AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

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AP 7th Class Science 5th Lesson Physical and Chemical Changes Questions and Answers

AP Board Solutions Class 7 Science Chapter 5 Physical and Chemical Changes

Exercise 

Question 1.
Classify the changes involved in the following processes as physical or chemical changes:
a) Photosynthesis
b) Dissolving sugar in water
c) Burning of coal
d) Melting of wax
e) Beating aluminium to make aluminium foil
f) Digestion of food
Answer:
a) Chemical change
b) Physical change
c) Chemical change
d) Physical change
e) Physical change
f) Chemical change

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Question 2.
State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
a) Cutting a log of wood into pieces is a chemical change. (True/False)
b) Formation of manure from leaves is a physical change. (True/False)
c) Iron pipes coated with zinc do not get rusted easily. (True/False)
d) Iron and rust are the same substances. (True/False)
e) Condensation of steam is not a chemical change. (True/False)
Answer:
a) False – Cutting a log of wood into pieces is a physical change.
b) False – The formation of manure from leaves is a chemical change.
c) True
d) False – Iron and rust are not same but different substances.
e) True

Question 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 ………….
b) The chemical name of baking soda is …………
c) Two methods by which rusting of iron can be prevented are ………… and …………
d) Changes in which only ………… properties of a substance change are called physical changes.
e) Changes in which new substances are formed are called ………… changes.
Answer:
a) Calcium carbonate
b) Sodium hydrogen carbonate
c) Galvanisation, painting or greasing
d) Physical
e) Chemical

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Question 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.
Answer:
When baking soda (Sodium hydrogen carbonate) is mixed with lemon juice (citric acid), bubbles are formed. These bubbles are formed due to the evolution of carbon dioxide gas. As chemical composition of substance is getting changed and also new product is formed after mixing. Hence, it is a chemical change. In this change, citric acid contained in lemon juice reacts with sodium hydrogen carbonate which results in the evolution of carbon dioxide gas. The reaction can be represented as follows:
Lemon Juice + Baking soda → Carbon dioxide + Other substances

Question 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.
Answer:
When a candle burns both physical and chemical changes occur:
a) Physical change : On heating, melting candle is a physical change since it again turns into solid wax on cooling.
b) Chemical change : The wax vapours burns to give new substances like carbon dioxide, carbon soot, water vapour, heat and light. This is a chemical change. Cooking of food is both physical and chemical change because raw vegetables get cooked which is a chemical change and the water changes into steam which is a physical change.

Question 6.
How would you show that setting of curd is a chemical change?
Answer:
Setting of curd is a chemical change because a new substance(lactic acid) is formed. Both curd and milk have different properties. Once the curd is formed, it cannot be reversed back into milk. A new substance is formed with different taste, smell and other chemical properties and also it is an irreversible process. Hence, setting of curd is a chemical change.

Question 7.
Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer:
Burning of wood is a chemical change because during burning new substances are formed. After burning, we cannot get original substance, (i.e., wood) back.But cutting it into small pieces is physical change because no new substance is formed. We can only reduce the size of wood.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Question 8.
Describe how crystals of copper sulphate are prepared.
Answer:
Crystals of copper sulphate are prepared by the method of crystallisation. A cup of water is taken in a beaker and add a few drops of dilute sulphuric acid.Heat the water. When it starts boiling add copper sulphate powder slowly with continuous stirring.Continue adding copper sulphate powder till saturation level.Filter the solution and allowed to cooling. The solution should be kept undisturbed. We can observe the crystals of copper sulphate after few hours.

Question 9.
Explain how painting of an iron gate prevents it from rusting.
Answer:
Rusting is aided by both moisture (water) and air (oxygen). Painting of an iron gate, prevents it from rusting because it cuts the direct contact of iron from the air and therefore there is no further exposure of iron to oxygen. Hence, rusting is prevented.

Question 10.
Explain why rusting of iron objects is fester in coastal areas than in deserts.
Answer:
Both the air and the moisture are required for rusting to take place.In coastal areas, air contains more moisture because of the sea or ocean, while in deserts air is, dry and hot.As the content of moisture in the air in coastal areas is higher than in the air in deserts. Therefore, rusting of iron objects is faster in coastal areas than in deserts.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Question 11.
The gas we use in the kitchen is called liquified petroleum gas (LPG).In the cylinder it exist 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.
i) Process – A is a chemical change.
ii) Process – B is a chemical change.
iii) Both processes A and B are chemical changes.
iv) None of these processes is a chemical change.
Answer:
ii) Process – B is a chemical change

Question given in the lesson

Page No. 104

Question 1.
Can we call photosynthesis a chemical change ?
Answer:
Yes, photosynthesis is a chemical change. In photosynthesis, process water and carbon dioxide combine to form glucose and oxygen in the presence of sunlight. Since, new chemical substances are formed. So, photosynthesis is clearly a chemical change.

Question 2.
Paheli said that even digestion is a chemical change.
Answer:
In the process of digestion, the various food materials break down to form new substances which can be absorbed by the body. So the process of digestion is a chemical change.

Question 3.
Can we call the breaking down of ozone a chemical change?
Answer:
Yes, the breaking down of ozone is a chemical change.During the reaction, ozone absorbs the radiation and breaks down into oxygen. Oxygen is different from ozone. Since, new substance is formed, we consider it as a chemical change.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Page No. 106

Question 4.
Paheli understood that why her friend Rita is always complaining about iron articles rusting so fast. She lives near the coast. Comment.
Answer:
The essential factors responsible for rusting is presence of both oxygen and water. The water or moisture content is more in coastal areas, hence, iron objects get both water and air for rusting. This makes the process of rusting faster.

Extended Learning – Activities and Projects

Question 1.
Describe two changes that are harmful. Explain why you consider them harmful. How can you prevent them ?
Answer:
Two changes which are harmful are :
a) Rusting of iron: When iron is left exposed to air it reacts with oxygen and water in the air to form a powdery substance called rust. Rusting of iron is harmful because it slowly destroys iron articles and makes them useless.
Prevention: Rusting can be prevented by oiling, greasing or painting, it can also be prevented by galvanisation.

b) Spoilage of food grains: Food grains get spoiled over a period of time. This causes a loss to us. It can be prevented in the following ways:

  • Food grains can be dried before storage.
  • Keeping the food grains in safe, cool and dry place.
  • Spraying insecticides in godowns.

Question 2.
Take three glass bottles with wide mouths. Label them A, B and C. Fill about half of bottle A with ordinary tap water. Fill bottle B with water which has been boiled for several minutes, to the same level as in A. In bottle C, take the same boiled water and of the same amount as in other bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle C so that it forms a film on its surface. Put the bottles away for a few days. Take out nails from each bottle and observe them. Explain your observations.
Answer:
The nails in bottles B rust a little, nails in A are the most rusted and that in C remain unchanged.
In bottle B : Formation of rust will takes place since air from the atmosphere will dissolve in the boiled water as it is uncovered.
In bottle C : No rust formation will occur since only water is present.The layer of oil present prevents dissolving of atmospheric air in the water. For rusting both water and oxygen are necessary. Both the factors are present in the bottle A since oxygen is dissolved in water.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Question 3.
Prepare crystals of alum.
Answer:
Prepare a super saturated solution of alum in a beaker by dissolving the powdered alum in water. For this, take a beaker half filled with water on a stand and heat it till it starts boiling. Add alum slowly to this solution. Keep on adding alum until no more alum can be dissolved. Filter the warm solution into another beaker and allowed to cool down. Crystals of alum slowly form at the bottom of the beaker.

Question 4.
Collect information about the types of fuels used for cooking in your area. Discuss with your teachers/parents/others which fuels are less polluting and why.
Answer:
Solid fuels : Coal, dung cake, wood etc.,
Liquid fuels : Kerosene oil.
Gaseous fuel: LPG.It is a less polluting fuel.

Activities

Activity 5.1 Page No : 94

Question 1.
Cut a piece of paper in four square pieces. Cut each square piece further into four square pieces. Lay these pieces on the floor or a table so that the pieces acquire the shape of the original piece of paper. Obviously, you cannot join the pieces back to make the original piece, but is there a change in the property of the paper ?
Answer:
Aim : To study the change in property of a paper.
Materials required : Piece of paper, table.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes 1

Procedure :

  • Cut a piece of paper in four square pieces.
  • Cut each square piece further into four square pieces.
  • Lay these pieces on the floor or a table so that the pieces acquire the shape of the original piece of paper.

Observations: We observe that we cannot join the piece back to make the original piece of paper. We also observe the property of the paper does not change.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Conclusion : From the above activity we can conclude that there is change in size of paper. There is no change in property of paper. No new substance is formed. Therefore the change is physical change.

Activity 5.2  Page No : 94

Question 2.
Collect the chalk dust lying on the floor near the chalkboard in your classroom. Or, crush a small piece of chalk into dust. Add a little water to the dust to make a paste. Roll it into the shape of a piece of chalk. Let it dry. Did you recover chalk from the dust ?
Answer:
Materials required : Chalk dust, water.
Procedure:

  • Collect the chalk dust lying on the floor near the chalkboard in your classroom.
  • Or crush a small piece of chalk into dust.
  • Add a little water to the dust to make a paste.
  • Roll it into the shape of a piece of chalk.
  • Let it dry.
  • Record your observations.

Observations: We observe that we can recover the chalk from the dust.

Conclusion: From the above activity we can conclude that there only change in shape of chalk.There is only change in size. No new substance is formed. Therefore the change is physical change.

Activity – 5.3 Page No : 94

Question 3.
Take some icedn a glass or plastic tumbler. Melt a small portion of ice by placing the tumbler in the sun. You have now a mixture of ice and water. Now place the tumbler in a freezing mixture (ice plus common salt).Does the water become solid ice once again ?
Answer:
Aim : To study the change in state of ice.
Materials required : Piece of ice, glass/plastic tumbler.

Procedure:

  • Take some ice in a glass or plastic tumbler.
  • Melt a small portion of ice by placing the tumbler in the sun.
  • Now we have a mixture of ice and water. .
  • Now place the tumbler in a freezing mixture.
  • Record your observations.

Observations: We observe on heating the ice, it is converted into water and on freezing, it again changes into ice.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Conclusion : From the above activity, we conclude that solid changes into liquid and ‘ changes back into solid. Thus, there is no change in the properties of water.

Activity 5.4 Page No: 96

Question 4.
Boil some water in a container. Do you see the steam rising from the surface of water? Hold an inverted pan by its handle over the steam at some distance from the hoiling water. Observe the inner surface of the pan.Do you see any droplet of water there ?
Answer:
Aim: To study the change in state of water.
Materials required : Water, beaker, burner and a pan with handle.

Procedure

  • Take some water in a container.
  • Water vapours rises from the surface of water.
  • Hold an inverted pan by its handle over the steam at some distance from the boiling 1 water.Yon can also keep some ice cubes over the pan.
  • Observe the inner surface of the pan.

Observations : We observe when the water is heated, it is converted into steam.On cooling clown, the steam it converted into water again.

Conclusion: From the activity, we can conclude that there is no change in the properties of water. No new substance is formed. So, the boiling water to form steam is a physical change.

Activity 5.5 Page No : 56

Question 5.
Hold a used hack-saw blade with a pair of tongs. Keep the tip of the free end of the blade on the gas stove. Wait for a few minutes.Does the colour of the tip of the blade change?
Remove the blade from the flame. Observe the tip once again after some time. Does it get back its original colour ?
Answer:
Aim : To study the hack-saw blade changed colour on heating.
Materials required : Hack-saw blade, gas stove.

Procedure:

  • Hold a used hack-saw blade with a pair of tongs.
  • Keep the tip of the free end of the blade on the gas stove.
  • Wait for a few minutes
  • Remove the blade from the flame.
  • Observe the tip once again after some time.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Observations : We observe that the colour of the tip of the blade kept near the flame changes.When we remove the flame from the blade we will get back its original colour.

Conclusion : From the above activity we can conclude that there is only change in colour in the tip of the blade.No new’ substance is formed.Therefore the change is physical change.

Activity 5.6 Page No: 98

Question 6.
Get a small piece of a thin strip or ribbon of magnesium. Clean its tip with sandpaper. Bring the tip near a candle flame. It burns with a brilliant white light. When it is completely burnt it leaves behind a powdery ash. Does the ash look like the magnesium ribbon?
Answer:
Aim : To study the burning of magnesium ribbon in air.
Materials required : A magnesium ribbon, sand paper, candle.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes 2

Procedure:

  • Get a small piece of a thin strip or ribbon of magnesium.
  • Clean its tip with sandpaper.
  • Bring the tip near a candle flame.
  • Hold it for some time and observe.

Observations: We observe the magnesium ribbon burns with a brilliant white light. When it is completely burnt, it leaves behind a powdery ash. It is called a magnesium oxide.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Conclusion : From the above activity, we can conclude that heat and light are given out in this process. It is not possible to recover magnesium ribbon in the original form from the powdery ash. Therefore burning of magnesium ribbon is a chemical change.

Activity 5.7 Page No: 100

Question 7.
Dissolve about a teaspoonful of copper sulphate (blue vitriol or neela thotha) in about half a cup of water in a glass tumbler or a beaker. Add a few drops of dilute sulphuric acid to the solution. You should get a blue coloured solution. Save a small sample of the solution in a test tube or a small glass bottle. Drop a nail or a used shaving blade into the remaining solution. Wait for half an hour or so. Observe the colour of the solution. Compare it with the colour of the sample solution saved separately. Do you see any change in the colour of the solution?
Answer:
Aim : To study the reaction between copper sulphate solution and iron (iron nail/ shaving blade).
Materials required : Test-tube, copper sulphate, two beakers, an iron nail, or shaving blade.

Procedure:

  • Dissolve about a teaspoonful of copper sulphate (blue vitriol or neela thotha) in about half a cup of water in a glass tumbler or a beaker.
  • Add a few drops of dilute sulphuric acid to the solution.
  • Save a small sample of the solution in a test tube or a small glass bottle.
  • Drop a nail or a used shaving blade into the remaining solution.
  • Wait for half an hour or so.
  • Observe the colour of the solution.
  • Compare it with the colour of the sample solution saved separately (figure)

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes 3

Change in colour of the copper sulphate solution due to reaction with iron

Observations: We observe when we take out the nail or the blade changes are noticed due to a reaction between copper sulphate and iron. The change in colour of the solution from blue to green is due to the formation of copper sulphate, a new substance. The brown deposit on the iron nail is copper, another new substance.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Conclusion: From the above activity, we can conclude that iron reacts with copper sulphate to form iron sulphate and copper. The equation can be written asp:
Copper sulphate solution (blue) + Iron → iron sulphate solution (green) + Copper(brown deposit)

Activity 5.8 Page No : 100

Question 8.
Take about a teaspoonful of vinegar in a test tube. Add a pinch of baking soda to it. You would hear a hissing sound and see bubbles of a gas coming out. Pass this gas through freshly prepared lime water. What happens to the lime water?
Answer:
Aim : To study the reaction between vinegar and baking soda.
Materials required : Vinegar, baking soda, test tube.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes 4

Procedure:

  • Take about a teaspoonful of vinegar in a test tube.
  • Add a pinch of baking soda to it.
  • You would hear a hissing sound and see bubbles of a gas coming out.
  • Pass this gas through freshly prepared lime water as shown in Figure.

Observations : We observe the bubbles of gas are seen in vinegar. This gas turns lime water Set up to pass gas through lime water milky white in colour.

AP 7th Class Science 5th Lesson Questions and Answers Physical and Chemical Changes

Conclusion : From the above activity, we can conclude that carbon dioxide passing through lime water makes calcium carbonate which makes lime water milky. It is a chemical change. The equation can be represented as :
Carbon dioxide (CO2) + Lime water [Ca(OH2)] → Calcium Carbonate (CaCO3) + Water (H2O)

Activity 5.9 Page No : 106

Question 9.
Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid. 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. 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?
Answer:
Aim : To show the process of crystallisation.
Materials required : Water, beaker, few drops of dilute sulphuric acid, copper sulphate powder.

Procedure :

  • Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid.
  • Heat the water.
  • When it starts boiling add copper sulphate powder slowly while stirring continuously (Figure).
  • Continue adding copper sulphate powder till no more 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.

Observations: We observe that the pure crystals of copper sulphate will form at the bottom of the beaker.
Conclusion : From the activity we understand that the technique of crystallisation.

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