PHYSICAL AND CHEMICAL CHANGES COMPLETE QUESTION ANSWERS | SCIENCE CLASS 7 ,CHAPTER 6

 b) Represented by symbols of elements and the number of atom of each element .

c) Eg: H2O , CO2  etc.       

3. What does the following equation explain 

       2H2O ——> 2H2 + O2

Ans: The following equation explains that two atoms of Hydrogen and one atom of Oxygen are produced from the compound water. 

4. Write the names of the following compounds.

    i) MgO      ii) HCl      iii) NaOH     iv) SO2    v) H2SO4

Ans:    i) Magnesium oxide

            ii) Hydrochloric acid.

            iii) Sodium hydroxide.

            iv) Sulphur dioxide. 

            v)  Sulphuric acid. 

V. 1. What are the evidences of a chemical reaction . 

Ans: Few evidences of a chemical  reactions are – 

         a) Production of gas :  When baking powder and lemon juice reacts with each other they produce carbon dioxide gas  and some other substances . Now, the bubbles which we see serves as an evidence for production of gas . 

        b) Formation of precipitate : When colourless solution of potassium iodide and lead nitrate react with each other a coloured substance settles at the bottom and is called precipitate . This precipitate formed acts as an evidence . 

       c) Change in temperature : When wood is burnt and  fireworks are exploded they produce heat , light and even sound . This acts as an  evidence for changes in temperature. 

       d) Change in colour : A chemical reaction between two colourless liquid gives  rise to  a new colourless liquid , but changes  in  colour are also seen in some chemical reactions and this acts as an evidence. 

  2. List difference between physical and chemical changes. 

  Ans:  Physical changes                                                                      

          a) Changes which doesn’t   give rise to a new substance.                                            

          b) These are reversible changes.                                                                                                                                   

          c) Substances formed have some properties.        

      Chemical changes             

a) Changes which give rise to a new substance. 

b) These are non – reversible .

 c) Substances formed have different properties.                                                                                                               

3. Name the types of chemical reactions . Explain any one of them with a chemical equation.

Ans: The types of chemical reactions are –

           a) Synthesis or combination reaction.

          b) Decomposition reaction.

           c) Single decomposition reaction.

          d) Double decomposition reaction. 

                                 Synthesis / Combination reaction : In a synthesis reaction two or more substances combined to form one substance.

                          Eg – 2H2 + O2 ———>  2H2O

       Here,  Hydrogen  and Oxygen react to form water. 

 4. Explain crystallisation .

Ans:  The process of forming crystals is called crystallisation. This process is used to obtain the pure solid form of certain substances. Pure sugar , urea, alum etc form crystals . 

                Crystals can be obtained by the following ways – i) First , the substance to be crystallized is dissolved in a solvent to obtain a saturated solution .

 ii) The saturated solution is  now filtered to remove impurities. 

 iii) Lastly , the solution is cooled to form crystals .