## Topic outline

• ### Chapter 2 - Is Matter Around Us Pure - 7

• Is Matter Around Us Pure

Q89. Suggest separation technique(s) one would need to employ to separate the following mixtures.

(a) Mercury and water

(b) Potassium chloride and ammonium chloride

(c) Common salt, water and sand

(d) Kerosene oil, water and salt

Ans. (a) Mercury and water - Separating funnel

(b) Potassium chloride and ammonium chloride - Sublimation

(c) Common salt, water and sand – Filtration will separate salt solution (salt and water) and sand. Evaporation of salt solution will separate salt from the solution.

(d) Kerosene oil, water and salt - Separating funnel will separate kerosene oil and salt solution. Evaporation of salt solution will separate salt and water.

Q90. A solution contains 40 ml of alcohol mixed with 120 ml of water. Calculate the concentration of this solution.

Ans. Volume of alcohol (solute) = 40 ml

Volume of water (solvent) = 120 ml

Volume of solution = Volume of solute + Volume of solvent

= 40 + 120 = 160 ml

Concentration of solution = volume of solute/volume of solution x 100

= 40/160x100

= 25%

Q91. The teacher instructed three students ‘A’, ‘B’ and ‘C’ respectively to prepare a 50% (mass by volume) solution of sodium hydroxide (NaOH). ‘A’ dissolved 50 g of NaOH in 100 mL of water, ‘B’ dissolved 50 g of NaOH in 100 g of water while ‘C’ dissolved 50 g of NaOH in water to make 100 ml of solution.

Which one of them has made the desired solution and why?

Ans. Mass by volume% = mass of solute/volume of solution x 100

50               = mass of solute/100ml x 100

(50x100)/100  = mass of solute

Mass of solute = 50 g

In the given question, student ‘C’ has made it correctly because 50% (mass by volume) means 50 g of solute for every 100 ml of solution

Q92. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.

Ans. Mass of solute (sodium chloride) = 36 g

Mass of solvent (water) = 100 g

Mass of solution = Mass of solute + Mass of solvent

= 36 g + 100 g = 136 g

Concentration = mass of solute/mass of solution x 100

= 36g/136g x100

= 26.47%

Hence, the concentration of solution is 26.47%

Q93. Differentiate between mixtures and compounds.

Ans.

 Mixtures Compounds 1. Elements or compounds just mix together to form a mixture and no new compound is formed. 1. Elements react to form new compounds. 2. A mixture has a variable composition. 2. The composition of each new substance is always fixed. 3. A mixture shows the properties of the constituent substances. 3. The new substance has totally different properties. 4. The constituents can be separated fairly easily by physical methods. 4. The constituents can be separated only by chemical or electrochemical reactions.

Q94. State the properties of solution.

Or

What are the properties of solution?

Or

What are the characteristics of a solution?

Ans. Properties of a solution

1. A solution is a homogeneous mixture.

2. The particles of a solution are smaller than 1 nm (10-9 metre) in diameter. So, they cannot be seen by naked eyes.

Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.

3. The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.

Q95. What would you observe when:

(a) a saturated solution of potassium chloride prepared at 60°C is allowed to cool to room temperature

(b) an aqueous sugar solution is heated to dryness

(c) a mixture of iron filings and sulphur powder is heated strongly?

Ans. (a) When a saturated solution of potassium chloride prepared at 60°C is allowed to cool at room temperature, crystals of potassium chloride will be formed.

(b) On heating sugar solution, water will evaporate first. Once the solution dries up, sugar gets charred and it turns black.

(c) Iron sulphide is formed when a mixture of iron filings and sulphur is heated strongly.

Q96. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25°C), which are miscible with each other?

Ans. A mixture of kerosene and petrol which are miscible with each other can be separated by distillation.

1. Take the mixture in a distillation flask. Fit it with a thermometer.

2. Heat the mixture slowly keeping a close watch at the thermometer.

3. The petrol vaporises, condenses in the condenser and can be collected from the condenser outlet.

4. Kerosene is left behind in the distillation flask.

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