Class 7 Science Ch 06: Physical and Chemical Changes

    Solved examples for class 7 science : Chapter 6

    Physical and Chemical Changes

    I. Multiple Choice Questions (MCQs).
    Q.1. A physical change is
    a) always reversible
    b) normally reversible
    c) irreversible
    d) none of these
    Answer: (b) normally reversible
    Q.2. When an iron article is exposed to air and moisture, a layer of brown flaky substance covers it, this substance is called
    a) rust
    b) iron sulphate
    c) water
    d) Grease
    Answer: (a) rust
    Q.3. A chemical change is
    a) always irreversible
    b) normally irreversible
    c) reversible
    d) none of these
    Answer: (b) normally irreversible
    Q.4. The process of photosynthesis is
    a) chemical change
    b) irreversible change
    c) both (a) and (b)
    d) none of these
    Answer: (a) chemical change
    Q.5. The carbon dioxide gas turns limewater
    a) milky
    b) colourless
    c) muddy
    d) none of these
    Answer: a) milky

    II. Fill in the blanks by selecting appropriate words from the given list: List : dazzling, state, ultraviolet, properties, new

    Q.1. The change of water into water vapour on boiling, and condensing of the water vapour back to water on cooling, are changes of ______________.

    Q.2. The physical _______________ of the substance changes during a physical change.

    Q.3. A small, thin strip of magnesium burns with a ______________ white light and changes to ash-like substance.

    Q.4. Chemical reactions always produce ______________ substances.

    Q.5. Ozone absorbs ______________ radiation and gets break down into oxygen, a new substance.
    Answer:
    1-sate
    2- properties,
    3- dazzling
    4-new
    5- ultraviolet

    III. Match the items of Column A with those of Column B.

    Column A

    Column B

    1.            
    Crystallisation

    (a) Depositing
    a layer of zinc on iron

    2.            
    Galvanisation

    (b) New substance is formed

    3.            
    Rusting

    (c) No new
    substance is formed

    4.            
    Physical
    change

    (d) Purification of common salt

    5.            
    Chemical
    change

    (e) Reddish
    brown material covered on iron

    Answer: 1-(d), 2-( a), 3- (e ), 4- (c),5- (b)

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. A piece of iron metal, held by a pair of tongs becomes red hot and starts to glow when heated strongly.
    Q.2. In all the physical changes, the shape, size, colour, state or position of a substance changes.
    Q.3. No amount of heat is required for a physical change.
    Q.4. When fruits or vegetables are cut and left exposed to the moisture, the cut surface turns brown in colour.
    Q.5. Galvanisation is carried out by a physical process known as electrolysis. Answer: 1-T, 2-T, 3-F, 4-T, 4-F
    V. Very Short Answer Questions.
    Q.1. Name the product formed, when a piece of magnesium is burnt in air. Ans: Magnesium oxide

    Q.2. Name a kind of energy that is required for a physical change.
    Answer: Heat energy
    Q.3. State two visible changes which occur during a physical change.
    Answer: Two visible changes during a physical change is the shape and colour. Some visible changes during physical changes are glowing of an electric bulb, magnetisation of an iron piece.
    Q.4. State two visible changes which occur during a chemical change.
    Answer: Two visible changes during a chemical reaction are
    (a) Absorption or evolution of heat and light energy
    (b) Change of colour
    Q.5. Why does the fermented food become warmer than the original food?
    Answer: The fermented food becomes warm i.e., energy is given out during the process.
    VI. Short Answer Questions.
    Q.1. How can you say that digestion of food is a chemical change?
    Answer: Digestion of food in the stomach is a chemical changes. This is because the food breaks down into simpler substances and it is not possible to get back the original food. The new substances have different chemical properties and different composition compared to the food.
    Q.2. Why is crystallisation considered as a physical change?
    Answer: Crystallisation is a physical change since impurities are removed from a substance by this method. We do not get any new substance by the crystallisation process.
    Q.3. Why is physical change known as temporary change?

    Answer: Physical changes are temporary changes because they are generally reversible. For example When ice melts it changes to water.
    This is a change from solid to Liquid state. The water can be converted to ice by freezing it. This is a change from the Liquid to the solid state. Similarly, the change of water into water vapour on boiling, and condensing of the water vapour back to water on cooling, are changes of state. No new substances are formed.

    Q.4. What do you mean by physical change? Discuss some examples with their
    characteristics.

    Answer: A change in which the physical properties of a substance changes and no new substances are formed is called a physical change.

    Characteristics of a physical change
    (i) The physical properties of the substance changes during a physical change. For example melting of wax, butter or ghee.
    (ii) No new substances are formed and the nature of the substance remains the same during the change. Heating of iron rod.
    (iii) It is a temporary change and generally reversible though a few of the changes may be irreversible. For example of chopping of wood is irreversible.
    (iv) Only a small amount of heat is absorbed or given off during the change. Freezing of water to get ice.

    Q.5. What do you mean by chemical change? Discuss some examples with their
    characteristics.
    Answer: A change in which one or more new substances are formed is called a chemical change.
    Characteristics of a Chemical Change
    (i) A chemical reaction takes place between two or more substances. For example rusting of iron.
    (ii) New substances with different properties are formed after the change. For example curding of milk.
    (iii) It is a permanent change and cannot be reversed by a simple physical methods. For example spoilage of food.
    (iv) A large amount of heat is either absorbed or evolved during a chemical change. For example, burning of LPG

    Q.6. What happens when magnesium is burnt? Name the new substance formed. Is it basic or acidic?
    Answer: (a) Burning of magnesium : When magnesium is held with a pair of tongs and brought near the tip of a candle flame. It burns with a dazzling white Light and changes to a powdery ash-Like substance. The ash looks completely different from the silvery white metal. Here, magnesium reacts with oxygen of the air to form a new substance, magnesium oxide. The chemical reaction can be represented by the equation:

    $Magnesium (Mg) + Oxygen(O_{2})\rightarrow Magnesium oxide (MgO) $

    Aqueous solution of magnesium oxide turns red Litmus blue, showing that the substance, magnesium hydroxide, is basic in nature.

    VII. Long Answer Questions.
    Q.1. (a) Describe the reaction taking place between vinegar and baking soda. How can you test the gas produced by the above reaction?
    (b) How can you observe the changes that take place when solution copper sulphate reacts with iron?
    Answer: (a) Reaction between vinegar and baking soda. A spoonful of vinegar is taken in a beaker and a small amount of baking soda is added to it. A reaction takes
    place instantly and a gas escapes with a hissing sound. The chemical reaction that takes place can be represented by the following equation :
    $Vinegar + Baking soda \rightarrow Carbon dioxide + Other substances $

    The gas produced is passed into freshly prepared Limewater. Limewater turns milky. Which shows that the gas produced is carbon dioxide. This chemical reaction can be represented by the equation
    Carbon dioxide $ [CO_{2}]$ + Lime water $[Ca(OH)_{2}] ->$ Calcium Carbonate $[CaCO_{3}] + Water[H_{2}O]$

    (b) To observe the changes that take place when copper sulphate reacts with iron we need copper sulphate crystals, a shining iron nail, a few drops of dilute sulphuric acid, water, a beaker and a test tube.
    Fill half the beaker with water. Dissolve a few small crystals of copper sulphate in it. Add a few drops of sulphuric acid to it. Pour a small amount of this solution in a test tube and keep aside. Drop the nail in the beaker and wait for some time.

    Observation: Copper sulphate dissolves in water to form a blue coloured solution. On dropping the iron nail, the blue solution changes to green after some- time. The surface of iron gets coated with a dull reddish brown deposit.

    Copper sulphate reacts with iron to form iron sulphate, a new substance. Therefore, the colour changes from blue to green. The dull reddish brown deposit formed on the iron nail is due to the formation of another new substance, copper. The reaction is written as:
    Copper sulphate [blue) + Iron -> Iron sulphate [green] + Copper [brown deposit]

    Q.2. What do you mean by rusting of iron? How can it be prevented?

    Answer: Rusting is one of the ways of corrosion in metals. When iron articles react with oxygen of the air and moisture, to form rust. The chemical reaction that takes place can be represented by the equation:

    $Iron (Fe) + Oxygen (O_{2}) + Water (H_{2}O)-> Rust (iron oxide, Fe_{2}O_{3})$

    Rusting destroys the iron articles slowly. Huge monetary loss is suffered due to rusting
    of iron used in making bridges, bodies of ships, vehicles, etc.
    To prevent iron articles from rusting, iron articles should be prevented from coming in contact with air or water or both. This can be done by painting the iron articles at regular intervals. The surface of the articles can also be coated with a layer of oil or grease, to prevent its contact with air or moisture.
    The best method to prevent rusting is galvanisation. It involves the deposition of a layer of zinc on iron. The metal chromium can also be deposited on iron to prevent rusting.

    Q.3. What is crystallisation? How can you separate sugar crystals from sugar solution?
    Answer: Crystallisation is a process in which a solid separates as crystals from its supersaturated solution.
    To separate sugar crystals from sugar solution, we need beaker, sugar, water, filter paper, thread, etc.
    Procedure : Fill half a beaker with water. Make a saturated solution of sugar in it. It means that, there should be some undissolved sugar in the beaker which settles down at the bottom. Heat the solution till all the sugar dissolves. Filter it and allow it to cool.
    Sugar crystals are formed in the beaker.

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