Topic outline

    • Is Matter Around Us Pure 

      Q79. How can we separate cream from milk? 

      Ans. Steps

      1. Take some full-cream milk in a test tube.

      2. Centrifuge it by using a centrifuging machine for two minutes. 

      3. The centrifugal force acts on the milk and due to this, the milk separates into cream and skimmed milk. 

      4. The cream being lighter floats over the skimmed milk and can then be removed.


      Q80. Non-metals are usually poor conductors of heat and electricity. They are non-lustrous, non-sonorous, non-malleable and are coloured.

      (a)   Name a lustrous non-metal.

      (b)   Name a non-metal which exists as a liquid at room temperature.

      (c)   The allotropic form of a non-metal is a good conductor of electricity. Name the allotrope.

      (d)  Name a non-metal which is known to form the largest number of compounds.

      (e)   Name a non-metal other than carbon which shows allotropy.

      (f)   Name a non-metal which is required for combustion.


      (a) Iodine 

      (b) Bromine 

      (c) Graphite 

      (d) Carbon 

      (e) Sulphur, Phosphorus 

      (f) Oxygen 


      Q81. Which of the following are not compounds?

      (a) Chlorine gas

      (b) Potassium chloride

      (c) Iron

      (d) Iron sulphide

      (e) Aluminium

      (f) Iodine

      (g) Carbon

      (h) Carbon monoxide

      (i) Sulphur powder

      Ans. These are not compounds

      (a) Chlorine gas                                      

      (c) Iron

      (e) Aluminium                                        

      (f) Iodine

      (g) Carbon                                               

      (i) Sulphur powder


      Q82. Arun has prepared 0.01% (by mass) solution of sodium chloride in water. Which of the following correctly represents the composition of the solutions?

      (a) 1.00 g of NaCl + 100 g of water

      (b) 0.11 g of NaCl + 100 g of water

      (c) 0.01 g of NaCl + 99.99 g of water

      (d) 0.10 g of NaCl + 99.90 g of water

      Ans. Mass % of solution 

      = Mass of Solute/(Mass of Solute + Mass of Solvent) x 100

      (a) 1.00g/(1.00g+100g) x 100 = 0.99 %

      (b) 0.11g/(0.11g+100g) x 100 = 0.1098 % = 0.11 %

      (c) 0.01g/(0.01g+99.99g) x 100 = 0.01 %

      (d) 0.10g/(0.10g+99.90g) x 100 = 0.10 %


      Q83. What are the properties of non-metal?

      Ans. Non-metals usually show some or all of the following properties:

      • They display a variety of colours.

      • They are poor conductors of heat and electricity.

      • They are not lustrous, sonorous or malleable.

      Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc.


      Q84. What is a solution?

      Ans. A solution is a homogeneous mixture of two or more substances. Lemonade, soda water etc. are all examples of solutions. Usually we think of a solution as a liquid that contains either a solid, liquid or a gas dissolved in it. But, we can also have solid solutions (alloys) and gaseous solutions (air). In a solution there is homogeneity at the particle level. For example, lemonade tastes the same throughout. This shows that particles of sugar or salt are evenly distributed in the solution.


      Q85. During an experiment the students were asked to prepare a 10% (mass/mass) solution of sugar in water. Ramesh dissolved 10 g of sugar in 100 g of water while Sarika prepared it by dissolving 10 g of sugar in water to make 100 g of the solution.

      (a) Are the two solutions of the same concentration?

      (b) Compare the mass % of the two solutions.

      Ans. (a) No, the two solutions do not have the same concentration.

      (b) Mass % of solution prepared by Ramesh 

      = Mass of Solute/Mass of Solution x 100 

      = 10/(10+100) x 100 = 9.09 %

      Mass % of solution prepared by Sarika

      = Mass of Solute/Mass of Solution x 100 

      = 10/(100) x 100 = 10 %

      The solution prepared by Ramesh has less percentage (9.09%) by mass than that of Sarika (10%).


      Q86. Explain why particles of a colloidal solution do not settle down when left undisturbed, while in the case of a suspension they do.

      Ans. The size of colloidal particles in a colloidal solution is smaller than suspension. Because of the small size of colloidal particles, we cannot see them with naked eyes. These particles are in a random motion hence do not settle down when left undisturbed. The particles of suspension are bigger and can be seen by the naked eye. They tend to settle down under the effect of gravity.


      Q87. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.

      Ans. Steps

      1. Use water as a solvent and boil it for few minutes.

      2. Add half a tea-spoon of tea-leaves, it is insoluble in water.

      3. Add milk which is also soluble in water and boil.

      4. Now add sugar which is soluble in water.

      5. Boil it for few minutes.

      6. Filter the tea with the help of strainer, the tea collected in cup is filtrate and the tea leaves collected on the strainer is residue.


      Q88. On heating, calcium carbonate gets converted into calcium oxide and carbon dioxide.

      (a) Is this a physical or a chemical change?

      (b) Can you prepare one acidic and one basic solution by using the products formed in the above process? If so, write the chemical equation involved.

      Ans. (a) This is a chemical change because the composition of product formed is different from the substance taken.

      (b) When calcium oxide (CaO) dissolves in water, it forms calcium hydroxide which is basic solution.

      CaO + H2 Ca(OH)2

      When carbon dioxide (CO2) is dissolved in water, it forms carbonic acid which is acidic solution.

      CO2 + H2 H2CO3