PHYSICAL AND CHEMICAL CHANGES

POINTS TO REMEMBER
  1. Changes are taking place rapidly in our surrounding. All those changes are broadly classified into two types-i.e., physical change and chemical change.
  2. Physical properties: Properties such as shape, size, colour and state of substance are called its physical properties.
  3. Physical change: A change in which substance undergoes a change in its physical properties is called a physical change.                       
   
                          (a) Stretching of spring                                                                            (b) Change is states of water

                                                                  Fig. 6.1. Physical changes

 4.   Chemical change:
These are the changes in which chemical properties of substances are changed and new substances are formed                                      .



                   
                       (a) Charring of sugar of heating                                                                    (b) Burning of paper
 
  5.  Characteristics of physical changes:
  1. No new substances are formed.
  2. Products are identical to the reactants.
  3. These changes are reversible.
 
  6.  Characteristics of chemical changes:
  1. Properties of products are different from the properties of reactants.
  2. Most of the chemical changes are irreversible.
  3. These changes always result in energy changes.
 
   7.  Reactants: Substances which are present before a reaction (or change) are called reactants.
 
   8.  Products: Substances which are formed after the reaction (or change) are called products.
   
   9.  Chemical reaction: The process of converting reactants into products by applying certain criteria is called a chemical reaction.
 
 10.  In addition to new products the following may accompany a chemical change.
  1. Heat, light or any other radiation may be given off or absorbed.
  2. Sound may be produced.
  3. A change in smell may take place or a new smell may be given off.
  4. A colour change may take place.
  5. A gas may be formed.
 
11.  Rusting of iron:  if a piece of iron is left open for sometime, it acquires a coating of brownish substance. This substance is called rust and the process is called rusting.
       Rusting takes place in the presence of both oxygen and water (or water vapour). One simple   way to prevent rusting is to apply a coat of paint or grease over the metallic items.
       Another way is to coat a layer of a metal like chromium or zinc on iron.
 
12.  Galvanisation: The process of coating a thin layer of zinc on iron objects is called galvanisation.
.
13.  Crystallisation: It is the process of getting crystals of pure substance from their solutions.
 
 
 
 
 
Q.1. Classify the changes involved in the following processes as physical or chemical changes:
  1. Photosynthesis
  2. Dissolving sugar in water
  3. Burning of coal
  4. Melting of wax
  5. Beating aluminium to make aluminium foil
  6. Digestion of food
Ans:      a) Chemical change                             b) Physical change
              c) Chemical change                             d) Physical change
              e)  Physical change                              f) Chemical change
 
Q.2. State whether the following statement are true or false. In case a statement is false, write the correct statement in your notebook.
  1. Cutting a log of wood into pieces is a chemical change.     (True/False)
  2. Formation of manure from leaves is a physical change      (True/False)
  3. Iron pipes coated with zinc do not get rusted easily.           (True/False)
  4. Iron and rust are the same substances.                                 (True/False)
  5. Condensation of steam is not a chemical change                (True/False)
Ans:      a) False
        Correct statement: Cutting a log of wood into pieces is a physical change.
              b) False
        Correct statement: Formation of manure from leaves is a chemical change.
              c) True
              d) False
        Correct statement: Iron and rust are two different substances.
              e) True.
 
Q.3. Fill in the blanks in the following statement:
  1.  When carbon dioxide is passed through lime water, it turns milky due to the formation of ………………
  2.  The chemical name of baking soda is …………..
  3.  Two methods by which rusting of iron can be prevented are …………… and ………………
  4. Changes in which only ………….. properties of a substance change are called physical changes.
  5. Changes in which new substances are formed are called ………… changes.
Ans: a) calcium carbonate                              b) sodium hydrogen carbonate
         c) Painting or greasing, galvanization   d) physical
         e) Chemical.
       
 
Q.4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Ans:
The reaction between baking gas and lemon juice is given below:
 
Lemon juice (citric acid) + Backing soda (sodium hydrogen carbonate)   ?  carbon dioxide (bubbles) +  Other substances.
 
Since, in this reaction a new substance is formed, therefore, it is a chemical change.
 
Q.5. When a candle burns, both physical and chemical change take place.
        Identify these changes. Give another example of a familiar process in  which
        Both the chemical and physical changes take place.


Ans:
When a candle burns, both physical and chemical changes occur as given below:
  1. Physical change.  Melting of wax, vapourisation of melted wax.
  2. Chemical change. Burning of the vapours of wax to give carbon dioxide, heat and light.
Burning of LPG in our kitchen is another such example in which a physical change occurs when LPG come out of cylinder and is converted from liquid to gaseous state and a chemical
change occurs when this gas burn in air.
 
Q.6. How would you show that setting of curd is a chemical change?

Ans:
Achange is said to be a chemical change, if:
  1. We cannot get back the substance in original state.
  2. A new substance is formed as a product.
         In setting of curd:
  1. The curd cannot be reversed into milk.
  2. A new substance is formed with different taste, smell and other properties.
        Hence, we can say that setting of curd is a chemical change.
Q.7.  Explain why burning of wood and cutting it into small pieces are       considered as two different types of changes.

Ans:
Burning of wood is a chemical change because in burning new substance are formed as given below:

       Wood + oxygen  ?           Coal + carbon dioxide + heat + light.

While cutting it into small pieces is physical change because by cutting we can only reduce the size of the log of wood and no change in its chemical properties
occur neither a new substance is formed.
 
Q.8. Describe how crystals of copper sulphate are prepared.

Ans:
We can prepare the crystals of copper sulphate by following activity:
Take a cup of water in a beaker and add a few drops of dilute sulphuric acid and heat it. When it starts boiling, add copper sulphate powder slowly and stir continuously (fig. 6.3).
Continue adding of copper sulphate powder till no more
 
 
 
 

 
                                                        Fig. 6.3. Crystal of copper sulphate.
 
powder can be dissolved. Filter the solution. Allow it to cool. Do not disturb the solution when it is cooling. Look at the solution after some time. Can you see the crystals of copper sulphate?
If not, wait for some more time.   
   
Q.9   Explain how painting of an iron gate prevents it from rusting.

Ans:
We know that the process of rusting requires both oxygen and water (or moisture). Painting prevents the surface of iron gate from coming in contact with oxygen and moisture thus prevent it from rusting.
 
Q.10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Ans:
Since content of moisture in the air in coastal areas is higher than in the air in deserts, so the process of rusting becomes faster in the coastal areas.
 
Q.11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist as a liquid. When it come out from the cylinder it become a gas (change –A) then it burns (change-B). The following statements pertain to these changes. Choose the correct one.
  1. Process A is a chemical change.
  2. Process B is a chemical change.
  3. Both process A and B are chemical changes.
  4. None of these processes is a chemical change.
Ans:   ii. Process B is a chemical change.
 
Q.12. Anaerobic bacteria digest animal waste and produce biogas (change-A). The biogas is then burnt as fuel (change-B). The following statements pertain to these changes. Choose the correct one.
  1. Process A is a chemical change.
  2. Process B is a chemical change.
  3. Both processes A and B are chemical changes.
  4. None of this processes is a chemical change.
Ans: iii. Both processes A and B are chemical change.
 
 
 
ADDITIONAL IMPORTANT QUESTIONS
 
 
Q.  1. How many kinds of changes are occurring around us?

Ans. 
Two kinds of changes occur around us: (1) Physical change and (2)
          Chemical change.         
 
Q.  2. Name some physical properties of a substance.

Ans. 
The shape, size, colour and state of a substances are called its physical     properties.

Q. 3. We cannot obtain wood from saw dust. What kind of change occurs in the process of sawing?


Ans.
Physical change.
 
Q. 4. What kind of changes involve the change in physical properties only?

Ans. 
Physical changes.
 
Q. 5. Which type of changes involve energy changes?

Ans.
Chemical changes.
 
Q. 6. What kind of change is rusting?

Ans. 
Chemical change.
 
Q. 7. What are the essential conditions for rusting?

Ans. 
Presence of moisture and air (oxygen) are the two conditions.
 
Q. 8. What is galvanisation?

 Ans.
The process of coating a layer of zinc on iron is called as galvanisation.
 
Q. 9. Define crystallisation?

Ans.
Crystallisation is the process of getting crystal of pure substances from their solutions.
 
Q. 10. What is a physical change?

Ans. 
A change in which a substance undergoes a change in its physical properties only is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.
 
Q. 12. Give some examples of physical changes.


Ans
.  Examples of physical changes are:
  1. Tearing of sheet of paper into pieces.
  2. Melting of ice.
  3. Change of water into steam.
  4. Breaking of glass tumbler.
  5. Lighting of electric bulb.
  6. Dissolution of sugar or salt in water.
 
Q. 13. What are the characteristics of a chemical change?

Ans.   
Characteristics of a chemical change are
  1.  Most of the chemical changes are irreversible.
  2.  These always result in energy change.
  3.  New substances are formed by the result of a chemical change.
  4.  Properties of the products are different from the properties of reactants.
 
 
Q.14. When a chemical change takes place what change accompany the formation of new substances?

Ans: 
In addition to new product the following may accompany a chemical change:
  1. Heat, light or any other radiation may be given off or absorbed.
  2. Sound may be produced.
  3. A change in smell may take place or a new smell may be given off.
  4. A colour change may take place.
  5. A gas may be formed.
 
Q.15. Give some examples of chemical changes.

Ans:
Examples of chemical changes are:
  1. Burning of paper, wood, candle, etc.
  2. Setting of curd.
  3. Cooking.
  4. Rusting of iron.
  5. Mixing of vinegar with baking soda.
  6. Ripening of fruit.
 
Q.16. In the following boxes, add a reverse arrow wherever a change is physical and can be reversed.
 
          
 
Ans
        


      
Q.17.  A magnesium strip is burnt. The ash so obtained is dissolved in water.  What kind of changes are there? Write chemical equations for these.
Ans: Both these changes are chemical changes and can be represented by following equation:
  1. Magnesium (2Mg) + Oxygen (O2)   ?             Magnesium oxide (2MgO)
  2. Magnesium oxide (MgO) + Water (H2O)   ?          Magnesium hydroxide       
[ Mg(OH)2]
 
Q.18. When an iron nail is dipped in copper sulphate and kept undisturbed, which type of change occurs? Represent this change by an equation.

Ans:
A brown layer of copper gets deposited on iron nail. This change is due to a chemical reaction between copper sulphate and iron. Also the colour of solution changes from blue to green due to the formation of iron sulphate.
This reaction can be represented by following equation:

         Copper sulphate solution (blue) + Iron   
?          Iron sulphate solution (green) + copper (brown)
 
Q.19. Most physical changes are reversible. Give reasons with two examples.

Ans:

  1. Melting of ice. During this change the water changes from its solid form to liquid form. It can be solidified again. The water remains water in both cases hence reversible.
  2. Lighting of an electric bulb. During this change, electricity is passed through the filament which becomes white hot and glows, but when switch is off, the filament returns to its original shape and condition, hence totally reversible.
 
Q.20. Formation of clouds is a physical change. Explain.

Ans:
Formation of clouds is a physical change as it is phase of transformation of natural water from liquid to gas during water cycle and them, gas to liquid. Hence, the property of water never changes in clouds form.
 
Q.21. Explosion of a cracker is a chemical change. Explain.

Ans:
Explosion of crackers is a chemical change because the explosive reactants are transformed into gaseous product along with heat, light and sound which cannot be reversed. Hence, it is a chemical change.
 
Q.22. What is a charring of sugar? It is a chemical change?

Ans:
When sugar continuously heated in the porcelain dish, then it starts evaporating and become foggy due to water vapour. A black powdery substance is left behind, which is charcoal. This process is also called charring of sugar and this is a chemical change, as it cannot be reversed.
 
Q.23. What is crystallization?

Ans:
Large crystals of pure substances can be formed from their impure solution. This process of obtaining crystals is called crystallization. It is an example of a physical change.
 
Q.24. If you put some glucose in your mouth, it feels cool.State whether energy is evolved or absorbed in this example.

Ans:
When we put some glucose on our tongue, the glucose dissolves and our tongue feels cool because some heat energy is required for glucose to dissolve. The glucose absorbs heat from our tongue. So tongue loses some heat and we feel cool.
 
 
 
Q.25. Identify the type of change and state whether energy is absorbed or evolved in each of the following:
         burning of a candle; lighting of a bulb; preparation of food by green plants; volcanic eruption; lightening.
Ans:   
      
1
2
3
4
5
Burning of candle
Lighting of bulb
Preparation of food by green plant
Volcanic eruption
Lightening
Chemical change
Physical change
Chemical change
Chemical change
Physical change
Energy is evolved
Energy is evolved
Energy is absorbed
Energy is evolved
Energy is evolved

 
Q.26. A fraction of ship’s iron has to be replaced every year.

Ans:
We know that ships are made of wood with same fraction of iron and a part of them remains under water. On the part above water also water drops keep clinging to the ship’s outer surface. Moreover, the water of the sea contains many salts. The salt water makes the process of rust formation faster. Therefore ships suffer a lot of damage from rusting inspite of being painted. So, that a fraction of ship’s iron has to be replaced every year.
 
Q.27. What is rusting? How to we prevent rusting?

Ans:
If we leave a piece of iron in the open for some time, it acquires a coating of brownish substance. This substance is called rust and the process of its formation is called rusting. This is the only change that effects iron articles and slowly destroys them. Since iron is used in making bridges, ships, cars, truck bodies and many other articles. The monatory loss due to rusting huge.
For rusting the presence of both oxygen and water (or water vapour) is essential.
In fact, if the moisture content in air is high which means if it is more humid, rusting become faster.

Methods of preventing rust formation:

         Rusting of an iron object can be prevented or reduced by not allowing air and moisture to come in contact with the surface of iron object. Following are the methods of prevention from rusting:
  1. Putting oil or paint over the surface of iron implements.
  2. Galvanisation. In this method, surface of iron is covered with layer of more active metal like zinc. Zinc metal loses electrons in preference to iron and hence, prevents the rusting of iron.
  3.  by tin-painting.
  4. Use of anti-rust solution: Alkaline chromate and alkaline phosphate solution act as anti-rust solution. When iron objects are dipped into a boiling and strongly alkaline solution of sodium phosphate, a protective insoluble film of iron phosphate is formed on them. This film protects the article from rusting.
 
 
 
 
Q.28. What kind of changes are these? Choose from the boxes.


InflatingPhysical/chemicalBursting of a crackerPhysical/chemical
Ripening of a fruitPhysical/chemicalBurning of paperPhysical/chemical
Cutting of paperPhysical/chemicalRusting of ironPhysical/chemical
Lighting an electric bulbPhysical/chemicalGrowth of a plantPhysical/chemical
Spinning of wheels of a cyclePhysical/chemicalFogginess of glass surfacePhysical/chemical
Germination of seedPhysical/chemicalFormation of coalPhysical/chemical
 
Ans:
 
InflatingPhysicalBursting of a crackerchemical
Ripening of a fruitchemicalBurning of paperchemical
Cutting of paperPhysicalRusting of ironchemical
Lighting an electric bulbPhysicalGrowth of a plantchemical
Spinning of wheels of a cyclePhysicalFogginess of glass surfacePhysical
Germination of seedchemicalFormation of coalchemical
 
 
 
OBJECTIVE TYPE QUESTIONS
Q.1. Match the items given in column I with those given in column II.
Column IColumn II
  1. Rust
  2. Vinegar
  3. Banking soda
  4. Lime water
  5. Neela thotha
 
  1. Alkaline solution of sodium phosphate
  1. Calcium hydroxide (Ca(OH)2)
  2. Ferric oxide (Fe2O3)
  3. Copper sulphate (CuSO4)
  4. Acetic acid (CH3COOH)
  5. Sodium hydrogen carbonate (NaHCO3)
  6. Anti-rust solution
 
Q.2 Fill in the blank space in the following statement.
  1. Changes that lead to the formation of new substance are called …………. .
  2. Melting of candle wax is a ………… change but its burning is a ……….. change.
  3. Energy is ………. In the formation of curd from milk.
  4. A brown layer formed when an iron article is left exposed in an open area is called …………  .
  5. The brown layer deposited on an iron nail dipped in copper sulphate solution is due to…………
  6. Large crystals f pure substance are obtained by ……………..  .
  7. Lighting of an electric bulb is ……….change.
  8. Magnesium burns in air give ………….. .
 
Q.3. Choose the true and false statements from the following:
  1. The cooking of rice is a physical change.
  2. When you cut an apple with knife interactions between the apple and the knife takes place.
  3. When catechu (kattha) is applied on lime while making a pan (bettel) a chemical change takes place.
  4. Eruption of volcano is reversible change.
  5. Formation of clouds is a reversible change.
  6. The salt obtained by evaporation is pure.
  7. Both oxygen and water are essential for rusting.
  8. In chemical changes new substances are formed.
 
Q.4. choose the correct option in the following:


(i) In which type of change a new substance is formed

a) Physical change                                    b) Chemical change
b) In both                                                  d) In neither of these
 

(ii) Which among following is a physical change?


a) Cutting a log of wood in small pieces  b) Burning of wood
c) Ripening of fruit                                   d) Cooking of food
 


(iii) Which of the following is a chemical change?

a) Burning of cracker                                b) Germination of seed
c) Coal formation from burried trees        d) All of these
 

(iv) Which is a method to prevent rust?

a) Crystalliation                                        b) Sedimentation
c) Galvanisation                                       d) Evaporation 
 

(v) How crystal of pure substances are obtained?


a) Crystalliastion                                       b) Chromatograghy
c) Peptisation                                            d) By all of these methods

(vi)   What is the chemical formal of rust?

       a) Fe2O3                                                      b) Fe3O4
       c) FeO                                                          d) Ca(OH)2
 

(vii) What will happen if carbon dioxide gas is passed through lime water?

a) Calcium carbonate is formed                   b) Lime water turns milky
c) both (a) and (b)                                        d) Lime water turns red
 

(viii) When an iron nail is dipped in copper sulphate solution and kept undisturbed for haft an hour or more?


          (a) The solution turns from blue to green

          (b) A brown layer is deposited on the iron nail

          (c) A chemical reaction takes place

           (d) All of the above

(ix) Which of the following is a periodic change?

         a) blowing of wind                                    b) evaporation of water
         c) growth of plant                                     d) rotation of earth
 

(x) Which of the following is a not a physical change
 

      a) Tearing of paper                                   b) Formation of curd
       c) Breaking of glass                                 d) Dissolution of sugar
 

(xi) Depositing a layer of zinc on iron is called
 

      a) galvanisation                                        b) crystallisation
       c) sublimation                                          d) layering
 
(xii) Which of the following is not a man made change?
       a) Formation of curd                                b) burning of petrol
       c) change of day and night                      d) breaking of a brick
 

(xiii) The reaction H2 + Cl2
?                  2HCl is a     
     
       a) Addition reaction                                b) Reduction reaction
       c) Substitution reaction                           d) Isomerisation reaction
 

(xiv) The reaction in which the two reacting substance exchange their corresponding ion is called

a) Displacement reaction                        b) Addition reaction
c) Double displacement reaction            d) Decomposition reaction

(xv) The rate of chemical reaction is not affected by the 

    a) Temperature                                        b) Pressure
     c) Catalyst                                               d) colour
 
 
 
ANSWER TO OBJECTIVE TYPE QUESTIONS
Q.1. Match the item Column I with Column II:
Column IColumn II
  1.    Rust
  2. Vineger
  3. Banking soda
  4. Lime water
  5.    Neela thotha
  6. Alkaline solution of
Sodium phosphate
  1. Ferric oxide (Fe2O3)
  2. Acetic acid (CH3COOH)
  3. Sodium hydrogen carbonate (NaHCO2)
  4. Calcium hydroxide (Ca(OH)2)
  5. Copper sulphate (CuSO4)
  6. Anti-rust solution
 
 
Q.2. Fill in the blanks:
        i. Chemical changes           ii. Physical, chemical        iii. Evolved
       iv. rust                                 v. copper metal                  vi. Crystallisation
      vii. reversible                    viii. Magnesium oxide.
 
Q.3. True/False
       i. False                 ii. True                 iii. True              iv. False
      v. True                 vi. False                vi. True             vii. True.
 
Q.4. Choose the correct option:
      i. (b)               ii. (a)                iii.  (d)              iv. (c)            v. (a)
    vi. (a)             vii. (c)               viii. (d)              ix. (d)            x. (b)
    xi. (a)             xii. (c)               xii.  (a)            xiv. (c)          xv. (d).