Chapter 2: Is Matter around us Pure?

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Is Matter around us pure?


Q.3: Pragya tested the solubility of three different substances at different temperatures and collected, the data as given below (results are given in the following table, as grams of a substance dissolved in 100 grams of water to form a saturated solution).

(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K?

(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain.

(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

(d) What is the effect of change of temperature on the solubility of a salt?


(a) The solubility of potassium nitrate at 313K is 62. This means that at 313 K, 62g of potassium nitrate is dissolved in 100g of water.

∴ Mass of potassium nitrate in 50g water =

 =frac{62}{100}	imes50=31g

Therefore, the mass of potassium nitrate will be 31 g.

(b) The solution which contains the maximum concentration of a solute that is dissolved in a solvent is called a saturated solution. The maximum solubility of potassium chloride is at 353K. So, when the saturated solution of potassium chloride is cooled at a temperature below 353K, the solubility of potassium chloride decreases, and hence, it separates as a crystal.

(c) Solubility is the ability of a compound to dissolve in a solvent. The solubility of salt in 100g of water at 293K is as follows:

(i) Potassium nitrate = 32g

(ii) Sodium chloride = 36g

(iii) Potassium chloride = 35g

(iv) Ammonium chloride = 37g

This means that the solubility of ammonium chloride is highest at 293K.

(d) The solubility of salt in water increases with an increase in temperature because the intermolecular forces of attraction are disrupted with an increase in temperature and hence salt molecules can mix easily with the water molecules.

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