CBSE Class 10th Science Chapter 2 Acid Base Salt Part-2

Questions

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

Ans: Curd & Sour substances are acidic in nature, it will react with brass & copper these acidic substances combine with metal. This reaction turns food to poison which damage people’s health.

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? 

Ans: When an acid reacts with any metal, salt and hydrogen gas are formed. Metal + Acid → Salt + Hydrogen gas

3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence(bubbles in liquid). 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: As metal compound released is Calcium Chloride the gas evolved here is CO2. Hence metal A should be Calcium Carbonate. Hence the reaction between Calcium Carbonateand HCl is

CaCO3 (s) + 2HCl (Aq) → CaCl2( Aq) + CO2 (g) + H2O (l)

If the red litmus paper changes to blue colour the solution is a basic solution. If the red litmus paper experience no change in acidic solution. If the red litmus paper changes to purple colour the solution is distilled water.

2.2 WHAT DO ALL ACIDS AND ALL BASES HAVE IN COMMON?

Hydrogen

Activity 2.8:

􀂄 Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, etc.

􀂄 Fix two nails on a cork, and place the cork in a 100 mL beaker.

􀂄 Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in Fig. 2.3.

􀂄 Now pour some dilute HCl in the beaker and switch on the current.

􀂄 Repeat with dilute sulphuric acid.

􀂄 What do you observe?

􀂄 Repeat the experiment separately with glucose and alcohol solutions. What do you observe now?

􀂄 Does the bulb glow in all cases?

bulb will start glowing in the case of acids, glucose and alcohol solutions do not conduct electricity. Glowing of the bulb indicates that there is a flow of electric current through the solution. The electric current is carried through the solution by ions. Since the cation present in acids is H+, this suggests that acids produce hydrogen ions, H+(aq), in solution, which are responsible for their acidic properties.

Since Bases also have +ve & -ve ions for eg. Na+, OH-, so it will also conduct electricity & hence Bulb will glow.

 2.2.1 What Happens to an Acid or a Base in a Water Solution?

Activity 2.9: 

􀂄 Take about 1g solid NaCl in a clean and dry test tube and set up the apparatus as shown in Fig. 2.4.

􀂄 Add some concentrated sulphuric acid to the test tube.

􀂄 What do you observe? Is there a gas coming out of the delivery tube? 

NaCl(S)+H2SO4(Conc)→Na2SO4(s)+HCl(g)

􀂄 Test the gas evolved successively with dry and wet blue litmus paper. Doesnt Turn Red Solid sodium chloride reacts with concentrated sulphuric acid and forms dry hydrochloric acid gas. Here HCl is produced in gaseous form as there is no water 

􀂄 In which case does the litmus paper change colour?  litmus paper works on H+  or OH- ion. Since there is no water in HCl gas, HCl does not dissociate into its ion and does not turn the blue litmus red. If we use moist blue litmus paper then it will turn in red.

􀂄 On the basis of the above Activity, what do you infer about the acidic character of: (i) dry HCl gas, Concentrated acids and bases need an aueous medium to turn litmus paper.

(ii) HCl solution?

This experiment suggests that hydrogen ions in HCl are produced in the presence of water. The separation of H+ ion from HCl molecules cannot occur in the absence of water.

HCl + H2O → H3O+ + Cl–

Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as

H+(aq) or hydronium ion (H3O+).

(H+) + H2O → H3O+

We have seen that acids give H3O+ or H+(aq) ion in water. 

Note: If the climate is very humid, you will have to pass the gas produced through a guard tube (drying tube) containing calcium chloride to dry the gas.

Let us see what happens when a base is dissolved in water.

NaOH(s)----H2O---> Na⁺ (aq) +OH^-  (aq)

KOH(s)----H2O----> K+(aq) +OH-(aq)

Mg(OH)2(s)---H2O------> Mg2+ (aq)+2 OH (aq) 

Bases generate hydroxide (OH–) ions in water. Bases which are soluble in water are called alkalis.All bases do not dissolve in water. An alkali is a base that dissolves in water. They are soapy to touch, bitter and corrosive. Never taste or touch them as they may cause harm.

all acids generate H+(aq) and all bases generate OH–(aq).

Acid + Base → Salt + Water

H X + M OH → MX + HOH

H+(aq) + OH– (aq)→H2O(l)

 Activity 2.10

􀂄 Take 10 mL water in a beaker.

􀂄 Add a few drops of concentrated H2SO4 to it and swirl the beaker slowly.

􀂄 Touch the base of the beaker.

􀂄 Is there a change in temperature? Yes it gets Hot

􀂄 Is this an exothermic(Heat release into atmosphere) or endothermic(heat absorbed from atmosphere) process? Exothermic 

hydrogen ion interacts with a water molecule and forms hydronium ion. In this process, heat evolves which make the beaker hot.

H2SO4 ——> H+ + HSO4–

H+ + H2O ——> H3O+

􀂄 Repeat the above Activity with sodium hydroxide pellets

The process of dissolving an acid or a base in water is a highly exothermic(Heat released) 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.

Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH–) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.

Questions:

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?

Ans: Release of H+ ion in water will make a compound acidic or non-acidic. Acids are the

substance which produce H+ ions in water. acids like HCl, HNO3 show acidic character cos they produce H+ ion.

Compounds similar to glucose or alcohol do contain hydrogen element but they do not show signs of acidic nature because hydrogen in them will not separate as like the hydrogen in the acids. They will not separate to become hydrogen ions, on dissolving in the water. So dissociation of hydrogen gas will decide the acidic or non-acidic nature of a compound.

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

Ans: Charged particles are responsible for the conductance of electricity in acid. These charged particles called as ions are the reason behind conductance of electricity in acid.

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

Ans: hydrogen ions in HCl are produced in the presence of water. The separation of H+ ion from HCl molecules cannot occur in the absence of water. So litmus paper willnt change its color with HCl gas.

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

Ans: Because if water is added to concentrated acid, it release huge amount of heat which may result in explosion and can cause acid burns on face, clothes and body parts. Hence it is safe to add acid to water but not water to acid.

5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?  2 Acids, Bases and Salts

Ans: When acid is added to water there will be a fixed amount of hydronium present in the fixed volume of solution. If we dilute the solution hydronium ion per volume of solution decrease, this in-turn decreases Hydronium concentration in the solution.

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

Ans: When base is dissolved in sodium hydroxide(NaOH) solution its hydroxide ions(OH-) increase but it will reach saturation at some point. After saturation point hydroxide ion concentration is not affected even after adding base further.

Mg(OH)2(s)---H2O------> Mg2+ (aq)+2 OH (aq)

NaOH(s)----H2O---> Na⁺ (aq) +OH^-  (aq)

KOH(s)----H2O----> K+(aq) +OH-(aq)

scale for measuring hydrogen ion concentration in a solution, called pH scale. p in pH stands for ‘potenz’ in German, meaning power. pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline) Higher the hydronium ion concentration, lower is the pH value.

Activity 2.11 Check pH

Saliva: Before meal 7.4, after meal if taken is basic in nature, value will increase, if acidic food is taken value will decrease.

Lemon Juice: 2.2

Colourless aerated drink: 4

Carrot Juice: 6

Coffee: 4.5

Tomato Juice: 4.3

Tap water: 6 to 8.5

1M NaOH: 14

1M HCl: 0                                    

Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be

weak acids.

When pH of rain water is

less than 5.6, it is called acid rain. When acid rain flows into the rivers, it

lowers the pH of the river water. The survival of aquatic life in such rivers

becomes difficult.

Activity 2.12

􀂄 Put about 2 g soil in a test tube and add 5 mL water to it.

􀂄 Shake the contents of the test tube.

􀂄 Filter the contents and collect the filtrate in a test tube.

􀂄 Check the pH of this filtrate with the help of universal

indicator paper.

our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach.

During indigestion the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, people use bases called antacids for eg. Baking Soda. These antacids neutralize the excess acid.

Magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose. 

when the pH in the mouth is below 5.5. Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.

Honey Bee-sting leaves an acid which causes pain and irritation. Use of a mild base like baking soda on the stung area gives relief. Stinging hair of nettle leaves inject methanoic acid causing burning pain.

Nettle is a herbaceous plant which grows in the wild. Its leaves have stinging hair, which cause painful stings when touched accidentally. This is due to the methanoic acid secreted by them. A traditional remedy is rubbing the area with the leaf of the dock plant, which often grows beside the nettle in the wild. 

Q U E S T I O N

1.   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? pH of any solution is inversely proportional to the hydrogen ion concentration”.

Ans: Therefore, it means that the solution that has a lower pH number will have a higher hydrogen ion concentration. Hence, solution A will have a higher hydrogen ion concentration. In addition, solution B will be basic and A will be acidic.

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

Ans: It decides solution is acidic or basic in nature. If Hydrogen ion

concentration increase then solution turn acidic and similarly if Hydrogen ion concentration decreases then solution turn basic.

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

Ans: Basic solutions have H+ ions, but hydroxide ions present in them are more. Hence Hydroxide ions turn solution to basic.

 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)?

Ans: If the soil is acidic in nature (PH below 7) then such field should be treated with

quick lime (calcium oxide CaO2) or slaked lime (calcium hydroxide CaHO) or chalk (calcium carbonate CaCO3).

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