CBSE Class 7 Science Physical and Chemical Changes VBQs

Choose The Correct Answer.

Question. When acetic acid is mixed with baking soda, the gas evolved is:
a) hydrogen
b) oxygen
c) carbon dioxide
d) carbon mono oxide
Answer: C

Question. The chemical formula of rust is:
a) Fe₂O₃
b) FeCO₃
c) Fe₂O₃.xH₂O
d) FeCO₃.xH₂O
Answer: C

Question. For rusting of iron objects, the essential requirement is:
a) air (oxygen)
b) moisture (water)
c) open surface of object
d) All of the above
Answer: D

Question. The example of chemical change is:
a) Eruption of volcano
b) burning of candle
c) cooking of food
d) all of the above
Answer: D

Question. For prevention of rust, we use:
a) A coat of oil and grease
b) A coat of paint
c) galvanization
d) All of the above
Answer: D

Fill In The Blanks.

(i) A change in which only physical properties of a substance are changed is a physical change.

(ii) Changes that lead to formation of new substances are called chemical changes.

(iii) Fossil fuels produce carbon dioxide gas on burning.

(iv) When carbon dioxide is passed through lime water, it turns milky.

(v) Galvanization is the method for the prevention of rusting of iron objects.

Match The Column ‘A’ With Column ‘B’.

Answer: (i) - (d), (ii) - (b), (iii) - (a), (iv) - (c), 
 

Write True Or False.  

Question. Iron and rust are the same substances. (False)
Answer: True

Question. Formation of manure from leaves is a physical change.
Answer: False

Question. Iron pipes coated with zinc do not get rusted easily.
Answer: True

Question. Cutting a log of wood into pieces is a chemical change.
Answer: False 

Question. Condensation of steam is not a chemical change.
Answer: True

ACTIVITY- Cutting of paper is a physical change. 

Question. Is the cutting of paper a physical or a chemical change?
Answer: Physical change.

Question. Mention whether the cutting is reversible or irreversible change.
Answer: Irreversible change.

ACTIVITY- Crushing of chalk is physical change. 

Question. What is the nature of above change? Is it physical or chemical?
Answer: Physical change.

Question. Can you recover chalk from dust?
Answer: Yes.

Question. Does chalk powder dissolve in water?
Answer: No.

ACTIVITY- Change in a state of water is a physical change. 

Question. Water changes to ice with …………… in temperature.
Answer: Decrease.

Question. Ice melts into water with …………… in temperature.
Answer: Increase.

ACTIVITY-Change in a state of water is a physical change. 

Question. Water changes into gaseous form with ………. in temperature.
Answer: Increase.

Question. Water changes from gaseous state to liquid state with ………… in temperature.
Answer: Decrease.

Question.What is evaporation?
Answer: Process of converting liquid into its vapour form is known as evaporation.

Question. Define condensation.
Answer: Process of converting vapours into its liquid form is known as condensation.

ACTIVITY- Colour change is a physical change. 

Question. Define a physical change. 
Answer: The change in physical properties like state, length, position, colour, shape etc. is known as a physical change.

Question. Explain any two examples of physical change from your surroundings.
Answer: Converting water into ice and melting of candle’s wax are physical changes.

Question. Is this physical change reversible or irreversible?
Answer: Reversible.

Question. Is a new substance formed during a physical change?
Answer: No.

Question. Which colour do you notice on heating the free end of hacksaw blade?
Answer: It turns orange/red.

ACTIVITY- Burning of magnesium ribbon is a chemical change.

Question. What is the colour of magnesium oxide formed on heating magnesium?
Answer: White.

Question. What type of metal oxide is formed on burning of magnesium ribbon?
Answer: Basic.

ACTIVITY- Reaction of copper sulphate with iron is a chemical change.

Question. Write the colour and chemical formula of ferrous sulphate.
Answer: Light green ; FeSO₄.

Question. What is the common name of copper sulphate?
Answer: Blue vitriol.

ACTIVITY- Evolution of gas is a chemical change. 

Question. How the presence of CO₂ gas can be detected?
Answer: Carbon dioxide (CO₂) gas turns lime water milky.

Question. What is the formula of lime water?
Answer: Ca(OH)₂.

Question. What is the colour and formula of copper sulphate?
Answer: Colour- Blue ; Formula- CuSO₄.

Question. Write two observations to see chemical change.
Answer: (1) Evolution of heat or light, (2) Evolution of a gas.

Question. In chapter 5, you neutralized an acid with a base. Is neutralization a chemical change?
Answer: Yes.

Question. Name the gas evolved when acetic acid (vinegar) reacts with baking soda.
Answer: Carbon dioxide (CO₂) gas.

Question. What is a chemical change? 
Answer: A change in which one or more new substances are formed is called a chemical change.

Question. Name some compounds formed as a result of chemical reactions.
Answer: Calcium hydroxide, Calcium carbonate, Carbon dioxide etc.

Question. What is lime water?
Answer: Aqueous solution of calcium hydroxide-Ca(OH)₂.

Question. What happens when carbon dioxide is passed through lime water?
Answer: Carbon dioxide (CO₂) gas turns lime water milky due to the formation of calcium carbonate.

ACTIVITY- Preparation of crystals of copper sulphate. 

Question. When will you stop dissolving copper sulphate powder to hot water?
Answer: Till no more powder can be dissolved i.e. to get saturated solution.

Question. What is end product in the process of crystallization?
Answer: Pure crystals of the given salt.

Very Short Answer Type Questions.

Question. What is rust? Write its chemical formula.
Answer: On reacting with air and moisture, a brown-red coloured layer is formed on iron objects, this layer is known as rust. Chemical formula of rust is Fe₂O₃.xH₂O.

Question. What is galvanization?
Answer: The process of depositing a layer of non-reactive metal (like zinc) over the surface of iron is known as galvanization.

Question. Why burning of fireworks s harmful?
Answer: Because burning of fireworks may cause blast, which produce many harmful gases.

Question. Why are iron objects painted frequently?
Answer: For rusting of iron, contact of air and water is necessary. So iron objects are painted to prevent the contact of iron with air and moisture.

Question. Name two metals which are deposited on the surface of iron objects during the process of galvanization.
Answer: Zinc or aluminium.

Question. What is crystallization?
Answer: Method of getting pure solid from an impure sample of solid by making crystals is known as crystallization.

Question. Burning of candle is an example of which type of change-physical or chemical change? Give reasons.
Answer: Burning of candle is a chemical process as it produces gases (new substance). Besides this melting of wax of candle is a physical change.

Question. Write the conditions necessary for rusting of iron.
Answer: Presence of: (1) Air (oxygen), (2) moisture (water), (3) open surface of an iron object.

Short Answer Type Questions.

Question. Explain how cutting and burning of wood are different type of change?
Answer: Cutting of wood is a physical change, because here only shape and size of wood are changed. But
burning of wood is a chemical change, because here a new substance carbon dioxide gas is produced.

Question. Why does the colour of copper sulphate (CuSO₄) solution change, when an iron nail is dipped into it? Write chemical equation also.
Answer: When iron nails are dipped in copper sulphate solution, solution turns green from blue, because more reactive iron displaces copper forming green coloured iron sulphate. Chemical equation is as follows:
Fe + CuSO₄(aq)                                   →       FeSO₄(aq) + Cu
Iron nails + Copper sulphate solution → Iron sulphate + copper.
(blue)                                                                (light green)

Question. Mixing of baking soda or vinegar is a chemical change or a physical change? Discuss.
Answer: Chemical change, because a new substance carbon dioxide gas is produced.

Question. What will happen when carbon dioxide is passed through lime water?
Answer: Carbon dioxide (CO₂) gas turns lime water milky due to the formation of calcium carbonate.

Question. Magnesium ribbon is burnt and the ash formed is mixed with water. Now answer the following:

(a) Write equation for the burning of magnesium.
Answer: 2Mg + O₂ → 2MgO

(c) Name the substance formed by mixing ash and water. Is it acidic or basic?
Answer: Magnesium oxide solution is basic.

(b) What will happen when the mixture of ash and water is added to (1) blue litmus solution and (2) red litmus solution?
Answer: (1) Blue litmus solution will remain blue.
(2) Red litmus solution will turn blue.

Long Answer Type Questions.

Question. What are physical and chemical changes? Write differences between physical and chemical changes.
Answer:

Question. Give detail of process of crystallization of copper sulphate (CuSO₄).
Answer: Take 200ml of water in a beaker and add a few drops of dilute sulphuric acid into it. Heat the water. When it starts boiling, copper sulphate powder is added slowly while stirring continuously to make a saturated solution. The solution is filtered and allowed to cool down. Crystals of copper sulphate slowly form at the bottom of the beaker.

Question. What do you understand by rusting of iron? Write the necessary conditions required for rusting of iron objects. How rusting of iron and iron objects can be prevented?
Answer: Rusting of iron- On reacting with air and moisture, a brown-red coloured layer is formed on iron objects; this process is known as rusting.
Necessary conditions for rusting- Presence of: (1) Air (oxygen), (2) moisture (water), (3) open surface of an iron object.
Prevention of rusting- (1) By greasing or oiling, (2) By painting, (3) By galvanization.