NCERT solution for class 10th //Science // lesson 2 - Acid bases and salt

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NCERT solution for class 10th  //Science  // lesson 2  - Acid bases and salt 


 lesson 2  - Acid bases and salt

                Page number 18

Q1. 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? 

Ans- We take a small amount of liquid in three separate test tube  now dip the red litmus paper in each test tube if the liquid turns the red litmus to Blue then  It is the basic solution keep that test tube side and out of these remaining two test tubes at one drop of each  put on the  the same litmus paper which turns blue due to basic solution if that litmus paper turn red then it is the acidic solution and the remaining test tube contain the distilled water .

                Page number 22 

Q1. Why should curd and sour substances not be kept in brass and copper vessels.

Ans- Curd and sour substances contain acids which react with copper and brass and form the salts which are poisonous in nature and they can cause the food poisoning hence the sour substance and curd not kept in the brass and copper vessels

Q2. 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. 

Ans - When acid react with metal then hydrogen gas is liberated 

Zn + H2SO4   →   ZnSO4 + H2 

For the  H2 gas test if the hydrogen gas passed from the soap solution then it burns with pop sound

Q3. 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.

Ans -  CO2  gas is is found fire extinguishes a burning candle CO2is produced when the metal carbonates react with acid is the one of the product of them is calcium chloride so the compound should be calcium carbonate the chemical equation is

CaCO3 + 2 HCl    →  CaCl2+ CO2 + H2O

                Page number  25

Q1.  Why do HCl,  HNO3 etc show acidic characters in aqueous solutions while solutions of compounds well control and glucose do not show acidic character 

Ans- If the H + ions are released in aqueous solution then the compound shows the acidic characters in case of HCl and HNO3 the H+ ions are replaceable and that's why they show the acidic character but in glucose and alcohol do not have a replaceable H + ions hence they  not show the acidic characteristics

Q2. Why does an aqueous solution of an acid conduct electricity? 

Ans- Aqueous solution acid released H + ions because they have a replaceable H + ions and the free ions have a capacity to conduct the electricity so an aqueous solution of an acid conduct electricity

Q3. Why does dry HCl gas not change the colour of the dry litmus paper

Ans- Dry HCl does not contain the free H + ions so it does not show any acidic property hence the  colour of litmus paper  does not change on dry HCl  gas

Q4. While diluting an acid why is it recommended that the acid should be added to water not water to the acid? 

Ans- Dilution of concentrated acid is highly exothermic reaction while mixing the concentrated acid with water the acid is added slowly to water with constant stirring  . If water is added to the concentrated acid the heat generated may cause the mixture splash out and cause burns .The glass container may also break due to excessive local heating

Q5. How is the concentration of hydronium ions H3O affected when a solution of an acid is diluted?

Ans- An acid dissociate into hydronium ion when dissolved in water when the solution is diluted the volume of the solution increases but the number of the ions remains same so the concentration of hydronium ions per unit volume decreases

Q6. How is the concentration of hydroxide ion OH-  affected when excess base is dissolved in the solution of sodium hydroxide ? 

Ans- Sodium hydroxide is a strong base in the excess of solution of NaCl is added in the Sodium hydroxide then the concentration of hydroxide ions per unit volume increases due to dissociation of NaOH as well as the other base in aqueous solution

NaOH   →     Na+  + OH- 

Any other base 

Mg(OH) -→ Mg+  x  OH- 

                Page number 28

Q1.  You have two solutions A and B the pH of solution a is 6 and the pH of the solution b is at which solution has more hydrogen ion concentration which of these is acidic in which of one is basic

Ans- i) pH = 6 has more H + ion concentration 

 ii) Solution A having pH =  6 is  acidic solution B having pH= 8 is basic

Q2. What effect does the concentration of h + ion have on the nature of the solution? 

Ans- Higher the concentration of H + ion in a solution more acidic is the solution

Q3. Do basic solution also have H+ ions if yes then why are these basic? 

Ans-. Yes basic solution also contain H + ions but in basic solution the concentration of  OH-  ion is higher than that of H + ion that's why they are basic in nature

Q4. Under what soil conditions do you think a farmer would treat the soil of his field with quicklime( calcium oxide )or salked lime( calcium hydroxide) or chalk (calcium carbonate)

Ans- Calcium oxide, calcium hydroxide or calcium carbonate are basic in nature therefore they are beneficial to use in soil if the soil is acidic neutralize the acidity of the soil

            Page number 33. 

Q1. What is the common name of the compound CaOCl 2

Ans - Bleaching powder

Q2. Name the substance which on treatment with chlorine yields bleaching powder.

Ans-Calcium hydroxide  ( salked lime ) in dry state

Q3. Name the sodium compound which is used for softening hard water 

Ans- Sodium carbonate

Q4. What will happen if a solution of sodium hydrocarbonate is heated give the equation of the reaction involved

Ans- When the sodium hydrogen carbonate is heated the carbon dioxide gas is produced with sodium carbonate

            2NaHCO3    →     Na2 CO3 + H2O + CO2

Q5. Write an equation to show the reaction between plaster of Paris and water.

Ans- (CaSO4)2 . H2O + 3 H2O → 2 CaSO4 . 2 H2O

                    Exercise

Q1. A solution turns red litmus blue its pH is likely to be.

a) 1.    b) 4      c) 5     d) 10 

Q2. A solution reacts with crushed egg shells to  give a gas that turns lime water milky the solution contains 

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

Q3. 10 ml of a solution of NaOH is found to be completely neutralized by 8 ml of the given solution of HCL if we take 20 ml of the same solution of NaOH the amount of HCl solution required to neutralize it will be 

a) 4ml    b) 8ml     c) 12 ml.   d) 16ml 

Q4. Which of the following types of medicine is used for treating in digestion

a)Antibiotic.  b)  analyser   c) antacid   d) antiseptic 

Q5 . Write word equation and then balanced equation for the reaction taking place when

a) Dilute sulphuric acid react with zinc granules

b) Dilute hydrochloric acid reacts with magnesium ribbon

c) Dilute sulphuric acid react with aluminium powder.

d) Dilute hydrochloric acid react with an iron

 filings 

Ans- a)  Dilute Sulphuric acid + Zinc granules ------  Zinc sulphate + Hydrogen gas 

H2SO4+ Zn   →     ZnSO4+ H2   ↑

b) Dialute Hydrochloric acid + Magnesium----- Magnesium chloride + Hydrogen gas 

2HCl + Mg    →     MgCl2 + H2  ↑        

c) Dilute Sulphuric acid + Aluminium  ----- Aluminium sulphate + Hydrogen gas 

  3H2SO4  + 2Al   →    Al2( SO4) 3 + 3H2   ↑   

d) Dilute Hydrochloric acid + Iron ----  Iron chloride + Hydrogen gas 

  2HCl + Fe    →   FeCl2 + H2    ↑

Q6 . Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acid describe an activity to prove it.

Ans- Alcohol and glucose both contain hydrogen atom but in their aqueous solution they do not ionize the H + ions hence they can't conduct electricity  so they are not categorized as acid it is explained by the following activity 

Set the apparatus as shown in figure 


Observation 

  • The bulb glow when the dilute HCl was added in beaker 
  • the bulb did not glow when the alcohol or glucose solution was added to the beaker 

Conclusion 

  • Alcohol and glucose are not ionized in a solution and hence can't conduct electricity 
  • Dilute HCl solution contain H + ions and these are responsible to carry the electric current through the solution


Q7 . Why does distilled water not conduct electricity whereas rainwater does? 

Ans - The distal water does not contain any type of ions so it does not  conduct electricity but during the process of raining the atmospheric acidic oxide like SO2 and NO2 dissolved in water and make the rainwater little bit acidic in nature so it is better conductor of electricity

Q8. Why do acids not show acidic behaviour in the absence of water.

Ans- Acid does not conduct acidic behaviour in absence of water because the ions are produced only in aqueous medium and presence of h + ions are responsible for the showing acidic property

Q9 . Give solutions a b c d and e when tested with universal indicator shows pH as 4,1,11 ,7 and 9 respectively which solution is 

Neutral 

strongly alkaline

 strongly acidic 

 weakly acidic 

weakly alkaline

 arrange the pH in increasing order of hydrogen ion concentration .

Ans- Neutral d -  pH  = 7 

Strongly alkaline  c -   pH =11

Strongly acidic b - pH =  1

Weakly acidic  a - pH  = 4  

Weakly alkaline e  - pH  = 9 

c <  e <  d <  a  < b

Q10. Equal lengths of magnesium ribbons are taken in test tube A and B hydrochloric acid HCl is added to test tube A while acetic acid ch3cooh is added to test tube B amount of concentration taken for both this acids are same in which test tube will be freezing occur more vigorously and why  ? 

Ans- Reaction occurs in test tube  a 

2HCl + Mg →   MgCl2 + H2  ↑

Reaction occurs in test tube b

2 CH3COOH+ Mg  →  (CH3COOH )2 Mg + H2  ↑

Fizzing occurs in test tube a is more faster than B because of the greater extent of dissociation of HCl as compared to to ch3cooh it is a weak acid while HCl is a strong acid 


Q11. Fresh milk has a pH of 6 how do you think the pH will change as it turns into curd explain your answer

Ans- Fresh milk has a pH of 6 but whenever the it changes itself into curd then lactic acid is produced and which decreases the the pH of the milk less than 6

Q12. 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 a card

Ans- a) Alkaline medium does not allow milk to turn sour easily 

b) Whenever the milk is set as a cut the presence of alkali are not easily become it into acidic medium hence the milk take time to set as a curd

Q13. Plaster of paris should be stored in a moisture proof container explain why

Ans- Plaster of paris is chemically calcium sulphate hemihydrate when it comes in contact with water it become hard and solid it called gypsum so to prevent this condition it must be stored in moisture proof container

CaSO4  H2O + 1   H2O  →    CaSO4. 2 H2O

Q14. What is a Neutralisation reaction give two example

Ans- When the acid are combined with base then salt and water is formed in that reaction is called neutralization reaction 

acid + base --- --- salt + water 

a) HNO3 + KOH   →   KNO3+ H2O .

b) HCl + NaOH  →  NaCl + H2O

Q15. Give two important uses of washing soda and baking soda 

Ans- 

Uses of washing soda are

It is used to remove the permanent hardness of water 

it is used to use as a cleansing agent detergents 

Uses of baking soda

It is used in bakery 

It is used in fire extinguisher

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