PHYSICAL AND CHEMICAL CHANGE ( LESSION-6)

 

Q.1.  What is physical change?

Ans. A change in which no new substance is formed and the substance undergoes a change in its physical properties like shape, size, colour, texture, state, electrical and magnetic properties etc. is called physical change. It is a temporary change and is easily reversible. In this change the molecular composition remains unchanged.  or

 

A change in which no new substance is formed and the physical properties like shape, size, colour, state etc of the substance are changed is called physical change. It is a temporary change and is easily reversible. In this change the molecular composition remains unchanged. 

 

Q.2.  What is a chemical change?

Ans. A change in which one or more new substances are formed and the substance undergoes a change in its chemical properties is called chemical change. It involves a chemical reaction.  The molecular composition is changed. It being a permanent change is not easily reversible.  Or

 

A change in which the chemical properties of a substance are changed and one or new substances are formed is called chemical change. It involves a chemical change. The molecular composition is changed. It being a permanent change is not easily reversible.

 

Q.3.  Write some examples of physical changes:

         1.  Breaking of glass

         2.  Conversion of water into ice (freezing of water) and                   conversion of ice into water ( melting of ice)

         3. Sugar solution (dissolution of sugar in water)

         4. Ironing of cloth

         5. Dissolution of sulphur in carbon disulphide

         6. Dissolution of rubber in petrol

         7. Dissolution of resin in alcohol

         8. Evaporation of water

         9. Formation of dew, frost, snow, hail, mist and fog.                          (Condensation of water)

         10. Glowing of an electric ball

         11. Magnetization of a bar of steel.

         12. Changing of water into steam by boiling                                     (evaporation)

         13.  Production of sound when two materials are hit                           together

         14.  Expansion or contraction of metal on heating

         15.  Formation of solutions of soluble substance in water.

         16.  Change of colour due to heat as in case of zinc or lead                   monoxide.

         17. Beating of metals into sheets or drawing metals into                   wires

         18. Shaping of glass by heat.

         19. Crystallization

Q.4.  Characteristics of physical change

1.      No new substance is formed during the physical change.

2.      It can be generally reversed.

3.      No change in the weight during physical change.

4.      Only a little heat ( if any) is absorbed or given off during                physical change.

 

 

Q.5.  Write some example of chemical change.

         1.         Burning of things

         2.         Formation of curd from milk

         3.         Growth of human

         4.         Rusting of iron

         5.         Cooking of food

         6.         Digestion of food by living beings.

         7.         Respiration / photosynthesis

         8.         Rotting of fruits and vegetables

         9.         Preparation of medicines

         10.       Preparation of plastics

         11.       Food turning bad after a few days

         12.       Fading the colours of clothes

         13.       Ripening of fruits

         14.       Lightening of a match stick

         15.       Blackening of silver ware

         16.       Hardening of cement to form concrete blocks

 

Q.6.  Characteristics of a chemical change

1.      One or more new substances with entirely new properties          are produced during a chemical change.

2.      Chemical change cannot be easily reversed.

3.      Usually a change in weight during chemical change.

4.      Lot of heat is usually given off or absorbed during a                       chemical change.

 

Q.8.  Write some properties of chemical change.

Ans. Some properties of chemical change are:1. Heat, light or any other radiation like ultraviolet may be given off or absorbed.

  1. Sound may be produced
  2. A change in smell may take place or a new smell may be given off.
  3. A colour change may take place
  4. A gas may be formed.

 

Q.9   Explain any two chemical changes with the help of chemical equation.

Ans. Take copper sulphate solution in a beaker. Put an iron nail into it. After half an hour we can observe that the colour of the solution changed from blue to green due to the formation of iron sulphate, which is a new substance formed during the chemical reaction between iron and sulphate and a brown coating of copper on the iron nail.  This chemical equation is as below:

       CuSO4                                  +      Fe     -----→ FeSO4            + Cu

Copper sulphate solution      Iron            Iron sulphate     Copper

 

      Mg       +    O2       -----→   MgO

Magnesium   Oxygen              Magnesium oxide

 

Q.10. What do you know about the rusting of iron? How can we prevent rusting of iron?

Ans. If the iron materials are exposed to air and moisture for a long time, they get coated with a brown powdery substance called rust on the surface of the iron materials. The process due to which iron changes into rust is called rusting. For rusting, the presence of both Oxygen and water is essential. It is a chemical reaction which can be represented by the following equation:

 

       Fe      +      O2    +   H2O     -----→   Fe2O3

      Iron        Oxygen     water                                (rust)    

 

 

The different methods to prevent rusting of iron are

1.     By oiling or greasing the surface                          

2.     By painting the surface

3.     By galvanization                                                       

4.    By making the alloys of iron

 

Q.11. Define galvanization.

Ans. The process of depositing a layer of zinc on iron is called galvanization. This layer of zinc on iron does not allow the moist air (oxygen and water) to come in contact with the iron. Hence, galvanization prevents corrosion / rusting of iron.

 

Q.12. What is crystallization? How will you get the crystals of copper sulphate?

Ans. The process of obtaining crystals of a soluble substance from its hot saturated solution is called crystallization.

 

To get the crystals of copper sulphate we have to perform the following.

 

Take a cupful of water in a beaker and add a few drops of dilute sulphuric ( H2SO4) . Heat the water. When it starts boiling add copper sulphate powder slowly while stirring continuously. Continue adding copper sulphate powder till no more powder can be dissolved (saturation). Then filter the solution. Allow it to cool and don not disturb it. After sometimes we can see the crystals of copper sulphate.

 

 

NCERT book questions

Q.1.  Classify the changes involved in the following processes as physical or chemical change;

 

a)     Photosynthesis                                     :Chemical Change

b)     Dissolving sugar in water                    : Physical Change

c)     Burning coal                                           : Chemical change

d)    Melting wax:                                           :Physical change

e)    Beating aluminium to make aluminium foil : Physical change

f)     Digestion of food                                    : Chemical change

 

Q.2.     state whether the following statements are true or false. In case a statement is false, write the corrected statement in your note book.

 a)  Cutting a log of wood into pieces is chemical change.    False

It is a physical change. Though the shape of the wood has changed but there is no change in the chemical properties.

b)  Formation of manure from leaves is a physical change. False

It is a chemical change. A new substance called manure has been formed.

a)  Iron pipes coated with zinc do not get rusted easily. True

b)   Iron and rust are the same substances. False

Iron is a metal where as rust is an oxide of iron Fe2O3, which occurs as a brown deposit on the surface of the iron when it rusts.

c)     Condensation of steam is not a chemical change. True.

 

Q.3.     Fill in the blanks in the following statements:

 

a)     When carbon dioxide is passed through lime water, it turns milky due to the formation of  Calcium Carbonate.

b)     The chemical name of baking soda is Sodium bicarbonate  (NaHCO3).

c)      Two methods by which rusting of iron can be prevented are painting and galvanization.

d)     Changes in which only physical properties of a substance change are called physical change.

e)     Changes in which new substances are formed are called chemical change.

 

Q.4.     When baking soda Sodium bicarbonate (NaHCO3) is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Ans.    When baking soda ( sodium bicarbonate or sodium hydrogencarbonate  NaHCO3 ) is mixed with lemon juice ( citric acid ),bubbles are formed with the evolution of Carbon dioxide ( CO2). A hissing sound is also heard. This can be represented by the following equation.

NaHCO3              + Citric Acid          -----→          CO2                 + other substance.

Baking soda       Lemon juice                 Carbon dioxide 

Since, during this change Carbon dioxide (CO2), a new substance having different chemical property is formed, it is a chemical change.

 

Q.5.     When a candle burns, both physical and chemical changes 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, melting of wax and evaporation of melted was take place. These are physical changes.  But the burning of wax is a chemical change.

Another example of a familiar process in which both the chemical and physical changes take place is burning of LPG. When LPG comes out of cylinder and is converted from liquid to gaseous state, it is a physical change. When the gas burns in air, it is a chemical change.

 

Q.6.     How would you show that setting of curd is a chemical change?

Ans.    Curd is produced from milk. When a small amount of lemon juice ( citric acid) is added to milk, it changes to curd. The curd formed is a new substance having entirely different chemical properties. It is irreversible, as milk can not be get back from curd. Hence, 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 produces ash, smoke which are new substances having entirely different chemical properties. It is irreversible as wood can not be got back from the ash and smoke.  Hence, burring of wood is a chemical change. But when wood is cutting into small pieces, no new substance is produced. It undergoes a change in the physical properties such as its shape and size. Hence, it is a physical change.

 

Q.8. Describe how crystals of copper sulphate are prepared.

Ans.Take a cupful of water in a beaker and add a few drops of dilute sulphuric ( H2SO4) . Heat the water. When it starts boiling add copper sulphate powder slowly while stirring continuously. Continue adding copper sulphate powder till no more powder can be dissolved (saturation). Then filter the solution. Allow it to cool and don not disturb it. After sometimes we can see the crystals of copper sulphate.

 

Q.9. Explain how painting of Iron Gate prevents it from rusting.

Ans. For rusting both oxygen and water are essential. When oxygen and water present in the atmosphere come in contact with the iron, it rusts. Painting of iron gate does not allow the oxygen and moisture present in the atmosphere to come in contact with the iron and prevents it from rusting.  

 

Q.10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Ans.    In costal areas, the humidity of air is much higher than that of in the deserts. Or in other words, in coastal areas, the air present has much higher water percentage of water vapour than in deserts. Hence, rusting of iron objects is faster in coastal areas than in deserts.

 

Q.11. The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder, it becomes a gas ( change A) then it burns ( change-B). The following statements pertain to these changes. Choose the correct one.

a)     Process A is a chemical change.

b)     Process B is a chemical change.

c)      Both processes A and B are chemical changes.

d)     None of the processes are chemical changes.

 

Q.12.   Anaerobic bacteria digest animal waste and produce bio gas ( change A). The biogas is then burnt as fuel (change-B). The following statements pertain to these changes. Choose the correct one.

a)     Process A is a chemical change.

b)     Process B is a chemical change.

c)     Both processes A and B are chemical changes.

d)     None of the processes are chemical changes.

 

 

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