light; its particles cannot be seen by naked eyes. A solution is the basis for many products that are used in daily life like shampoos, glue, soda, and medicines. Molarity (number of moles per litre) is employed to measure the concentration of a solution.

Solute is that component (usually solid) that gets dissolved in a solvent to produce a homogeneous mixture. The amount of solute is usually less in quantity compared to the solvent. The concentration represents the amount of solute present in a chemical solution with respect to the amount of solvent. Some examples of solutes include sodium chloride, sugar (See figure 1), and carbon dioxide.
Dissolution of a solute in a solvent to form a solution                                                         

The term 'solvent' usually refers to a substance that dissolves the solute. It can be solid, liquid or gas. Solvent comprises a major part of the solution compared to solute. Some common examples are as follows:

• Water
• Ethanol
• Cyclohexane
• Heptane
• Xylene
• Hexane
• Toluene
• Acetone
• Petroleum spirits
• Dichloromethane
• 1.1.1-trichloroethane
• Methanol
• Trichloroethylene
• Methyl ethyl ketone
• Perchloroethylene
• White spirit
• Chloroform
• Pentane
• Tetrachloroethylene
• Benzene
• Turpentine

Strength of Solutions

In general, “solubility” is a chemical property referring to the ability for a given substance, the solute, to dissolve in a solvent at any given temperature. A solution is said to be unsaturated as long as more solute can be dissolved in the solvent.

Dilute or Weak Solution – This solution contains a minor quantity of solute dissolved in solvent.

Concentrated Solution – This solution contains relatively larger amount of solute than the quantity of same solute that would be present in the dilute solution.

Saturated Solution – In saturated solution, the quantity of solute dissolved in solvent is the maximum possible, keeping the existing temperature and pressure constant. All the dissolved and undissolved solutes stay at equilibrium, leaving no possibility of dissolving any more solute at this temperature and pressure.

Supersaturated Solution – This solution is more concentrated with solute than does a saturated solution, such that the solutes are not in equilibrium with the solvent. Carbonated water is supersaturated solution such that the moment the pressure is released the liquid fizzes out and gradually loses the dissolved gas till it attains equilibrium.

Water is the Universal Solvent -Water is a good solvent. This property of water is because of its polarity. It is important in biology, since the biochemical reactions occur when reactants are in aqueous state. Sometimes, however, water may not help to dissolve substances. For example, oil is not miscible in water; water and oil form an emulsion when shaken briskly, instead of getting properly mixed.

Solutions are differentiated into various types based on the following criteria:

• The amount of solvent added

Types of solutions based on presence or absence of water:

The solutions which have water as the solvent are called “aqueous” solutions. An example is a solution of salt and water. The substances that are dissolved in water are separated into individual components called ions. In saltwater, the ionization results in sodium (Na⁺) and chloride (Cl^-) ions. Svante Arrhenius identified the ionic nature of liquid solutions and was awarded the Nobel Prize for his contribution in the year 1903. Water has polarity and is considered an ideal solvent and this property is important for many biochemical reactions. Although water is called a universal solvent it cannot dissolve many compounds like oil.
Some solutions have solvents other than water like benzene, ether, methanol, and petrol and are called non-aqueous solutions. An example of such a non-aqueous solution is sulphur dissolved in carbon disulphide.

Types of solutions based on the amount of solute:

• Unsaturated solution: The solution can take in more solute at a definite temperature.

• Saturated solution: It is a solution that has the maximum amount of solute and cannot dissolve any more of it in the solvent at the present temperature and pressure. In chemistry, the term “solubility” refers to the maximum amount of solute that dissolves in a solvent at equilibrium – this means that no more solute can be dissolved in the solvent in the set conditions (temperature, pressure). Such a solution is called a saturated solution. Solubility is affected by temperature, pressure and molecular structure. An increase in temperature can increase the solubility of many compounds.

• Supersaturated solution: In this type of solution, the amount of solute is in excess quantities than what the solvent can dissolve at that temperature and pressure. Now if the solution is heated or excess pressure is applied, the excess solute is dissolved. However, at normal temperatures and pressures, this excess solute will crystallize and get deposited at the bottom. A supersaturated solution is normally unstable.

Types of solutions based on the quantity of solvent:

• Concentrated solution: This means that a lot of solute is dissolved in the solvent. Since the solubility depends on temperature, a solution that is concentrated at one temperature may not be concentrated at a higher temperature.

• Dilute solution: These have a small amount of solute dissolved in a large amount of solvent.

Examples of Solutions:

Liquid solutions

Liquids solutions can dissolve all forms of matter including solids, liquids, and gases as they have liquid as the solvent. Some examples of liquid solutions are provided below:

• Gas in liquid – Soda water (made of carbon dioxide and water), oxygen in water.

• Liquid in liquid – Vinegar (made of acetic acid and water), and alcoholic beverages (made of ethanol in water). Gasoline is another common example of a liquid solution.

• Solid in liquid – Seawater (made of salt and water), sucrose solution (made of sugar in water).

Solid solutions

In these solutions, the solvent is solid and can dissolve all forms of matter including solids, liquids, and gases. Some examples of solid solutions are provided below:

• Liquid in solid – Amalgamation of mercury in gold, the formation of moist solids and the dissolution of hexane in paraffin wax.

• Solid in solid – Brass (Zinc and Copper), steel, brass alloy and polymers.

• Gas in solid – Dissolution of hydrogen gas in metals like palladium.