Living Science 2019 2020 Solutions for Class 7 Science Chapter 8 - Physical And Chemical Changes

Answer:

In chemical change molecules of substances undergo changes, as in a chemical change molecules reacts with other molecules. Sometimes they combine with other molecules and sometimes they degenerate into smaller molecules. While in a physical change, only the shape size, and color of the substance changes, which does not affect the molecular structure of the substance. Hence, we can say in a chemical change, molecules of the substance undergo changes.

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Reactants → Products

The following representation shows a chemical reaction, in which reactants are converting into products.

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(a) Melting of wax is an example of physical change.
In a physical change, the substance undergoes only change in physical properties such as shape size, colour, and state. 

(b) Burning of wax is an example of chemical change.
In a chemical change, reactants get converted to give new products/substances.

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When two chemicals react in their solution form, an insoluble solid is formed which settles down to the bottom. That insoluble solid is known as a 'precipitate'.

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Heat is released in exothermic reactions.

Exothermic reactions are those reactions in which heat is released.
For eg. $\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2+\mathrm{Heat}$

Endothermic reactions are those reactions in which heat is absorbed.
For eg.
            $$\begin{gathered} \mathrm{C}+2\mathrm{S}+\mathrm{Heat}\to {\mathrm{CS}}_2 \\ \mathrm{C}+2\mathrm{S}\to {\mathrm{CS}}_2-\mathrm{Heat} \end{gathered}$$

Answer:

Following reaction is an example of a combination reaction.
When two or more elements combine together to give a compound, then it is known as a combination reaction.
Eg. ${\mathrm{N}}_2+2{\mathrm{O}}_2\to 2{\mathrm{NO}}_2$

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Following reaction is an example of double displacement reaction.
MN + PQ → MQ + PN
In double displacement reaction, compounds react by exchanging their elements or groups.
All neutralization reactions are an example of double displacement reaction.
 

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Oxidation can be defined as the addition of oxygen or removal of hydrogen from the substance. So, both are true for the oxidized substance in a chemical reaction.

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Yes, atoms or molecules are arranged in a regular pattern.
Solids have a regular arrangement of atoms, they are tightly arranged and are not free to move.
 

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We can separate a solid from a liquid by the process of 'Evaporation'. This process is based on the fact that liquids vaporize easily while solid do not. In this process, a solution of a solid and a liquid is heated, molecules of liquid start evaporating and solid substance starts to settle down in the jar. After complete evaporation of liquid, the solid substance can easily be extracted.

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The process of obtaining crystals of a substance from its solution in water is called crystallization.

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(a) A substance with different chemical properties is formed.
A chemical change occurs when two substances react chemically to form new substances with different chemical properties.

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(d) breaking of a glass tumbler
Breaking of a glass tumbler is an irreversible physical change. Rusting of iron is a chemical change. Freezing of water is a reversible physical change. A double decomposition reaction is a chemical change.

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(b) formation of precipitate
In a chemical reaction, an arrow pointing downwards shows the formation of a precipitate and an arrow pointing upwards shows the evolution of a gas.

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(a) exothermic reaction
Exothermic reactions are accompanied with release of heat. Endothermic reactions are accompanied with absorption of heat.

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(c) one only
In a combination reaction, a compound is formed from its elements. For example, water and oxygen combine to form water.

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(a) A more reactive element replaces a less reactive element.
In a displacement reaction, a more reactive element replaces a less reactive element from its compound.

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(d) double displacement reaction.
A neutralisation reaction is a double displacement reaction in which an acid reacts with a base, and the radicals are exchanged, to form salt and water.

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(c) oxygen or removal of hydrogen
An oxidation reaction involves addition of oxygen or removal of hydrogen from a substance.

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(b) decomposition reaction
In a decomposition reaction, a compound (reactant) breaks up into two or more substances (products).

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(a) A will replace B from a salt of B.
The given reaction is an example of displacement reaction in which a more reactive element replaces a less reactive element from its salt solution.

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In chemical changes, substances undergo chemical transformation to form new substances.

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False.
Rust is a compound. Its chemical formula is Fe₂O₃.H₂O.

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The process of depositing zinc on iron to prevent rusting is called galvanization.

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When calcium carbonate is heated, calcium oxide is formed and carbon dioxide is evolved.
        CaCO₃ → CaO + CO₂

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When two soluble substances in solution react to give an insoluble substance, a precipitate is formed.

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Heat is absorbed in an endothermic reaction.

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'+ heat' sign signifies that the reaction is accompanied with the evolution of heat. The reaction is exothermic.

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(a) Combination reaction
(b) Decomposition reaction
(c) Double displacement reaction
(d) Single displacement reaction
(e) Double displacement reaction
(f) Neutralisation reaction

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An oxidising agent provides oxygen or removes hydrogen from a substance.

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We will use crystallisation method to obtain a pure liquid if it has dissolved impurities in it.

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False.
Salt is obtained from sea water by evaporation.

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True.
A supersaturated solution contains more solute than it can remain dissolved in water. It is highly unstable and crystallises easily.

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Water pipes are made up of iron. Rusting of iron occurs when iron interacts with air and water. Therefore, to prevent rusting of iron, water pipes are galvanized, i.e. iron pipes are coated with metals such as zinc or chromium to prevent iron from interacting with air.

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In a physical change, only the physical properties of a substance change and no new substances are formed.
Freezing of water is a physical change because only the physical state of water is changed; i.e. water is converted into ice. Hence, freezing is a physical change.

Answer:

 (a) Zn    +    H₂SO₄        →         ZnSO₄     +     H₂↑
​     Zinc     Sulphuric                    Zinc          Hydrogen
                   acid                       sulphate
 (b) CuCO₃       →       CuO     +      CO₂↑​
       Copper               Copper        Carbon
    carbonate              oxide            dioxide
     (green)                (black)

(c) CuSO₄      +       H₂S      →       CuS↓       +      H₂SO_4
​     Copper            Hydrogen        Copper             Sulphuric
  sulphate              sulphide         sulphide               acid

(d) 2H₂O (l)     →      2H₂ (g)     +     O₂ (g)
     Water                Hydrogen       Oxygen

(e) â€‹2H₂ (g)       +   O₂ (g)       →       2H₂O (l)
   Hydrogen         Oxygen                 Water

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Answer:

In a combination reaction, a compound is formed from its elements or from simpler substances. For example, magnesium reacts with oxygen on heating to form magnesium oxide.
 2Mg + O₂ → 2MgO

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 In a chemical reaction, a substance is said to be reduced if hydrogen is added to it or oxygen is removed from it.

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Crystallisation is a process in which pure solid is separated from a solution in the form of its crystals.
For example, common salt obtained from sea water contains soluble impurities such as magnesium chloride. To purify common salt, crystallisation process is applied.

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A solution in which more solute is dissolved by increasing the temperature of a saturated solution is called supersaturated solution. When a supersaturated solution is cooled to room temperature, it crystallises easily.

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Rusting of iron can be prevented by following methods:
(a) Painting: A coat of paint applied on the surface of an iron object prevents oxygen and moisture present in the atmosphere from interacting with the iron object.
(b) Oil or grease: Applying oil or grease on the surface of iron tools helps to keep out the oxygen and moisture of the atmosphere.
(c) Galvanization: Galvanization is a process of coating iron objects with a thin layer of metal such as zinc or chromium. A thin coating of zinc or chromium protects the iron from rusting.

Answer:

Carbon dioxide is given off when dilute hydrochloric acid is added to calcium carbonate. The equation involved is:
                             CaCO₃    +     HCl     →     CaCl₂     +     H₂O     +     CO₂↑

Carbon dioxide can be tested by passing it over lime water. Lime water turns milky. The equation involved is:
                      Ca(OH)₂      +        CO₂        →        CaCO₃      +      H₂O
                        ​Lime                  Carbon               Calcium            Water
                                                  dioxide              carbonate

The calcium carbonate formed in the reaction is white in colour and not much soluble in water. It gives milky appearance and is used as a test for the presence of carbon dioxide.

Answer:

The following steps are involved in preparing the base magnesium hydroxide from the metal magnesium:
(a) Take a small piece of magnesium ribbon.
(b) Clean its tip with sandpaper.
(c) Hold it with a pair of tongs and bring it near the flame. It burns with a dazzling flame and forms a powdery ash (magnesium oxide).
(e) Collect the ash in a test tube and mix it with a small amount of water and stir.
When magnesium oxide dissolves in water, it forms magnesium hydroxide. The equation involved is:
                    2MgO       +       2H₂O       →       2Mg(OH)₂
                Magnesium           Water                Magnesium
                    oxide                                            hydroxide

Answer:

The reaction in which a compound breaks down to give two or more simpler substances is called a decomposition reaction.
For example, on passing an electric current through water in which a small amount of sulphuric acid has been added, water decomposes to give hydrogen and oxygen gases.
2H₂O → 2H₂ + O₂

The reaction in which two or more elements combine to form a single compound is called a combination reaction.
For example, hydrogen combines with oxygen to form water.
2H₂ + O₂ → 2H₂O 

From the above examples, it is clear that a decomposition reaction is the opposite of a combination reaction.

Answer:

The two conditions under which decomposition reaction normally takes place are as follows:
(a) Heating: When calcium hydroxide is heated, calcium oxide and steam are produced.
                      Ca(OH)₂       →         CaO      +     H₂O
​                      Calcium                Calcium         Water
                     hydroxide                oxide
(b) Electrolysis: When an electric current is passed through a molten aluminium oxide, it undergoes decomposition to form aluminium and oxygen gas.
                  â€‹    â€‹2Al₂O₃    →    4Al   +  3O₂

Answer:

In a displacement reaction, one element replaces another element from a compound. Generally, a more reactive element replaces a less reactive element from its compound. For example, iron is more reactive element than copper. If a piece of iron is added to a solution of copper sulphate, iron sulphate is formed. Copper metal gets deposited on the iron piece.

                  CuSO₄       +      Fe      →      FeSO₄      +      Cu
                 Copper              Iron               Iron                Copper
                 sulphate                                  sulphate 

Answer:

A reaction in which two compounds react to form two other compounds by mutual exchange of atoms or group of atoms is called double displacement reaction. There are two types of double displacement reaction.

(a) Precipitation reaction: In this reaction, two compounds react in their aqueous state to form an insoluble product, which appears in the form of a precipitate. For example, aqueous sodium chloride and silver nitrate react to form a white precipitate of silver chloride.
   
            NaCl      +      AgNO₃     →      AgCl↓​     +      NaNO₃
         Sodium               Silver              Silver              Sodium
         chloride              nitrate          chloride             nitrate  

(b) Neutralisation reaction: When a base and an acid react, they exchange their ions and form compounds called salt, water and sometimes a gas. For example, hydrochloric acid reacts with potassium hydroxide to form potassium chloride (salt) and water.
                 HCl        +        KOH         →      KCl       +       H₂O
         Hydrochloric        Potassium         Potassium        Water 
                acid                hydroxide          chloride

Answer:

Oxidation is a chemical reaction that involves addition of oxygen or removal of hydrogen from a substance. For example, when carbon reacts with oxygen to form carbon dioxide, it is said to be oxidised by gaining oxygen.
            C      +      O₂       →     CO₂
      Carbon       Oxygen       Carbon
                                            dioxide

Similarly, when hydrogen sulphide reacts with chlorine, hydrogen sulphide is said to be oxidised to sulphur by loss of hydrogen.
 
         H₂S       +      Cl₂       →     S        +       2HCl
    ​Hydrogen       Chlorine      Sulphur         Hydrochloric
      sulphide                                                     acid 
 
An oxidising agent is one that oxidises other substances either by providing oxygen to them or by removing hydrogen from them.

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Two conditions are required for rust formation: oxygen and moisture content in the atmosphere. The climate of Delhi is hot and dry, whereas the climate of Mumbai is humid. Hence, rusting is a greater problem in Mumbai because there is a greater presence of moisture and oxygen in the atmosphere.

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Melting of wax is a physical change and formation of carbon dioxide and water vapour is a chemical change. Therefore, both physical and chemical changes occur when a candle burns.

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LPG in a cylinder exists as a liquid. When it comes out of the cylinder, it becomes a gas, which is a physical change. Burning of LPG is a chemical change.

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No, the chemical reaction does not occur. When a soda bottle is opened, carbon dioxide is released. The carbon dioxide is already dissolved inside the soda bottle. On removing the cap, the pressure inside is released and carbon dioxide escapes.

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Steel reacts with acids present in the pickle and forms rust. Hence, we should never put stainless steel spoons in pickle jars.

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As hydrochloric acid is not easily available and is too dangerous to use in normal daily use, so, in place of hydrochloric acid, Meeta can use citric acid which is easily available and does not cause much harm. Citric acid can easily be obtained from the lemon.