Saturated and unsaturated solutions - MeroPaper

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Saturated and unsaturated solutions

Definition of Saturated and unsaturated solutions

When a Crystal of a solute  such as copper sulphate are mixed with water in a beaker. Each copper sulphate crystal is sourrounded by a number of water molecules and dispersed throughout the solution.

also read : Behaviour of saturated solutions

This process is known as dissolution. If little more copper sulphate crystal is added it also dissolves. When the process of dissolving the solute is continued in the fixed amount of water, some of the undissolved copper sulphate remains in the solution. Thus, there is a limit to the power of dissolving.

A solution which is unable to dissolve any more of the solute at a particular temperature is called saturated solution.

A saturated solution is that solution in which an equilibrium exists between dissolved solute and the undissolved excess solute.

A solution which can dissolve any more solute at the same temperature is known as unsaturated solution. Generally, a saturated solution becomes unsaturated if the temperature is raised because more of the substance is soluble at the higher temperature.

At lower temperature dissolving power of the solvent as lower. When the temperature of saturated solution, prepared at higher temperature is allowed to cool, the extra amount of solute that was dissolved during heating will be thrown out as crystals. But it takes some time to do so and during this interval of time the solution holds more amount of solute than required to saturate the solution at that temperature. Such as solution is known as supersaturated solution.

In other words, a solution which contains more amount of solute than required to saturated the solution at that temperature is known as supersaturated solution.

Saturated and unsaturated pogil

To Test whether a given solution is saturated, unsaturated or supersaturated at a given temperature a crystal of solute is added to the sample solution. If the Crystal dissolves, The solution is unsaturated, if the crystal of solute is added to the sample solution. If the crystal dissolves, the solution is unsaturated, if the crystal doesnot dissolve, the solution is saturated, and if the crystal grown in size, it is supersaturated.

Concentration of solutions

Solubility

A given amount of solvent can dissolves a particular amount of solute at a given temperature. The solubility of a substance is defined as the amount of solute in grams which when dissolved in 100 grams of a solvent from a saturated solution at that particular temperature.

Solubility= Weight of solute in grams/ Weight of solvent in grams *100

For example, The solubility of copper sulphate at 20 Degree celcious is 20.7. It means, at 20 degreec 100gm of water can dissolves 20.7gm of copper sulphate to a given a saturated solution.

Determination of solubility

The solubility of a solute can be determined by preparing a saturated solution of the given solute at laboratory temperature. Then the solution is filtered into a clean porcealain basin, The weight of which is previously determined. The weight of the basin with saturated solution is determined and the solution is evaporated to dryness and weighted again. The process of heating, cooling and weighing is repeated till the constant weight is obtained. The solubility of the solute is then calculated as follows.

Let, weight of empty porcelein basin= w1gm

Weight of basin+saturated solution = w2gm

weight of basin + solute = w3gm

: Weight of solution= (w2- w1) gm

weight of solute = (w3-w1)gm

weight of solvent = (w2-w1)-(w3-w1) gm = (w2-w3)gm

(w2-w3)gm of solvent dissolves= (w3-w1) gm of solute.

100 gm of solvent dissolves = w3-w1/w2-w3* 100 gm

solubility = w3-w1/w2-w3*100

Effect of temperature on solubility( solubility curve)

effect of temperature on solubility
effect of temperature on solubility

The way of expressing the relationship between solubility and temperature is to use a simple line graph. A curve obtained by plotting the solubility of a substance at different temperatures against these temperatures is known as the solubility curve. Such solubility curves for KNO3, NANo3, Na2SO4, NAcl, Ce2(SO4)3 are shown in figure. From the curve it is clear that the solubility of the most substances increases with the rise in temperature. But the effect of temperature is not same for all the substances. For example, The solubility of potassium nitrate and sodium nitrate increase in temperature while that of sodium chloride increase very slightly. On the other hand there are certain substance whose solubility decrease with increase in temperature.

A break in the solubility curve indicates change in the composition of the solute. For example, in case of the solubility curve of Na2so4.10H20, it is due to the formation of anhydrate.

Uses of Solubility curve

Solubility curve gives the following informations:

  1. IT can be used to find the solubility of a substance at a desired temperature.
  2. Idea about the variations of solubility with temperature.
  3. Helps to compare the solubility of a number of substances at different temperature.
  4. Helps to predict which substance will crystallise out first when a solution of two or more solutes is evaporated or cooled.
  5. Informs the temperature at which the change in composition of a solute takes place.

Colligative properties

Colligative properties are the properties of dilute solutions which depend entirely upon the number of the particles of the solute contained in definite amount of a given  solvent and not all upon the nature of the solute. The four important colligative properties shown by dilute solutions are,

  1. lowering of vapour pressure
  2. Elevation of boiling point
  3. Depression of freezing point
  4. Osmotic pressure

also read : Seprations of mixture questions

 

 

 

 

 

 

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