Q. 1 Classify the changes involved in thefollowing processes as physical or chemicalchanges:Ans: a) Photosynthesis – Chemical changeb) Dissolving sugar in water –Physical changec) Burning of coal – Chemical changed) Melting of wax – Physical changee) Beating aluminium to make aluminium foil –Physical changef) Digestion of food – Chemical change
Q. 2 State whether the following statements are true or false.In case a statement is false, write thecorrected statement in your notebook.(a) Cutting a log of wood into pieces is a chemical change.(True/False)= False(b) Formation of manure from leaves is a physical change.(True/False)= False(c) Iron pipes coated with zinc do not get rusted easily.(True/False)= True(d) Iron and rust are the same substances.(True/False)= False(e) Condensation of steam is not a chemical change.(True/False)= True
FILL IN THE BLANKS:(a) When carbon dioxide is passed through limewater, it turns milky due to the formation of calciumcarbonate (CaCO3).(b) The chemical name of baking soda is sodiumhydrogen carbonate (NaHCO3).(c) Two methods by which rusting of iron can beprevented are galvanizing and painting.(d) Changes in which only physical properties of asubstance change are called physical changes.(e) Changes in which new substances are formed arecalled chemical changes.
Q. 4 When baking soda is mixed with lemonjuice, bubbles are formed with the evolution ofa gas. Whattype of change is it? Explain.Ans: The formation of bubbles with evolutionof a gas indicates that a new substance isformed in thereaction. Thus, it is a chemical change.Lemon juice + Baking soda Carbon dioxide +Other substances.
Q. 5 When a candle burns, both physical and chemical changes takeplace. Identify these changes.Give another example of a familiar process in which both the chemicaland physical changes take place.Ans: When a candle burns, following physical and chemical changestake place:1. Physical change – melting of wax.2. Chemical change – burning of wax produces carbon dioxidegas, water vapour and heat.Formation of soil from rocks is another familiar process in whichboth, the chemical and physicalFormation of soil from rocks is another familiar process in whichboth, the chemicalPhysical change – Rocks broken down by weathering.Chemical change – New product soil is formed from rocks. The changeis irreversible.
Q. 6 How would you show that setting of curdis a chemical change?Ans: Setting of curd is a chemical change due tothe following reasons:1. In a chemical change, a new substance isformed. Here, curd is the new substanceobtained from milk.2. Both milk and curd have different properties.3. Milk cannot be re-obtained from curd.
Q. 7 Explain why burning of wood and cutting it intosmall pieces are considered as two different types ofchanges.Ans: 1) Chemical change – Burning of wood involvesthe formation of a new substance .i.e. coal, with theevolution of heat and unpleasant gases. Thus, it is achemical change.2) Physical change – Cutting of wood into small piecesdoes not form any new substance. Here, onlythe shape and size of the wood is changed. Thus, it isa physical change.
Q. 8 Describe how crystals of copper sulphate are prepared?Ans: Crystals of copper sulphate are prepared by the methodof crystallisation. The procedure is as follows:1. Take a cupful of water in a beaker.2. Add few drops of dilute sulphuric acid, and heat the water.3. Once the water starts boiling, add copper sulphate powderslowly by stirring it.4. Continue adding copper sulphate powder till no more canpowder be dissolved5. Now filter this solution and allow it to cool withoutdisturbing it.6. After sometime, crystals of copper sulphate are prepared.
Q. 9 Explain how painting of an iron gateprevents it from rusting.Ans: An iron object rusts, when its surfaceis exposed to oxygen and water, or watervapour . Painting an Iron gate forms aprotective coat on it. This coat prevents theiron gate from coming in contact withoxygen, water, or both. Thus, it preventsrusting.
Q. 10 Explain why rusting of iron objects isfaster in coastal areas than in deserts.Ans: An iron object rusts due to thepresence of oxygen and water, or watervapour. The content of moisture is high incoastal areas than in deserts. So, rusting ofiron objects is faster in coastal areasthan in deserts.
1. Broadly there are only two type of changes theyare chemical changes and physical changes.2. Cutting of paper in pieces is physical change.3. If you leave a piece of iron in the open for sometime ,it acquires a film of brownish substance.4. Heat, light or any other radiation (ultraviolet, for maybe example) given off or absorbed.6. Ozone layer protect us from ultra violent radiationwhich comes from the sun.5. The ash is the new substance formed whenmagnesium is burnt.
6. A change in which one or more new substances are iscalled a chemical change.7. A chemical change is also called a chemical reaction.8. All new substances are formed as a result of chemicalchanges.9. This process of depositing a layer of zinc on iron is calledgalvanisation.10. The iron pipes we use in our homes to carry water aregalvanised to prevent rusting.11. ships suffer a lot of damage from rusting in spite of beingsolutions. The process is called crystallisation.It is an example of a physical change.
12. salt can be obtained by the evaporation of seawater.13. Physical changes are changes in the physicalproperties of substances. No new substances areformed in thesechanges. These changes may be reversible.14 In fact, if the content of moisture in air is high, whichmeans if it is more humid, rusting becomes faster.15. Ozone absorbs the radiation and breaks down tooxygen.16. Ozone acts as a natural shield against thisradiation.
What we have learnt:1Changes can be of two types, physical and chemical.2 Physical changes are changes in the physicalproperties of substances.No new substances are formed in these changes.These changes maybe reversible.3 In chemical changes new substances are produced.4Some substances can be obtained in pure state fromtheir solutions by