2. Acids , Bases and Salts

CHAPTER  2.
ACIDS , BASES AND SALTS
PAGE No: 18

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?

Take the red litmus paper and cut it into three pieces. Dip  the pieces one each in one test tube.
Note the difference in colour

(a) The test tube in which the red litmus turns blue is a basic solution

 The blue litmus obtained from the first test is dipped in the rest of the two test tubes
(b)The test tube in which the blue litmus turns red is an acidic solution

(c)The test tube in which no change in colour occurs to the litmus is distilled water

PAGE No: 22

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

The curd and sour substances contains acids which reacts with metals.
Metals + Acids  gives  Salt + Hydrogen
This may spoil the food and even turns poisonous.

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?

Hydrogen gas is produced when a metal reacts with acids
Zinc granules reacts with hydrochloric acid to produce Zinc chloride and hydrogen

   Zn + 2HCl   à  ZnCl₂ + H₂

We can test  the presence of hydrogen by passing it through a delivery tube to  a container containing soap solution.
Introduce a lighted candle while the bubbles arises. The bubbles burns with a pop sound indicates hydrogen gas

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 reactions if one of the compounds formed is calcium chloride

Since the gas evolved extinguishes the burning candle,. the gas evolved is carbon dioxide

The products are calcium chloride and carbondioxide.
The given reactant is Hydrochloc acid , hence the other reactant is calcium carbonate 

CaCO₃   + 2HCl  à  CaCl₂  +   CO₂ + H₂O

     

PAGE No: 25

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

When acids are made dilute with water     H⁺   ions are produced  which combines with water to form hydronium ions ( H₃O⁺ )

This is responsible for the acidic character  of acids in aqueous solution

While alcohol and glucose in aqueous solution do not produce ions hence do not show acidic or basic character

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

Acids produce H⁺  ions in aqueous solution. This is responsible for the conduction of electricity.

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

Dry HCl gas does not have H⁺  ions. Hence dry HCl gas not change the colour of dry litmus paper.
If we dilute it with water  H⁺   ions are produced  which combines with water to form hydronium ions ( H₃O⁺ ) and hence the colour of  blue litmus paper change to red

4. While diluting an acid why is it recommend that the acid should be added to water and not         water to the acid?

If we dilute an acid  with water  H⁺   ions are produced  which combines with water to form hydronium ions ( H₃O⁺ ) . This reaction is highly exothermic.

If we add water to the acid  all the molecules of water take part in reaction and excess heat is produced , causing the acid to splash out and may cause burns.
Also the container may break due to high temperature

On the other hand if we add acid to water drop by drop we can control the reaction and hence avoid danger.  

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

When a solution of acid is diluted with water H₃O⁺ ions per unit volume decreases. 

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

If excess base is dissolved in a solution of sodium hydroxide, the concentration of  OH^-
 ions increases.
 This is because when another base dissolve in the solution another set OH^-  ions are produced

PAGE No: 28

1. You have two solutions, A and B, the  p^H   of  solution A is 6 and  p^H  of solution B is 8. Which solution have more hydrogen ion concentration ? Which of this is acidic and which one is basic?

 Solution A 
 More hydrogen ion concentration, since its p^H  is below seven.
 Solution is acidic

Solution B
 Less hydrogen ion concentration, since its p^H  is above seven
Solution is basic

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

The H⁺   ion  concentration determines the power of an acid. Higher the   H⁺   ion  concentration higher is its acidic character and vice versa.

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

Yes. Basic solution contain H⁺ ions. But in a basic solution the concentration of OH^-  ions is higher than that of H⁺  ions

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)?

Plants grow well between the p^H  range of 6  to 8  . If the soil become too acidic , to reduce the acidicity the farmers use bases like quick lime ( calcium oxide) or slaked lime ( calcium hydroxide) or chalk ( calcium carbonate)

PAGE No: 33

1. What is the common name of the compound CaOCl₂  ? 

Bleaching Powder

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

Dry slaked lime  Ca(OH)₂   

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

Washing soda   (Na₂CO₃.10H₂O )

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

When a solution of sodium hydrocarbonate is heated sodium carbonate, water and carbon dioxide are formed

2NaHCO₃   à  Na₂CO₃  + H₂O  + CO₂

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

The product is gypsum

CaSO₄ .½H₂O   +  1½H₂O   à   CaSO₄ .2H₂O   

EXERCISES 

PAGE No: 34

1. A solution turns red litmus blue, its p^H  is likely to be

10, since the solution is basic . All other option given are below seven.

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

HCl.  All other options are basic.

3. 10 mL of a solution of NaOH is found to be completely neutralized by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution ( the same solution as before) required to neutralize it will be

10 mL NaOH  neutralize  8 mL  HCl

20 mL NaOH  neutralize X mL  HCl

HCl  required  is    ( 8 x 20 )  /  10 

        =  16 mL

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

Antacid.  Antacid contain mild base,  normally sodium hydrogen carbonate ( NaHCO₃ )

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

(a)  Dilute sulphuric acid reacts with zinc granules

Zinc  +  Sulphuric acid ( dil)  à  Zincsulphate  + Hydrogen

Zn  + H₂SO₄   à  Zn SO₄  + H₂

(b) Dilute hydrocholoric acid reacts with  magnesium ribbon

Magnesium + Hydrochloric acid (dil)  à Magnesium chloride + Hydrogen

Mg  + 2HCl  à  MgCl₂  + H₂

(c) Dilute sulphuric acid reacts with aluminium powder

Aluminium + Sulphuric acid ( dil)  à  Aluminium sulphate  + Hydrogen

2Al  + 3 H₂SO₄  à  Al₂(SO₄) ₃  + 3H₂

(d) Dilute hydrochloric acid reacts with iron filings

Iron  + Hydrochloric acid (dil)  à Ironchloride + Hydrogen

Fe  + 2HCl  à  FeCl₂  + H₂

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

Materials required : dilute solution of ethanol and glucose solution
Apparatus required : beaker , a cork ,2 iron nails ,battery , bulb ,key , ethanol ,glucose solution , connecting wire
Procedure :

• take a cork
• fix two iron nails with a space between them 
• take the beaker and place this in the middle of the beaker
• now connect the two nails to the circuit containing a key a dry cell and a bulb
• now fill the beaker with glucose solution 
• note down the observation 
• drain the solution out and fill it with ethanol solution 
• note down your observation 

Observation : you would observe that the bulb does not glow in both the solution
Conclusion :from the above experiment we come to a conclusion that both the solution does not conduct electricity and this implies that both the solution does not contain any ions therefore it does not contain  H⁺  ions  thus it is not an acid

7. Why does distilled water not conduct electricity, where as rain water does?

 Rain water:
  Rain water contains impurities like mud acid etc therefore rain water is acidic thus it contains H⁺  ions and other charged particle   so it conducts electricity
Distilled water:
 Distilled  water does contain any  impurities or charged particles are not present  so it does not conduct electricity

8. Why do acids not show acidic behavior in the absence of water ?

Acids cannot produce H⁺  ions in the abscence of water . Only by diluting with  water it produces H⁺  ions

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

(a) Neutral ?

D , since its p^H is 7

(b) Strongly alkaline ?

C, since its p^H is 11

(c) Strongly acidic ?

B , since its p^H is 1

(d) Weakly acidic ?

A , since its p^H is 4

(e)  Weakly alkaline ?

E , since its p^H is 9

The increasing order is C<E< D< A< B

10. Equal length 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 and concentration taken for both the acids are same.In which test tube will the fizzing occur more vigorously and why ?

Fizzing  occurs more frequently in the test tube in which  HCl was added this is because HCl is strong acid and CH3COOH is a weak acid

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

when the milk changes to curd its p^H would decrease this is because the acidic character increases as lactic acid is formed when it changes into curd

12. A milkman added a very small baking soda to fresh milk

(a) Why does he shift the  p^H  of milk from 6 to slightly alkaline?
This is to prevent the milk from changing to curd . the formed lactic acid would get neutralized

(b) Why does this milk take a long time to set as curd?
this is because the baking soda reacts with the formed lactic acid and thus get neutralizes the effect . thus the milk which is added with baking soda takes a long time to set as curd


13. Plaster of Paris should be stored in a moister  proof container . Explain Why?

plaster of paris must be stored in moisture proof container because plaster of paris reacts with water to form gypsum which o not posses the property of setting

CaSO₄ .½H₂O   +  1½H₂O   à   CaSO₄ .2H₂O   

14. What is neutralization reaction? Give two examples.

the reaction between an acid and base to produce salt and water is called neutralization reaction

HCl + NaOH  à NaCl +H₂O

HCl + KOH  à KCl +H₂O

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

Washing soda 

• can be used as a cleaning agent for domestic purposes 
• removing permanent hardness of water 

Baking Soda  

• used in soda acid fire extinguisher 
• An active ingredient in antacids