What is a solution?
Solution is defined as a homogeneous mixture of two or more components. The components of a solution are as follows:Solute
A solute is a substance which is present in minor quantities in a solution. It may be otherwise called as dispersed phase. In a solution of salt and water, salt is taken in a smaller proportion and hence it is the solute.Solvent
A solvent is a substance which is present in major quantity. It may be otherwise called the dispersed medium. In a solution of ethanol and water, if we take 100ml of ethanol and 10ml of water, then ethanol is the solvent and water the solute. In a solution both the solute and solvent can be in any phase like solid, liquid and gas.
There are different types of solutions based on their distinct properties. They are
- Aqueous Solutions
- Non aqueous Solutions
- Concentrated Solutions
- Dilute Solutions
- Saturated Solutions
- Unsaturated Solutions
- Supersaturated solutions
- Solid solutions
- Gaseous solutions
1. Non - Aqueous Solutions
It is a solution where the solvent is something other than water. For example, iodine in alcohol, acetone in benzene are example of non-aqueous solution.
2. Concentrated Solutions and Dilute Solutions
Between two solutions, the solute quantity may be different. The solution that has more proportion of solute is said to be concentrated. If the proportion of solute is less, the solution is a dilute solution. The concentration of a solution is expressed as the amount of solute present in a given amount (mass or volume) of the solution; or the amount of solute dissolved in a given mass or volume of a solvent. It is expressed as a percentage of these quantities.
3. Supersaturated solutions
The solubility of the salt will increase with increase in temperature. Hence we can add more solute to the saturated solution at an elevated temperature (nearly at its boiling point). A solution where the solute cannot be added further even at elevated temperature is called a super saturated solution. It is highly unstable and the solute will crystallize when the temperature is reduced.
4. Gaseous solution
Gases diffuse uniformly to form a homogeneous mixture. Air is such a homogeneous mixture of many gases like nitrogen, oxygen, carbon dioxide etc. A gaseous solution is a uniform solution of two or more non reactive gases.
The concentration of solution is expressed in different terms.
- Volume/volume percent: It is the volume of solute present in 100ml of solution. It is the most common method used when both solute and solvent are liquids. It is denoted by v/v. For example, if it is shown that the concentration of ethanol is 15% v/v, it means that 15ml of ethanol is present in 100ml of solution.
- Mass/mass percent: It is the mass of solute present in 100g of solution. It is denoted as w/w. It is commonly used when the solute is in solid state. For example, if it is shown that the concentration of glucose solution in a bottle as 10% w/w. It means that 10g of glucose is present in 100g solution.
- Moles per liter/Molarity: It is the number of moles of solute present in 1 liter of solution. It is expressed in moles/liter.
- Moles per kg of solvent/molality: It is the number of moles of solute present in 1kg of solvent. It is expressed in moles/kg.
- Mole fraction: It is the ratio of the number of moles of one component to the total number of moles in the system. Generally the sum of the mole fractions of all components is equal to 1.
Solubility is a measure that denotes the maximum amount of substance that can be dissolved in a solvent in a saturated solution. It is the amount of solute present in 100ml of a saturated solution at a particular temperature. It depends on temperature, nature of substance, pressure.
The other term closely related with solubility of the substance is 'solubility product'. It is the product of the concentration of ions produced in a solution at a particular concentration.
- Temperature: Temperature is directly related with solubility. When we increase the temperature, more energy is given to break the molecules. Hence the solubility increases on increasing the temperature.
- Nature of substance: In solubility 'like dissolves like', which means that non polar substances can dissolve well in non polar solvents like benzene etc. Similarly polar substances like sodium chloride will dissolve in polar solvents like water.
- Pressure: Pressure will affect the solubility of a gas in a liquid. Hence the solubility of gas is directly related to the pressure of the gas over the liquid. It is well known as Henry's law.
- The solution is homogeneous, which means that both solute and solvent are in the same state which cannot be differentiated easily.
- The solution is stable, which means that the solute particles will not settle down in time.
- The properties of a solution are different from the solute and solvent. In the case of solid solutions like alloys, the properties like melting point, refractive index are entirely different from the parent metals. Similarly for liquid solutions, the melting point and boiling point are different from a pure solvent.
In the solution if the solvent is water it is called an aqueous solution. For example
sodium chloride in water and copper sulphate in water are some examples for aqueous solution. Water is a universal solvent. Due to the high dielectric constant and high intermolecular space, the solutes can undergo either dissociative dissolving or undissociative dissolving. For example
sodium chloride breaks up as sodium and chloride ions in water but sugar molecules retain their identify in solution level also.
Saturated solution is a solution to which no more solute can be added at a particular temperature. As temperature affects the solubility of a substance, a saturated solution at lower temperature may become a dilute solution at higher temperature.
It is a solution to which more solute can be added to dissolve. They are also otherwise called dilute solutions.
The concentration of a solution is very low in an unsaturated solution.
It is the type of solution in which both solute and solvent are solids. For example
metal alloys are a homogeneous mixture of two or more metals and their properties are entirely different from the parent metals.
In a solution if both the solute and solvent are liquid they are called as liquid solutions. Ethanol in water, Benzene in carbon tetra chloride are example for liquid solutions.