Distinguishing Between Solutions

Our Objective

To prepare:

  • A true solution of common salt, sugar and alum
  • A suspension of soil, chalk powder and fine sand in water
  • A colloidal of starch in water and egg albumin in water

Distinguish between the above solutions on the basis of:

  1. Transparency
  2. Filtration criterion
  3. Stability 

The Theory

What is a solution?

A Solution is a homogeneous mixture of two or more substances. The component that is present in a larger amount is called the solvent. The component present in a smaller quantity is called the solute. 

The maximum mass of substance that is dissolved in 100 g of a solvent under a particular condition of temperature and pressure (for example, the conditions of room temperature and pressure) is referred to as the solubility of that substance in that solvent under the specified conditions. This solution so obtained is said to be a saturated solution.

What is meant by a saturated solution?

It means that no more solute is dissolved in the solvent at the given temperature and pressure. Solubility varies with temperature and pressure. For example, solubility of salt/sugar increases with the rise in temperature. The solution in which no more solute is soluble in the solvent, at given temperature and pressure conditions is said to be a saturated solution under those specified conditions. 

The solutions which we generally come across are not saturated. They contain a smaller amount of solute. The compositions of such solutions are expressed as concentration, which refers to the quantity of a solute dissolved in a definite weight or volume of the solution. The concentration may be expressed in terms of either mass or moles. The number of moles of a solute present in one litre of a solution is called the molarity of that solution. If 58.5 g NaCl (Na = 23 + Cl = 35. 5) is present in 1000 ml of a solution, it will be known as 1M (1 molar) solution of NaCl. 

Dispersion may give rise to a homogeneous (solution) or heterogeneous material (a suspension or a colloid). The distinction

lies in the size of the dispersed particles. In suspension, the particle size is of the order of 10-5 cm or larger. The particles can be seen distinctly in the dispersion medium (solvent) by the naked eye or under an ordinary microscope. The suspensions are not very stable. The suspended particles may settle down after sometime, e.g.,muddy water or smoke in the air. 

Colloids have  particles smaller than 10-5 cm (generally 10-7 to 10-5 cm). The dispersed particles show very little tendency to separate. Colloid particles cannot be seen by the naked eye or under an ordinary microscope. Colloids are stable. Gum, milk and blood are typical colloids. 

In a solution, the particles are of molecular dimensions, smaller than 10-7 cm. Solutions are transparent and homogeneous. 

In a solution, mass percentage of solute is equal to 100 minus mass percentage of solvent 

        
 

To find out mass percentage of solute in its solution in water, we apply the following formula:

            

The mass of water is always equal to its measurement in ml at room temperature and pressure. For example, mass of 50 ml of water is equal to 50 g because 

  

Learning Outcomes

  1. Students understand the terms: true solution, suspension, colloid, transparency, filterability, stability, etc.
  2. Students distinguish true solutions, suspensions and colloids based on experiments testing:
    • Transparency
    • Filtration
    • Stability
  3. Students classify the mixtures given to them as true solutions, suspensions and colloids based on the information from the experiment.
  4. Students acquire skills to perform the experiments for testing the transparency, filtration criterion and stability of true solutions, suspensions and colloids.

 

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