Is Matter Around Us Pure: Important Questions And Solutions

Very Short Question-Answer (1 Mark)

Ques. List the two conditions essential for using distillation as a method for the separation of the components from a mixture. (CBSE 2011) 

Ans: Two conditions:-

The elements of the mixture must be mixable with each other.

Both the mixable elements are liquids, they must differ in boiling point by more than 25ºc

Ques. Identify solute and solvent in ‘tincture of iodine. ( NCERT) 

Ans: Alcohol is the solvent and Iodine is the solute.

Ques. Identify the solutions among the following mixtures. (CBSE 2015) 

1. Soil
2. Seawater
3. Air
4. Coal
5. Soda water.

Ans: The following mixtures are

Soda water, air, seawater

Ques. What is the mass percent of a solution? (NCERT) 

Ans: the mass in grams of the solute present in one hundred grams of a solution is known as the mass percent of a solution.

Ques. Define crystallisation. (CBSE 2012) 

Ans: A process that separates a pure solid in the form of its crystals from a solution known as crystallisation.

Ques. Why particles in a true solution cannot be seen with a naked eye? (CBSE 2011) 

Ans: No, it cannot be seen with naked eyes because the particles are too small to scatter the beam of light. Also, the particle size in a true solution is less than 10-9 or 1 nm.

Ques. Sucrose (sugar) crystals obtained from sugarcane and beetroot are mixed together. Will it be a pure substance or a mixture? Give reasons for the same. [NCERT Exemplar]

Ans: Chemical composition of sugar crystals is the same irrespective of their source. So, it is a pure substance.

Check Important Units for SI Units

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Short Question-Answer ( 2 Marks )

Ques. Try segregating the things around you as pure substances or mixtures. (NCERT) 

Ans: Mix both the soil and water and then separate them. Water is a pure substance because it's made up of one kind of substance whereas soil is a mixture because it contains more than one substance.

Ques. a) Identify the heterogeneous mixture from the following :

Air, soda water, soap solution, brass.

1. b) Write two components of a colloidal solution. Give an example. (CBSE 2011) 

Ans:  a) Soap is a heterogeneous mixture while the air, soda water, and brass are homogenous in nature.

1. b) Dispersed phase and dispersion medium are the two components of colloidal solution.

Example – A colloidal solution of starch in water. ( Water act as the dispersion medium and starch act as the dispersed phase.

Ques. Define a solution. If 10 ml of H₂SO₄ are dissolved in 90 ml of water, calculate the concentration of the solution. (CBSE 2011) 

Ans: Volume of water (solvent) = 90 ml

Volume of (solute) = 10 ml

Volume of H₂SO₄ = (90 +10) mL

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

= ( 10 ml )/( 100 ml ) X 100

Concentration of solution = 10%

Ques. Water is a compound and not a mixture. Justify the statement by giving two reasons. (CBSE 2010, 2011) 

Ans: two reasons are:-

1. Hydrogen (H) and oxygen (O) are compounded chemically to form H₂O i.e. water that boils at a fixed temperature.
2. The properties of water are completely different from the constituting elements.

For example – water extinguishes fire while molecular hydrogen (H₂ ) is combustible and molecular oxygen (O₂ ) supports combustion.

Ques. Distinguish between elements and compounds with one example of each. (CBSE 2010) 

Ans: A compound can be broken apart which results in a chemical reaction or bypassing electric current (electrochemical reaction) whereas an element cannot be broken into simpler or smaller constituents.

Eg – sodium chloride ( compound ), sodium ( elements ).

Ques. Calculate the mass of sodium sulphate required to prepare its 20% (mass percent) solution in 100 g of water. [NCERT Exemplar] 

Ans: a mass of sodium sulphate needed = x g.

Mass of solution should be = ( x+100)g

20= x/x +100 X 100

= 20x + 2000 = 100x

80x = 2000

x = 2000/80 = 25g

Also Check: Physics Practical

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Long Question-Answer ( 3 Marks )

Ques. A solution contains 60 g of sugar in 480 g of water. Calculate the concentration of the solution in terms of mass by the mass percentage of the solution. (CBSE 2010, 2012)

Ans: Mass of solvent ( water) = 480g

Mass of sugar ( solute ) = 60g

Mass of solution = 60 + 480

= 540g

Concentration in term of mass percentage

= mass of solute/mass of solution X 100

= 60g/540g X 100

= 11.1 %

So, the answer is 11.1 %

Ques. A solution contains 50 g of sugar in 450 g of water. Calculate the concentration in terms of mass by the mass percentage of the solution. (CBSE 2015)

Ans: The mass percent of sugar = mass of sugar/ mass of solution X 100

= (50g)/(500g) X 100

= 10%

Mass percent of water = 100-10 = 90%

So, the answer is 90%

Ques. a) What is a colloid?

b)What are its various properties? (NCERT) 

Ans:  The heterogeneous mixture of substances is colloids, which cannot be seen by the naked eyes because the particle size is too small.

The various properties are:-

1. It is a heterogeneous mixture but looks homogenous.
2. They make their path visible by scattering the beam of light passing through it.
3. It cannot be seen by naked eyes as the size of the particles is too small.
4. When the colloid is left undisturbed, its components do not settle down.

Read More: Physics Law Formulas

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Very Long Question-Answer ( 5 Marks )

Ques. Sudha tested the solubility of four salts X, Y, Z, and T at different temperatures and collected the following data. (Solubility refers to the amount in grams dissolved in 100 g of water to give a saturated solution.)

Answer the following questions from the table

1. Which salt has the highest and lowest solubility at 323 K?
2. A student prepared a saturated solution of X at 323 K and then added 25 g of water to it. What mass of X must be added to again make the solution saturated?
3. The solubility of which salt is least affected by an increase in temperature?
4. What mass of ‘T’ would be required to make a saturated solution in 200 g of water at 290 K? CBSE 2011) 

Ans: 

1. At 323 K, salt Y has the highest solubility in the water while salt Z has the lowest solubility.
2. Saturated solution definition

=100 g of water at 323K contains salt = 40g

=125 g of water at 323 K contain salt = (40g)/(100g) X (125g) = 50g

= mass of salt to be added to make the solution again saturated = 50g – 40g

= 10g

1. The survey shows that the solubility of salt Y is least affected by an increase in temperature.
2. At 290 K, the mass of T is required to make the saturated in 200g of water

=(25g)/(100g) X ( 200g) = 50g

Ques. Iron filings and sulphur were mixed together and divided into two parts ‘A’ and ‘S’. Part ‘A’ was heated strongly while Part ‘S’ was not heated. Dilute hydrochloric acid was added to both the parts and evolution of gas was seen in both the cases. How will you identify the gases that evolved? ( NCERT IMP QUESTION ) 

Ans: PART A

Fe(s)+S(s) → FeS(s)

FeS(s) + 2HCl(aq) → FeCl₂(2aq) + H₂S(g)

Part B

Fe(i) + S(s) → intermix of iron filings and sulphur

When dilute HCl is impart in it

Fe(s) + S(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Sulphur remains unreacted.

H₂S gas formed has a pongy smell and on passing through lead acetate solution, it turns the solution black. Hydrogen gas burns with a pop sound.

Ques. Classify each of the following as a physical or a chemical change. Give reasons.

1. Drying a shirt in the sun.
2. Rising of hot air over a radiator.
3. Burning of kerosene in a lantern.
4. Change the colour of black tea by adding lemon juice to it.
5. Churning of milk cream to get butter. [NCERT Exemplar] 

Ans: 

1. The water is converted from its liquid state to a gaseous state because of the sunlight or sun’s heat, so it is a physical change.
2. Water in the radiator is converted from a liquid state to a gaseous state which is a physical change.
3. It is a chemical change as the combustion of kerosene occurs and new products are formed.
4. There is a reaction between the citric acid in the lemon and the compounds of tea resulting in informing of a new product because of which there is a chemical change.
5. It is a physical change because of which there is a chemical change.