Chemical Reactions and Equations Class 10 Notes Science Chapter 1

Chemical reaction & Balancing Chemical Equations, Class 10, Science

 1. Introduction

Change is the law of nature. Scientist classify these changes as physical changes and chemical changes. When a chemical change occurs, a chemical reaction is said to have taken place.

< Physical change : - A change in which the physical properties of the substance changes but the chemical composition does not change. The substance is restored to its original state as soon as the cause of change is withdrawn.

< Chemical change : - In a chemical change, at least one of the reacting substance changes into a new substances with a different composition. The new substances can not be changed back to the original substance even if the cause of change is withdrawn.

< Chemical reaction :- The processes, in which a substance or substances undergo a chemical change to produce new substance or substances, with entire new properties, are known as chemical reaction.

The nature and identity of products totally changes from the reactants. Observations which determines whether the chemical reaction has taken place or not.

(a) Chemical reaction must be associated with change in temperature i.e. Heat should be either evolved or absorbed.

(b) The reaction must occur between fixed quantities of the reactants.

(c) The chemical reaction should follow the law of conservation of mass.

(d) The products obtained must have properties different from those of the reactants.

Example - (i) When potassium nitrate is heated, it gives potassium nitrite and oxygen.

(ii) When sodium reacts with water sodium hydroxide is produced and hydrogen gas is liberated.

As description of a chemical reaction in a sentence form is qute long so when it is written in shorter form by using words and some signs (+ for addition & arrow (®) to show the direction), then it is called Word equation.

< Word equation :- A chemical equation which represents a chemical reaction briefly in words is called word equation.

Ex. - For the example (ii) the word equation is

Sodium + water → Sodium hydroxide + Hydrogen

< Reactants :- The substance or substances which takes part in a chemical reaction are called reactants.

< Products :- The new substance or substances formed as a result of a chemical reaction are called products.

In the above reaction sodium & water are reactants and sodium hydroxide & hydrogen are products.

Rules for writing a word equation

(i) The substances taking part in chemical reaction reactants are always written on the left hand side of arrow.

(ii) The substances formed after the chemical reaction, products are always written on the right hand side of arrow.

(iii) A plus sign (+) is put in between the reactants or between the products. If their number is two or more.

(iv) An arrow ( is put between the reactants and products, the arrow shows the direction of the reacton in which the reaction proceeds. The arrow is read as "to yield" or "to form". 

In the word equation when symbols and chemical formulae of the reactants and products are used then it is called as chemical equation. 

Example - Na + H₂O → NaOH + H₂

i.e. A chemical equation is a statement that describes a chemical reaction in terms of symbols and formulae. In this equation the law of conservation of mass is not obeying. Such chemical equation is called skeletal equation or unbalanced equation.

A chemical equation expressed in symbols and formulae, such that the number of atoms of different elements towards the side of the reactants is not equal to the number of atoms of the products is called skeletal equation or unbalanced equation.

To make this equation meaningfull, this equation is balanced then it is called balanced chemical equation

Charge Table 

One which contains an equal number of atoms of each element on both sides of the equation.

< Balancing chemical equations :-

The simple equations are balanced by "hit and trial method". Which is done in following steps.

Step (i) - Count the no. of atoms of various elements on both sides of the equation

Example - Fe + H₂O → Fe₃O₄ + H₂

Step (ii) - Start balancing with the compound which contains maximum number of atoms. It may be a reactant or product. In that compound select the element which has maximum number of atoms.

According to this rule Fe₃O₄ has maximum number of atoms & oxygen has 4 atoms so it is selected.

So the partly balanced equation is

Fe + 4H₂O → Fe₃O₄ + H₂

Step (iii) - Fe and H are not balanced in the above reaction so the above reaction repeat the above process for both i.e.

Now the equation becomes as 

Fe + 4H₂O → Fe₃O₄ + 4H₂

Step (iv) - Balance the Iron atoms similarly.

Now the equation is

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Step (v) - Finally check the correctness of the balanced equation by counting the number of atoms on both sides of the equation.

Step (vi) - To make chemical equation more informative physical states of the reactants and products are mentioned as for solid (s), liquid (l), gas (g) and for aqueous solution of reactant or product (aq ) is written.

Now the equation becomes as

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

Symbol (g) with water is written to show that water is used in the form of steam in this equation.

If a gas is evolved in a reaction it can be shown by the symbol (­) after the formula i.e. arrow pointing upwards e.g.

2Na(s) + 2H₂O(l) → 2 NaOH(aq) + H₂(g) or H₂(­) 

The symbol (¯) or ppt is be written for precipitate.

NaCl + AgNO₃ → NaNO₃ + AgCl (¯) or ppt.

Reversible reaction is represented by () symbol and irreversible reaction by  symbol.

The heat evolved in chemical reaction is written on the right side by putting positive (+) sign and heat absorbed in the chemical reaction is written on the right hand side by putting negative (-) sign.

N₂ + 3H₂ 2NH₃ + 22400 calorie (Exothermic reaction)

N₂ + O₂ 2NO - 43200 calorie (Endothermic reaction)

Some times the reaction conditions, such as temperature, pressure, catalyst etc. are written above or below the arrow in the equaton e.g.

N₂ + 3H₂ 2NH₃ + 22400 calorie heat

450°C (Fe-Mo)

< Exothermic reaction :- The reaction in which heat is liberated (or given out) is called an exothermic reaction Ex - C(s) + O₂(g) ¾® CO₂(g) + Heat (393 kJ/mol)

< Endothermic reaction :- The reaction in which heat is absorbed (or taken in ) is called an endothermic reaction

Ex - C(s) + 2S(g) → CS₂(g) Heat (92 kJ/mol)

The reaction with + Heat term on the product side are called exothermic reaction, while those with -Heat term on the product side are called endothermic reactions.

During respiration, the digested food gets oxidised and the energy is released. That is why, it is considered as an exothermic reaction.

< Balancing of a chemical equation is necessary because no matter (hence, no atom) is lost or gained during a chemical reaction.

Q.1 Translate the following statements into chemical equations and then balance them.

(A) Hydrogen gas combines with nitrogen to form ammonia

(B) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(C) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

(D) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Ans. (A) N₂(g) + 3H₂(g)  2NH₃(g)

(B) 2H₂S + 3O₂ ¾® 2H₂O + 2SO₂

from air

(C) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) ¾® 2AlCl₃(aq) + 3BaSO₄(s)

White ppt.

(D) 2K(s) + 2H₂O(l) ¾® 2KOH + H₂(g)

Q.2 Balance the following chemical equations.

(A) HNO₃ + Ca(OH)₂ ¾® Ca(NO₃)₂ + H₂O

(B) NaOH + H₂SO₄ ¾® Na₂SO₄ + H₂O

Ans. (A) 2HNO₃ + Ca(OH)₂ ¾® Ca(NO₃)₂ + 2H₂O

(B) 2NaOH + H₂SO₄ ¾® Na₂SO₄ + 2H₂O

Q.3 Write the balanced chemical equations for the following reactions.

(A) Calcium hydroxide + Carbon dioxide ¾® Calcium carbonate + water

(B) Zinc + Silver nitrate ¾® Zinc nitrate + Silver

(C) Aluminium + copper chloride ¾® Aluminium chloride + Copper

(D) Barium chloride + Potassium sulphate ¾® Barium sulphate + Potassium chloride.

Ans. (A) Ca(OH)₂ + CO₂ ¾® CaCO₃ + H₂O

calcium carbon calcium

hydroxide dioxide carbonate

(B) Zn + 2AgNO₃  ¾® Zn(NO₃)₂ + 2Ag

zinc silver nitrate zinc nitrate silver

(C) 2Al + 3CuCl₂ ¾® 2AlCl₃ + 3Cu

aluminium copper aluminium copper

chloride chloride

(B) BaCl₂ + K₂SO₄   ¾® BaSO₄ + 2KCl

barium potassium barium potassium

chloride sulphate sulphate chloride

Q.4 Write the balanced chemical equation for the following and identify the type of reaction in each case -

(A) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

(B) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide(g)

(C) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

(D) Magnesium(s) + Hydrochloridc acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Ans. (A) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(aq)

This reaction is a double-displacement reaction.

(B) ZnCO₃(s) → ZnO(s) + CO₂(g)

This reaction is a decomposition reaction

Q.5 Why should a magnesium ribbon be cleaned before burning in air?

Ans. Magnesium reacts with the constituent gases of the atmosphere to form various compounds which get deposited over its surface. The ribbon is cleaned before burning to remove the layer of these compounds so that pure magnesium can burn in air.

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Types of Chemical Reactions - Chemical Reactions and Equations, Class 10, Science

< Types of chemical reactions :- Chemical reaction occurs as a result of breaking and making of bonds resulting in redistribution of atoms among various reacting species in different ways. Accordingly the reactions are classified in different types. They are -

(1) Combination reaction or synthesis reaction 

(2) Decomposition reaction/Analysis reaction 

(3) Displacement reaction 

(4) Double displacement reaction/Methasis reaction 

(5) Oxidation and Reduction reaction.

(1) Combination reaction :- The reaction in which two or more substances combine to form a single new substance are called combination or synthesis reaction.

C(s) + O₂(g) → CO₂(g)

< Combination reactions are of three common types :- 

(i) Combination of two elements to form a

Compound eg.

(a) H₂(g) + Cl₂(g)  2HCl(g)

(b) 2H₂(g) + O₂(g) → 2H₂O(l)

(ii) Combination of an element and a compound to form a new compound.

(a) 2SO₂(g) + O₂(g) → 2SO₃(g)

(iii) Combination of two compounds to form a new compound :-

(a) NH₃(g) + HCl(g) → NH₄Cl(g)

(2) Decomposition reaction :- The reaction in which a single compound breaks up into two or more simpler substances are known as decomposition reaction. The decomposition reaction generally takes place when energy in some forms such as heat, electricity or light is supplied to the reactants.

(a) Decomposition reactions by heat (Thermal decomposition)

(i) CaCO_3(s) CaO_(s) + CO_2(g)

(Lime stone) (Quick lime)

(iii) 2FeSO_4(s) Fe₂O_3(s) + SO_2(g) + SO_3g)

(Ferrous sulphate) (ferric oxide)

(iv) 2Pb(NO₃)_2(s) ¾® 2PbO + 4NO_2(g) + O_2(g)

(Lead nitrate) (Lead oxide) (Nitrogen dioxide)

(b) Decomoposition by electricity (Electrical decomposition or Electrolysis) 

(i) 2H₂O_(l) 2H_2(g) + O_2(g)

(c) Decomposition by sunlight (Photochemical decomposition)

(i) 2 AgCl_(s) 2Ag_(s) + Cl_2(g)

(silver chloride) (Silver) (chlorine)

(white) (Grey in colour)

(ii) 2AgBr(s)  2Ag_(s) + Br_2(g)

(silver bromide) (Silver) (Bromine)

The decomposition of a compound with light is called photolysis.

Note :- All the decomposition reaction requires energy i.e. these reactions are Endothermic reactions. These reactions are used in extractions of metals.

Decomposition reaction is called opposite of combination reaction. This can be supported by the following reactions:

combination reaction 2H₂(g) + O₂(g)  2H₂O(l)

hydrogen oxygen water

Decomposition reaction : 2H₂O(l) 2H₂(g) + O₂(g)

water hydrogen oxygen

Activity Series of Hydrogen and Common Metals

Potassium K Most Reactive

Sodium Na

Barium Ba

Calcium Ca

Magnesium Mg

Aluminium Al

Zinc Zn

Iron Fe

Nickel Ni

Tin Sn

Lead Pb

Hydrogen H

Copper Cu

Mercury Hg

Silver Ag

Gold Au Least Reactive

(3) Displacement reactions :- The chemical reaction in which one element takes the place of another element in a compound, are called displacement reactions e.g. -

(i) Fe_(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

(copper sulphate) (Iron sulphate)

(ii) Zn_(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

(copper sulphate) (Zinc sulphate)

Iron, zinc and lead are more reactive elements than so they displace copper from its compounds.

(iii) 2KBr + Cl₂ → 2KCl + Br₂

(potassium bromide) (potassium chloride) (Bromine)

(iv) Copper displaces silver from silver nitrate.

2AgNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + 2Ag(s)

(silver nitrate) (copper) (copper nitrate) (silver)

Q.6 In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Ans. 2AgNO₃(aq) + Cu(s) → 2Ag(s)  + Cu(NO₃)₂(aq)

(silver nitrate) (copper) (silver metal) (copper(II) nitrate) 

(4) Double displacement reactions :- The reactions in which two compounds react to form two different compounds by mutual exchange of ions, are called double displacement reactions. These reactions are also called as Metathesis reaction.

< Two common types of double displacement reactions are - 

(A) Precipitation reaction :- Any reaction that produces a precipitate, (the insoluble substance formed), is called precipitation reaction e.g. -

(i) Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) (¯) + 2NaCl(aq)

(Sodium sulphate) (Barium chloride) (Barium sulphate) (Sodium chloride)

(B) Neutralisation reaction :- When an acid reacts with base to form salt and water by exchange of ions e.g.-

(i) NaOH(aq) + HCl(aq) ¾® NaCl(aq) + H₂O(l) 

(Base) (Acid) (Salt) (Water) 

(5) Oxidation and Reduction reactions or Redox reaction :-

< Oxidation :-

(i) The addition of oxygen to an element or compound.

(a) 2Cu + O₂ 2CuO

(ii) Removal of hydrogen from a compound is known as oxidation.

2HCl Cl₂

< Reduction :-

(i) The addition of hydrogen to an element or compound

H₂ + O₂ ¾® 2H₂O

(ii) Removal of oxygen from a compound.

CuO + H₂ Cu + H₂O

< Oxidising agent :- The substance which gives oxygen or removes hydrogen for oxidation is called oxidising agent and the substance which gains oxygen during reaction is said to be oxidised.

< Reducing agent :- The substance which gives hydrogen or removes oxygen for reduction is called reducing agent. The substance which gains, hydrogen during reaction is said to be reduced.

Those reactions in which oxidation and reduction (both) occurs simultaneously are called redox reactions.

In the name Redox the term 'red' stands for reduction and 'ox' stands for oxidation. 

Example

· SO₂ is reduced to sulphur, so it is oxidising agent.

· H₂S is oxidised to sulphur, so it is reducing agent.

It should be noted that substance which undergoes oxidation acts as reducing agent where as the substance undergoes reduction act as oxidising agent.

There is an another concept of oxidation and reduction in terms of metals and nonmetals. This is as follows

(i) The addition of nonmetallic element (or removal of metallic element) is called oxidation.

(ii) The addiition of metallic element (or removal of nonmetallic element) is called reduction.

Q.7 A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.

Ans. An element on heating in air changes in its oxide. The brownish element which forms black oxide is copper. So,

Name of the element Copper (Cu)

Name of black compound: Copper(II) oxide, (CuO)

Reaction 2Cu(s) + O₂(g) 2CuO(s)

copper (brown) from air copper(II) oxide (black)

Electronic concept for oxidation and reduction

< Oxidation - The loss of an electron by atoms or ions is called oxidation.

Atom → Cation + electrons

A → Aⁿ⁺ + ne^-

Atom 'A' loses n electrons to become a positively changed ion Aⁿ⁺. It is called cation

< Reduction :- The gain of an electron by an atom or ion is called reduction.

B + ne^- → B^n-

The atom B gains n' electrons to become negatively charged ion B^n-, it is called anion.

Oxidation and reduction reactions occurs simultaneously and are called as redox reactions. Only oxidation or only reduction is called half reaction. i.e.

A → Aⁿ⁺ + ne^- - Oxidation

B + ne^- → B^n- - Reduction

A + B → A⁺B^- → AB Redox

e.g. Na + Cl → NaCl

In this process sodium loses one electron and oxidised to Na⁺, chlorine gains this electron and is reduced to Cl^-.

Na → Na⁺ + e^- (loss of an electron is oxidation )

Cl + e^- → Cl^- (Gain of an electron is reduction)

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Corrosion, Rusting and Rancidity - Chemical Reactions and Equations, Class 10, Science

< Effects of oxidation reactions in everyday life :- Oxidation has damaging effect on metals as well as on food. The damaging effect of oxidation on metals is studied as corrosion and that on food is studied as rancidity. Thus there are two common effects of oxidation reactions are as

(i) Corrosion of metals (ii) Rancidity of food 

(i) Corrosion of metals :- Corrosion is the process of deterioration of metals as a result of its reaction with air, moisture and acids. (Present in environment) surrounding it.

The corrosion causes damage to buildings, bridges, ships and many other articles especially made of iron.

Rust : Iron corrode readily when exposed to moisture and gets covered with a brown feaky substance called rust. It is called rusting of iron, Rust is a hydrated Iron (III) oxide Fe₂O₃ · 2H₂O. 

Rusting of iron takes place under the following conditions -

(a) Presence of air (or oxygen)

(b) Presence of water (or moisture)

It has been observed that

(a) Presence of impurities in the metal speed
up the rusting process. Pure iron does
not rust.

(b) Presence of electrolytes in water also speeds up the process of rusting

(c) The position of the metal in the electrochemical series determines the extent of corrosion. More the reactivity of the metal, there will be more possibility of the metal getting corroded. Other examples of corrosion are -

(i) Coppper reacts with moist carbon dioxide in the air and slowly loses its shiny brown surface and acquires a green coating of basic copper carbonate. 

(ii) Silver articles become black after sometime when exposed to air because it reacts with sulphur to form a coating of silver sulphide. 

(iii) Lead or stainless steel lose their lusture due to corrosion. 

Unreactive metals such as gold, platinum, palladium, titanium etc. do not corrode. 

< Rancidity :- Fresh foods containing fats and oils smell and taste pleasant but when it remains exposed in air for a long time it's smell and taste changes to unpleasant. It is said that the food has become rancid.

OR

It is due to the oxidation of fats and oils, butter, ghee, boiled rice etc, after prolonged exposure to air i.e. The condition produced by the aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.

< Prevention of rancidity :- 

(i) Rancidity can be prevented by adding antioxidants to foods containing fats and oils. Antioxidants are reducing agents so when they are added to food it do not get oxidised easily and hence do not turn rancid. The two common anti oxidants are -

(a) BHA (Butylated Hydroxy Anisole) 

(b) BHT (Butylated Hydroxy Toluene)

Vitamin-E and vitamin-C (ascorbic acid) are the two antioxidants occuring in natural fats. 

(ii) Rancidity can be prevented by packaging fat and oil containing foods in nitrogen gas.

(iii) It can be retarted by keeping food in refrigerator.

(iv) It can also be retarded by storing food in air tight containers.

(v) It can be retarded by storing foods away from light.

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Previous Year Solved Questions - Chemical Equations and Reactions, Class 10, Science 

Solved Questions 

1. What is the basis of a balanced chemical equation ?

Or

State the law on which a balanced chemical equation is based.

Or

State the law of conservation of mass.

Or

On what basis is a chemical equation balanced ?

Ans. The basis of balanced chemical equation is the law of conservation of mass. Mass can neither be created nor destroyed in a chemical reaction.

2. Would you call digestion of food in our body a chemical change ?

Ans. Yes. It is a chemical change.

3. Balance the following chemical equation :

FeSO₄ Fe₂SO₃ + SO₂ + SO₃

Ans. Balanced chemical equation is

2FeSO₄ Fe₂SO₃ + SO₂ + SO₃

4. Balance the following chemical equation :

MnO₂ + HCl  MnCl₂ + Cl₂ + H₂O

Ans. Balanced chemical equation is

MnO₂ + 4HCl  MnCl₂ + Cl₂ + 2H₂O

5. Write a combination reaction in which two gases combine.

Ans. Hydrogen and chlorine gases combine to form hydrogen chloride.

H₂(g) + Cl₂(g)  2HCl(g)

6. What change in colour is observed when white silver chloride is left exposed to sunlight ? What type of chemical reaction is this ?

Ans. When silver chloride is exposed to sunlight, the white colour of silver chloride changes to grey colour. This is a photochemical decomposition reaction.

2AgCl(s)  2Ag(s) + Cl₂(g)

7. Why do we apply paint on iron articles ?

Ans. We apply paint on iron articles to protect them from corrosion.

8. How can you help your mother in keeping the fried items so that they do not develop a bad smell and their shelf life is increased ?

Ans. Since we cannot pack the fried objects in the atmosphere of nitrogen at home but we can increase their shelf life by keeping them in airtight containers. In this way they do not come in contact with oxygen. Their shelf life can further be increased by keeping them in fridge at low temperature.

9. A solution of a substance 'X' is used for white washing.

(i) Name the substance 'X' and write its formula.

(ii) Write the reaction of the substance 'X' named in (i) above with water.

Ans. (i) Calcium oxide or quick lime, its formula is CaO.

(ii) CaO(s) + H₂O(l) Ca(OH)₂(aq)

Quick lime water Slaked lime

10. Write any two limitations of a chemical equation.

Ans. A chemical equation does not provide the following informations :-

(i) Whether the reaction is fast, slow or instantaneous i.e. the rate of the reaction is not shown by a chemical equation.

(ii) Whether the reaction goes to completion or is stopped in between i.e., the extent to which a reaction takes place is not known from a chemical equation.

11. Balance the following equations :

(i) FeCl₃ + NH₄OH  Fe(OH)₃ + NH₄Cl

(ii) Na₂S₂O₃ + I₂ Na₂S₄O₆ + 2NaI

Ans. Balanced equations :-

(i) FeCl₃ + 3NH₄OH Fe(OH)₃ + 3NH₄Cl

(ii) 2Na₂S₂O₃ + I₂ Na₂S₄O₆ + 2NaI

12. Distinguish between an exothermic and an endothermic reaction. Amongst the following reactions, identify the exothermic reaction and the endothermic reaction :

(i) Heating coal in air to form carbon dioxide.

(ii) Heating limestone in alime kiln to form quick lime.

Ans. Exothermic reactions are those reactions in which heat is evolved.

Endothermic reactions are those which involve absorption of heat.

(i) Heating coal in air to form carbon dioxide is an exothermic reaction.

(ii) Heating limestone in a lime kiln to form quick lime is an endothermic reaction.

13. What is an oxidation reaction ? Give an example of oxidation reaction. Is oxidation an exothermic or an endothermic reaction ?

Ans. Reactions involving addition of oxygen are classified as oxidation reactions. For example magnesium reacts with oxygen to form magnesium oxide.

2Mg + O₂ 2MgO

Oxidation reactions are generally exothermic in nature.

14. What is a redox reaction ? When a magnesium ribbon burnsin air with a dazzling flame and forms a white ash, is magnesium oxidised or reduced ? Why ?

Ans. Those reactions in which oxidation and reduction reactions occur simultaneously, are called redox reactions. In these reactions one substance is oxidised and another substance gets reduced.

When a magnesium ribbon burns to form a white ash, magnesium metal is oxidised in this reaction because it combined with oxygen to form magnesium oxide.

2Mg(s) + O₂ (g)  2MgO(s)

15. In the reaction :

MnO₂(s) + 4HCl(aq)  MnCl₂(aq) + Cl₂(g) + 2H₂O(l)

Identify the oxidising and reducing agents.

Ans. MnO₂ loses oxygen, therefore it is reduced, thus it acts as an oxidizing agent.

HCl loses hydrogen. It itself is oxidized, thus it acts as a reducing agent.

In the above reaction

MnO₂ is oxidizing agent.

HCl is reducing agent.

16. Give an example of a decomposition reaction. Describe an activity to illustrate such a reaction by heating.

Ans. Calcium carbonate decomposes on heating and forms calcium oxide and carbon dioxide.

CaCO₃ CaO + CO₂

17. Define a combination reaction. Give one example of a combination reaction which is also exothermic.

Ans. Reactions in which two or more substances combine to form a new compound are called combination reactions. An example of this type of reactions is the combustion of magnesium in air, where magnesium combines with oxygen to form magnesium oxide, it is an exothermic reaction also.

2Mg(s) + O₂(g) 2MgO(s) + heat

18. Dilute solution of ammonium hydroxide is added to aqueous solution of ferrous sulphate. Ferrous hydroxide is formed. What is the type of this reaction ? Write chemical equation.

Ans. It is a double decomposition reaction.

FeSO₄ (aq) + 2NH₄OH (aq)

Ferrous sulphate Ammonium hydroxide

Fe(OH)₂ (s) + (NH₄)₂SO₄ (aq)

Ferrous hydroxide Ammonium sulphate

19. (i) What is observed when a solution of potassium iodide is added to a sulution of lead nitrate taken in a test tube ?

(ii) What type of reaction is this ?

(iii) Write a balanced chemical equation to represent the above reaction.

Ans. (i) A yellow precipitate of lead iodide will be formed.

(ii) This is a double displacement reaction.

(iii) Pb(NO₃)₂(aq) + 2KI(aq)  PbI₂(s) + 2KNO₃(aq)

20. Why does the blue colour of copper sulphate solution changes to green colour when an iron nairl is dipped? Write chemical equation.

Ans. The blue colour of the copper sulphate solution changes to green colour because copper of copper sulphate is replaced by iron and forms ferrous sulphate which is green coloured.

Fe(s) + CuSO₄(aq)  FeSO₄(aq) + Cu(s)

Iron Copper sulphate Ferrous sulphate Copper

PREVIOUS YEARS BOARD QUESTIONS : 

1. What happens chemically when quick lime is added or water ? [CBSE Delhi 2008] 

Ans. Calcium hydroxide (or slaked lime) is formed accompanied by a hissing sound. So much heat is evolved during the reaction that the reaction mixture starts boiling. The chemical equation for the reaction is :

CaO(s) + H₂O(aq)  Ca(OH)₂(s) + heat

(Quick lime) (Slaked lime)

2. What is an oxidation reaction ? Identify in the following reactions : [CBSE Delhi 2008]

(i) the substance oxidised (ii) the substance reduced.

ZnO + C  Zn + CO

Ans. Oxidation involves the addition of oxygen or the removal of hydrogen in a chemical reaction while reduction involves the addition of hydrogen or removal of organ.

In the given reaction :

(i) Carbon (C) is oxidised to carbon monoxide (CO).

(ii) Zinc oxide (ZnO) is reduced to zinc (Zn)

3. Which gas is evolved when dilute hydrochloric acid reacts with zinc ? Write the molecular formula of the gas.

Ans. Hydrogen gas is evolved. Its molecular formula is H₂. [CBSE Delhi 2008] 

4. State any two observation in an activity which may suggest that a chemical reaction has taken place. Give examples in support of your answer. [CBSE Foreign 2008] 

Ans. (i) In a tube take small amount of solid sodium carbonate

(Na₂CO₃). To this add a few drops of hydrocloric acid.

(ii) A colourless and oduless gas will evolve which shows that

a chemical reaction has taken place. The gas will turn lime

water milky when passed through it.

5. Identify the type of reaction in the following examples : [CBSE Delhi 2008]

(i) Na₂SO₄(aq) + BaCl₂(aq)  BaSO₄(s) + 2NaCl(aq)

(ii) Fe(s) + CuSO₄(aq)  FeSO₄(aq) + Cu(s)

(iii) 2H₂(g) + O₂(g)  2H₂O(l)

Ans. (i) It is an example of double displacement reaction.

(ii) It is an example of displacement reaction.

(iii) It is an example of combination reaction.

6. Solid calcium oxide was taken in a container and water was added slowly to it [CBSE All India 2008 Comptt.]

(i) State two observations made in the experiment.

(ii) Write the name of the chemical formula of the product.

Ans. (i) Water will start boiling and hissing noise will be produced.

(ii) Calcium hydroxide (slaked lime) will be formed.

CaO(s) + H₂O(aq)  Ca(OH)₂(s) + heat

Calcium oxide Calcium hydroxide

7. A house wife wanted heer house to be white washed. She bought 10 kg of quick lime from the market and dissolved in 30 litres of water. On adding lime to water, she noticed that the water started boiling even when it was not being heated. Give reason for her observation. Write the corresponding equation and name the product formed.  [CBSE Delhi 2008 Comptt.] 

Ans. A supension of slaked lime also called calcium hydroxide is formed when water is added to quick lime.

CaO(s) + H₂O(l)  Ca(OH)₂(s) + heat

(Slaked lime)

Since the reaction is highly exothermic, the solution started although it was not being heated. The suspension of slaked lime is allowed to cool for sometime, preferably overnight. It is then decanted and the liquid obtained is used for white washing.