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#### Question 1:

In which of these do the molecules of substances undergo changes–physical change or chemical change?

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.

#### Question 2:

What kind of change does this represent?
Reactants → Products

Reactants → Products

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

#### Question 3:

What kind of reaction is
a. melting of wax
b. burning of wax?

(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.

#### Question 1:

Two chemicals in solution form react, forming an insoluble solid that settles down. What is the settled solid known as?

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'.

#### Question 2:

In which reaction will heat be released–exothermic reaction or endothermic reaction?

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.
$\mathrm{C}+2\mathrm{S}+\mathrm{Heat}\to {\mathrm{CS}}_{2}\phantom{\rule{0ex}{0ex}}\mathrm{C}+2\mathrm{S}\to {\mathrm{CS}}_{2}-\mathrm{Heat}$

#### Question 3:

Nitrogen + oxygen → nitrogen dioxide:
What type of reaction is this?

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. ${\mathrm{N}}_{2}+2{\mathrm{O}}_{2}\to 2{\mathrm{NO}}_{2}$

#### Question 4:

What is the characteristic of the reaction:
MN + PQ → MQ + PN
What is the general name given to such reactions?

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.

#### Question 5:

Which of the following is true for a substance that is oxidized in a chemical reaction–oxygen is added to it, hydrogen is removed from it, or both are true?

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.

#### Question 1:

The atoms or molecules of a solid are always arranged in a regular pattern. Do you agree? Give reasons.

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.

#### Question 2:

You are given a solution of a solid and a liquid. How can you obtain the solid from it?

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.

#### Question 3:

What is the process of obtaining crystals of a substance from its solution in water called?

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

#### Question 1:

Which of these is a chemical change?

(a) A substacne with different chemical properties is formed.
(b) A substance in a different state but similar chemical properties is formed.
(c) The molecules of the substances do not undergo any change.
(d) A substance with a different shape and size but similar chemical properties is formed.

(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.

#### Question 2:

Which of these is an irreversible physical change?

(a) rusting of iron
(b) freezing of water
(c) a double decomposition reaction
(d) breaking of a glass tumbler

(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.

#### Question 3:

An arrow pointing downwards in a chemical reaction shows

(a) evolution of gas
(b) formation of precipitae
(c) oxidation
(d) displacement

(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.

#### Question 4:

A chemical reaction in which heat is given out is called

(a) exothermic reaction
(b) endothermic reaction
(c) decomposition reaction
(d) neutralization reaction

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

#### Question 5:

In a combination reaction how many products are formed?

(a) two only
(b) one or two only
(c) one only
(d) number cannot be specified

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

#### Question 6:

What happens in a displacement reaction?

(a) A more reactive element replaces a less reactive element.
(b) A less reactive element replaces a more reactive element.
(c) An element replaces one which is equally reactive.
(d) Any element can replace any other element with no relation to reactivity.

(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.

#### Question 7:

A neutralization reaction is a

(a) decomposition reaction.
(b) displacement reaction.
(c) combination reaction.
(d) double displacement reaction.

(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.

#### Question 8:

An oxidation reaction involves addition of

(a) oxygen only
(b) hydrogen only
(c) oxygen or removal of hydrogen
(d) hydrogen or removal of oxygen

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

#### Question 9:

If which of the following reaction is there only one reactant?

(a) combination reaction
(b) decomposition reaction
(c) single displacement reaction
(d) double displacement reaction

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

#### Question 10:

If A is more reactive than B, which of the following is true?

(a) A will replace B from a salt of B.
(b) B will replace A from a salt of A.
(c) Both can displace each other from their salts.
(d) Both cannot displace each other from their salts.

(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.

#### Question 1:

In which changes do substances undergo chemical transformation-physical or chemical?

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

#### Question 2:

Rush is a mixture and does not have a chemcial formula. True of false?

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

#### Question 3:

The process of depositing zinc on iron to prevent rusting is called__________

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

#### Question 4:

Which gas is given off when calcium carbonate is heated?

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

#### Question 5:

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

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

#### Question 6:

Heat is absorbed in an _________ reaction.

Heat is absorbed in an endothermic reaction.

#### Question 7:

If an equation has '+ heat' written on its right-hand side, what type of reaction is it?

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

#### Question 8:

Identify what kind of reaction each of the following is:

(a) X     +  Y       →   XY
(b)            XY     →   X      +   Y
(c) AB   +  XY     →   AY   +   XB
(d) AB   +  X       →   AX   +   B
(e) AB   +  CD     →  AD   +   BC
(f) Acid +  base   →  salt  +   water

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

#### Question 9:

An oxidizing agent provides ________ or removes _________ from a substance.

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

#### Question 10:

What method will you use to get a pure liquid if it has dissolved impurities present in it?

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

#### Question 11:

Salt is obtained from sea water by crystallization. True or false?

False.
Salt is obtained from sea water by evaporation.

#### Question 12:

A supersaturated solution of a solid in a liquid crystallizes easily. True or false?

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

#### Question 1:

Why are water pipes galvanized?

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.

#### Question 2:

When water is frozen, it forms ice that has different properties. Why then do we say that freezing is physical change?

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.

#### Question 3:

Give one example each of a chemical reaction showing the following characteristics.

(a) evolution of a gas
(b) change of colour
(c) formation of precipitate
(d) change of state from liquid to gas
(e) change of state from gas to liquid

(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↓       +      H2SO4
​     Copper            Hydrogen        Copper             Sulphuric
sulphate              sulphide         sulphide               acid

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

(e) ​2H2 (g)       +   O(g)       →       2H2O (l)
Hydrogen         Oxygen                 Water

#### Question 4:

Distinguish between endothermic reactions and exothermic reactions.

 Endothermic Reaction Exothermic Reaction These reactions are accompanied with absorption of heat. These reactions are accompanied with evolution of heat. Melting of solids, fusion, evaporation and vaporisation are examples of endothermic reaction. Combustion of petrol and oxidation of ammonia are examples of exothermic reaction.

#### Question 5:

What is a combination reaction? Give an example.

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

#### Question 6:

When is a substance said to be reduced in a chemical reactions?

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

#### Question 7:

What is crystallization?

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.

#### Question 8:

What is a supersaturated solution?

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.

#### Question 1:

Discuss the methods by which rusting of iron can be prevented.

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.

#### Question 2:

Which gas is given off when dilute hydrochloric acid is added to calcium carbonate?
How will you test the gas? Write the chemical equations for both reactions.

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.

#### Question 3:

Explain the steps involved in preparing the base magnesium hydroxide from the metal magnesium.

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

#### Question 4:

A decomposition reaction is the opposite of a combination reaction. Explain with the help of an example.

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 → 2H2

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

#### Question 5:

Under which two conditions do decomposition reactions normally take place? Give one example of each.

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.
​    ​2Al2O3    →    4Al   +  3O2

#### Question 6:

What is a displacement reaction? Which rule governs displacement of an element in a compound by another element? Explain with an example.

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

#### Question 7:

What is a double decomposition reaction? How many types of double decomposition reaction do you know of? Give one example of each.

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

#### Question 8:

Explain the terms-oxidation and oxidizing agent.

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.

#### Question 1:

In which city do you expect rusting to be a greater problem-in Delhi or in Mumbai? Why?

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.

#### Question 2:

When a candle burns, both physical and chemical change occur. What are these changes?

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.

#### Question 3:

LPG (liquefied petroleum gas) in a gas cylinder is in the form of a liquid. Which physical and chemical changes occur when it comes out of the cylinder and burns?

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.

#### Question 4:

When a soda water bottle is opened, a gas is released. Does a chemical reaction occur? Explain.

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.

#### Question 5:

Why should we not put stainless steel spoons in pickle jars?

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

#### Question 1:

Meeta's mother complained that white deposits form on all her utensils used for cooking. Meeta knows that the deposits are of calcium carbonate that gets left behind when water in the utensils evaporates. She also knows that calcium carbonate reacts with hydrochloric acid. However, she realises that hydrochloric acid is not easily available and is too dangerous for her to use in the kitchen to remove the stains.
Suggest to Meeta a method that uses the same principle as she thought of, but solves the two problems she has with hydrochloric acid.