CLASS 10 NCERT SOLUTION CHEMISTRY CHEMICAL REACTION AND EQUATIONS ,ACID BASES AND SALTS

                 

Chemical Reactions and Equations

Class X

Question 1:

Which of the statements about the reaction below are incorrect?

(a)  Lead is getting reduced.

(b)   Carbon dioxide is getting oxidised.

(c)  Carbon is getting oxidised.

(d)   Lead oxide is getting reduced.

(i)   (a) and (b)

(ii)   (a) and (c)

(iii)  (a), (b) and (c)

(iv)   all Answer 1: (i)(a) and (b)

Question 2:

 

The above reaction is an example of a

(a)  combination reaction.

(b)   double displacement reaction.

(c)  decomposition reaction.

(d)   displacement reaction.

Answer 2:

(d) The given reaction is an example of a displacement reaction.

Question 3:

What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

(a)  Hydrogen gas and iron chloride are produced.

(b)   Chlorine gas and iron hydroxide are produced.

(c)  No reaction takes place.

(d)   Iron salt and water are produced.

(a) Hydrogen gas and iron chloride are produced. The reaction is as follows

Question 4:

What is a balanced chemical equation? Why should chemical equations be balanced?

Answer 4:

A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation. The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reac- tion, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

Question 5:

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.

Answer 5:

(A)

(B)

(C)

(D)

Question 6:

Balance the following chemical equations.

(a)

(b)

(c)

(d)

A 

B

C 

                                                             

  D                                                                                                                                                                

Question 7:

Write the balanced chemical equations for the following reactions.

(a)   Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b)   Zinc + Silver nitrate → Zinc nitrate + Silver

(c)   Aluminium + Copper chloride → Aluminium chloride + Copper

(d)   Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer 7:

A

B

C

D

Question 8:

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) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Answer 8

a )      

                                                         Double displacement reaction

b)              

                                           Decomposition reaction

c)

combination reaction

d)

Displacement reaction

Question 9: What does one mean by exothermic and endothermic reactions? Give examples.

Answer 9:

Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example: Mixture of sodium and chlorine to yield table salt

In other words, combination reactions are exothermic.

Reactions  that  absorb  energy  or  require  energy  in  order  to  proceed  are  called endothermic reactions.

For example: In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

Question 10:

Why is respiration considered an exothermic reaction? Explain.

Answer 10:

Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

Question 11:

Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer 11:

Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.

Decomposition reaction:

Combination reaction:

Question 12:

Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer 12:

(a)  Thermal decomposition:

(a)   Decomposition by light:

(a)  Decomposition by electricity:

Question 13:

What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer 13:

In a displacement reaction, a more reactive element replaces a less reactive element from a compound.

A+BX-------->AX+B  where A is more reactive than B

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds

AB+CD--------->AD+CB

For example: Displacement reaction:

Double displacement reaction:

Question 14:

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

Answer 14:

Question 15:

What do you mean by a precipitation reaction? Explain by giving examples.

Answer 15:

A  reaction  in  which  an  insoluble  solid  (called  precipitate)  is  formed  is  called  a precipitation reaction.

For example:

In  this  reaction,  calcium  carbonate  is  obtained  as  a  precipitate.  Hence,  it  is a precipitation reaction.

Another example of precipitation reaction is

In this reaction, barium sulphate is obtained as a precipitate.

Question 16:

Explain the following in terms of gain or loss of oxygen with two examples each.

(a)   Oxidation

(b)   Reduction

Answer 16:

(a)  Oxidation is the gain of oxygen. 

For example:

 i)

ii) 

In equation (i), CO2 is reduced to CO and in equation (ii), CuO is reduced to Cu.

Question 17: A shiny brown-coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.

Answer 17:

'X' is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

Question 18: Why do we apply paint on iron articles?

Answer 18:

Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevent- ed their presence is essential for rusting to take place.

Question 19: Oil and fat containing food items are flushed with nitrogen. Why?

Answer 19:

Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat contain- ing food items is avoided.

Question 20:

Explain the following terms with one example each.

(a)   Corrosion

(b)   Rancidity

Answer 20:

(a)  Corrosion:

Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc. For example, iron, in the pres- ence of moisture, reacts with oxygen to form hydrated iron oxide.

This hydrated iron oxide is rust.

(a)   Rancidity: The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long. Rancidity can be avoided by:

1.   Storing food in air tight containers

2.   Storing food in refrigerators

3.   Adding antioxidants

4.   Storing food in an environment of nitrogen

Question 1:

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

Answer 1:

Magnesium is very reactive metal like (Na, Ca, etc.). When it expose to air it reacts with oxygen to form a layer magnesium oxide (MgO) on its surface.

This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be used for the reaction.

Question 2:

Write the balanced equation for the following chemical reactions.

(i)       Hydrogen + Chlorine → Hydrogen chloride

(ii)    Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

(iii)   Sodium + Water → Sodium hydroxide + Hydrogen

Question 3:

Write  a  balanced  chemical  equation  with  state  symbols  for  the  following reactions.

(i)    Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

(ii)   Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Question 1:

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.

Answer 1:

(i). The substance ‘X’ is calcium oxide. Its chemical formula is CaO.

(ii). Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime)

Question 2:

Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer 2:

During the Electrolysis of water, hydrogen and oxygen is get separated by the electricity. Water (H2O) contains two parts hydrogen and one part oxygen. Since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double of the amount collected in the other.

Question 1:

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer 1:

When an iron nail is dipped in a copper sulphate solution, iron (which is more reactive than copper) displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.

Therefore, the blue colour of copper sulphate solution fades and green colour appears.

Question 2:

Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer 2:

Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.

In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction.

Question 3:

Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(i) 4Na(s) + O2(g) → 2Na2O(s)

(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

Answer 3:

(i). Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(ii). Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).

            (Chapter 2)  

Acids, Bases and salts)

                                                      

Question 1

A solution turns red litmus blue, its pH is likely to be

(a) 1                                (b) 4                                    (c) 5                             (d) 10

(d) Bases turn red litmus blue and acids turn blue litmus red. Basic solution has a pH value more than 7. Since the solution turns red litmus blue, its pH is likely to be 10.

Question 2

A solution reacts with crushed egg Bshells to give a gas that turns limeBwater milky. The solution contains

(a)  NaCl                           (b) HCl                              (c) LiCl                                (d) KCl

(b)   The solution contains HCl.

Question 3

10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise it will be

(a) 4 mL                        (b) 8mL                           (c) 12 mL                         (d) 16 mL

(d) 16 mL of HCl solution will be required.

Question 4

Which one of the following types of medicines is used for treating indigestion?

(a)  Antibiotic

(b)   Analgesic

(c)  Antacid

(d)   Antiseptic

(c)   Antacid is used for treating indigestion

Question 5

Write word equations and then balanced equations for the reaction taking place when −

(a)   dilute sulphuric acid reacts with zinc granules.

(b)   dilute hydrochloric acid reacts with magnesium ribbon.

(c)   dilute sulphuric acid reacts with aluminium powder.

(d)   dilute hydrochloric acid reacts with iron filings.

(a)  Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

(b)   Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

(c)   Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen

(d)   Hydrochloric acid + Iron → Ferric chloride + Hydrogen

Question 6

Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

answer

Two nails are fitted on a cork and are kept it in a 100 mL beaker. The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch. Some dilute HCl is poured in the beaker and the current is switched on. The same experiment is then performed with glucose solution and alcohol solution.

Observation

It will be observed that the bulb g lows in the HCl solution and does not g low in the glucose solution.

Result

HCl dissociates into H+ and Cl− ions. These ions conduct electricity in the solution resulting in the g lowing of the bulb. On the other hand, the g lucose solution does not dissociate into ions. Therefore, it does not conduct electricity.

Conclusion

From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids.

That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids.

Question 7:

Why does distilled water not conduct electricity, whereas rain water does?

  

Distilled water is a pure form of water and is devoid of any ionic species. Therefore, it does not conduct electricity. Rain water, being an impure form of water, contains many ionic species such as acids and therefore it conducts electricity.

Question 8

Why do acids not show acidic behaviour in the absence of water?

  

Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions from an acid occurs in the presence of water only. It is the hydrogen ions that are responsible for the acidic behaviour.

Question 9

Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is

(a)   neutral?

(b)   strongly alkaline?

(c)   strongly acidic?

(d)   weakly acidic?

(e)   weakly alkaline?

Arrange the pH in increasing order of hydrogenBion concentration.

   answer   9:

(a)   Neutral → Solution D with pH 7

(b)   Strongly alkaline → Solution C with pH 11

(c)   Strongly acidic → Solution B with pH 1

(d)   Weakly acidic → Solution A with pH 4

(e)   Weakly alkaline → Solution E with pH 9

The pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1

Question  10:

Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why?

   w   10:

The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen gas at a faster speed due to which fizzing occurs.

       QUESTION   11:

Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

  ANSWER  11:

The pH of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in nature. The acids present in it decrease the pH.

 QUESTION 12

A milkman adds a very small amount of baking soda to fresh milk.

(a)   Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

(b)   Why does this milk take a long time to set as curd?

   ANSWER 12:

(a)    The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in alkaline condition, milk does not set as curd easily.

(b)    Since this milk is slightly basic than usual milk, acids produced to set the curd are neutralized by the base. Therefore, it takes a longer time for the curd to set.

     QUESTION 13

Plaster of Paris should be stored in a moistureBproof container. Explain why?

   ANSWER 13:

Plaster of Paris (POP) should be stored in a moistureBproof container because Plaster of Paris, a powdery mass, absorbs water (moisture) to form a hard solid known as gypsum.

     QUESTION     14:

What is a neutralization reaction? Give two examples.

  ANSWER 14

A reaction in which an acid and base react with each other to give a salt and water is termed as neutralization reaction. In this reaction, energy is evolved in the form of heat. For example:

(i)   

(ii) During indigestion (caused due to the production of excess of hydrochloric acid in the stomach), we administer an antacid (generally milk of magnesia,           which is basic in nature). The antacid neutralizes the excess of acids and thus gives relief from indigestion.

QUESTION  15:

Give two important uses of washing soda and baking soda.

  ANSWER 15

Two important used of washing soda and baking soda are as

follows:

(1) WASHING SODA 

(a)   It is used in glass, soap, and paper industries.

(b)   It is used to remove permanent hardness of water. 

(2) BAKING SODA 

(a)   It is used as baking powder. Baking powder is a mixture of baking soda and a mild acid known as tartaric acid. When it is heated or mixed in water, it releases CO2 that makes bread or cake fluffy.

(b)   It is used in soda-acid fire extinguishers.

Question 1:

You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

Answer 1:

Ifthe colour of red litmus paper gets changed to blue, then it is a base and if there is no colour change, then it is either acidic or neutral. Thus, basic solution can be easily identified.

Let us mark the three test tubes as A, B, and C. A drop of the solution in A is put on the red litmus paper. Same is repeated with solution B and C. If either of them changes colour to blue, then it is basic. Therefore, out of three, one is eliminated. Out of the remaining two, any one can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then the nature of the drops of the mixtures is checked. If the colour of red litmus turns blue, then the second solution is neutral and if there is no change in colour, then the second solution is acidic.

This is because acidic and basic solutions neutralize each other. Hence, we can distinguish between the three types of solutions.

 Question 1:

Why should curd and sour substances not be kept in brass and copper vessels?

Answer 1:

Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.

M etal + Acid        Salt + Hydrogen gas

Question 2:

Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Answer 2:

Hydrogen gas is usually liberated when an acid reacts with a metal.

Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.

We can test the evolved hydrogen gas by its burning with a pop sound when a

candle is brought near the soap bubbles.

 Question 3:

Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

 Answer 3:

Question 1:

Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

 Answer 1:

The dissociation of HCl or HNO3 to form hydrogen ions always occurs in the presence of water. Hydrogen ions (H+) combine with H2O to form hydronium ions (H3O+).

The reaction is as follows:

HCl + Water  → H + + Cl –

H + + H2O → H3O +

Although aqueous solutions of glucose and alcohol contain hydrogen, these cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.

 Question 2:

Why does an aqueous solution of an acid conduct electricity?

Answer 2:

Acids dissociate in aqueous solutions to form ions. These ions are responsible for conduction of electricity.

Question 3:

Why does dry HCl gas not change the colour of the dry litmus paper?

Answer 3:

Colour of the litmus paper is changed by the hydrogen ions. Dry HCl gas does not contain H+ ions. It is only in the aqueous solution that an acid dissociates to give ions. Since in this case, neither HCl is in the aqueous form nor the litmus paper is wet, therefore, the colour of the litmus paper does not change.

Question 4:

While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Answer 4:

The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating

 Question 5:

How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Answer 5:

When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.

 Question 6:

How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?

Answer 6:

The concentration of hydroxide ions (OH−) would increase when excess base is dissolved in a solution of sodium hydroxide.

Question 1:

You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

Answer 1:

A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.

Question 2:

What effect does the concentration of H+(aq) ions have on the nature of the solution?

Answer 2:

Concentration of H+(aq) can have a varied effect on the nature of the solution. With an increase in H+ ion concentration, the solution becomes more acidic, while a decrease of H+ ion causes an increase in the basicity of the solution.

Question 3:

Do basic solutions also have H+(aq) ions? If yes, then why are these basic?

Answer 3:

Yes, basic solution also has H+(aq) ions. However, their concentration is less as compared to the concentration of OH− ions that makes the solution basic.

Question 4:

Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Answer 4:

If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quick lime or slaked lime or chalk.

Question 1:

What is the common name of the compound CaOCl2?

Answer 1:

The common name of the compound CaOCl2 is bleaching powder.

Question 2:

Name the substance which on treatment with chlorine yields bleaching powder.

Answer 2:

Calcium hydroxide [Ca(OH)2], on treatment with chlorine Cl2, yields bleaching powder.

Question 3:

Name the sodium compound which is used for softening hard water.

Answer 3:

Washing soda (Na2CO3.10H2O) is used for softening hard water.

Question 4:

What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Answer 4:

When a solution of sodium hydrocarbonate is heated, sodium carbonate and water are formed with the evolution of carbon dioxide gas.

𝐻𝑒𝑎𝑡

2NaHCO3 →                Na2CO3 + H2O + CO2

Question 5:

Write an equation to show the reaction between Plaster of Paris and water.

Answer 5:

Plaster of Paris is a white powder and on mixing with water, it changes to gypsum giving a hard solid mass.