Acids, Bases and Salts

Acid – Base Indicators

Indicate the presence of an acid or base in a solution.

Types of Indicators
(i) Natural Indicators

Those indicators which are given by nature e.g. Red cabbage, china rose petals etc.

Litmus solution – It is a natural indicator. It is a purple day extracted from Lichens. Other examples are Red Cabbage and coloured petals of Petunia and turmeric.

Blue litmus solution To red in acid and No change in base.

Blue Litmus + Acid = Change to Red colour

Blue Litmus + Base = No Change

Red litmus solution No change in acid and To blue in base.

Red Litmus + Acid = No Change

Red Litmus + Base = Chage to Blue colour

Turmeric No change in acid but To red in base.

(ii) Synthetic Indicators

Those indicators which man made is called synthetic indicators.

(a) Litmus Paper

Red litmus paper No change in acid and To blue in base.

Blue litmus paper To red in acid and No change in base.

(b) Methyl Orange

Methyl orange is a liquid which is orange in colour.

Methyl orange To red in acid and To yellow in base.

(c) Phenolphthalein

Phenolphthalein is a colourless solution.

Phenolphthalein (colourless) No change in acid and change To pink in base.

(iii) Olfactory indicators

Show odour changes in acidic or basic media. eg.onion and clove.

Acids

• Sour in taste

• Change the blue litmus to red

• eg.Hydrochloric Acid (HCl), Sulphuric Acid (H2SO4), Nitric Acid (HNO3), Acetic Acid (CH3COOH).

• Some Naturally occuring acids

(i) Vinegar – Acetic Acid

(ii) Orange – Citric Acid

(iii) Lemon – Citric Acid

(iv) Tamarind – Tartaric Acid

(v) Tomato – OxalicAcid

(vi) Sour milk (Curd) – LacticAcid

(vii) Ant and Nettle sting – Methanoic Acid

• Dilute Acid Contains only a small amounts of acid and a large amount of water.

• Concentrated Acid A concentrated acid contains a large amount of acid and a small amount of water.

• Chemical Properties of Acids and Bases

• When acid react with metal gives salt and hydrogen gas.

• Acid + Metal ⇒ Salt + Hydrogen

e.g 2HCl + Zn ⇒ ZnCl2 + H2

2HNO3 + Zn ⇒ Zn (NO3)2 + H2

• Pop test When a buring candle is brought near a test tube containing hydrogen gas it burns with a ‘Pop’ sound. This test is conducted for examining the presence of hydrogen gas.

• Action of Acids with metal Carbonates and metal bicarbonates

• Metal Carbonate + Acid ⇒ Salt + Carbondioxide + Water

Na2CO3(s)+ 2HCl(aq) ⇒ 2NaCl(aq) + H2O(l) + CO2(g)

• Metal bicarbonate + Acid ⇒ Salt + Carbondioxide + Water

NaHCO3 + HCl ⇒ NaCl + CO2 + H2O

• Lime water Test On passing the CO2 gas evolved through lime water,

Ca(OH)2 (aq) + CO2(g) ⇒ CaCO3(s) + H2O(l)

• Lime water White precipitate

On passing excess CO2 the following reaction takes place

CaCO3(s) + H2O(l) + CO2(g) ⇒ Ca(HCO3)2 aq Soluble in water.

• Neutralisation Reactions

Base + Acid ⇒ Salt + Water

NaOH(aq) + HCl(aq) ⇒ NaCl(aq) + H2O(l)

• Neutralisation reacton takes place when the effect of a base is nullified by
an acid and vice versa to give salt and water.

• Reactions of metal oxides with acids

• Metal Oxide + Acid ⇒ Salt + Water

CuO + HCl ⇒ CuCl2 + H2O

• Copperoxide Hydrochloric acid ⇒ Copperchloride + Water

• Note : Appearance of blue green colour of the solution because of formation of CuCl2.

• Metallic oxides are said to be basic oxides because they give salt and water on reacting with acids.

• All acidic solutions conduct electricity

• Glowing of bulb indicates that there is a flow of electric current through the solution.

Acids produce H+ions in the presence of water

HCl + H2O ⇒ H3O+ + Cl–

H3O+ – Hydronium ion.

H+ ion cannot exist alone. It exists as H+(aq) or (H3O+) hydronium ion.

H+ + H2O ⇒ H3O+

Bases

• Bitter in taste

• Change red litmus to blue

• eg. Sodium hydroxide(NaOH), Potassium hydroxide(KOH).

• When base is react with metal it gives salt and hydrogen gas.

Base + Metal ⇒ Salt + Hydrogen

NaOH + Zn ⇒ Na2ZnO2 + H2

• Reaction of Non Metallic Oxide with Base

Non metallic oxide + Base ⇒ Salt + Water

Ca(OH)2 + CO2 ⇒ CaCO3 + H2O

• Note : NonMetallic oxides are said to be acidic in nature because on reacting with a base they produce Salt and Water.

• Bases in a Water Solution

Bases provide (OH–) ions in the presence of water

NaOH(s) ⇒ Na+ + OH–

KOH(s) ⇒ K+ OH–

Mg(OH)2(s) ⇒ Mg2+ + 2OH–

Alkalis

• All bases do not dissolve in water.An alkali is a base that dissolves in water. Common alkalis are

NaOH Sodiumhydroxide

KOH Potassiumhydroxide

Ca(OH)2 Calciumhydroxide

NH4OH :Ammonium hydroxide

Note : All alkalis are bases but all bases are not alkalis. Precaution must be taken while mixing acid or base with water. The acid must always be added to water with constant stirring as it is highly exothermic reaction.

• When an acid or a base is mixed with water they become dilute. This results in the decrease in the concentration of H3O+ or OH– per unit volume in acids and bases respectively.

Strength of an Acid or Base

• Strength of acids and bases depends on the no. of H+ions and OH–ions produced respectively.

• With the help of a universal indicator we can find the strength of an acid or base. This indicator is called PH scale.

• pH = Potenz in German means power.

• This scale measures from 0 (very acidic) to 14 (very alkaline) 7 Neutral(water in Neutral).

• pH paper : Is a paper which is used for measuring PH.

• Variation of PH

0 Dark red Highly acidic very high very low

4 Orange or yellow Acidic high low

7: Green Neutral Equal Equal

10 Bluish green or blue Alkaline low high

14 Dark blue or voilet highly basic very low very high

• strongAcids give rise to more H+ions.

eg. HCl, H2SO4 and HNO3.

• Weak Acids give rise to less H+ ions

eg. CH3 COOH, H2 CO3 (Carbonic acid)

• Strong Bases – Strong bases give rise to more OH– ions.

eg. NaOH, KOH, Ca(OH)2

• Weak Bases : give rise to less OH– ions.

eg. NH4OH

More about Salts
salts and their derivation

1. Potassium Sulphate K2SO4 is formed from KOH and H2SO4

2. SodiumSulphate Na2SO4 is formed from NaOH and H2SO4

3. SodiumChloride NaCl is formed from NaOH and HCl

4. AmmoniumChloride NH4Cl is formed from NH4OH and HCl

• Note : NaCl and Na2 SO4 belong to the family of sodium salts as they have the same radicals. Similarly NaCl and KCl belong to the family of chloride salts.

Importance of pH in our daily life

Importance of pH in our digestive system – pH level of our body regulates our digestive system. In case of indigestion our stomach produces acid in a very large quantity because of which we feel pain and irritation in our stomach.

• To get relief from this pain antacids are used. These antacids neutralises the excess acid and we get relief.

pH of Acid Rain : When pH of rain water is less than 5.6 it is called Acid Rain.When this acidic rain flows into rivers these also get acidic, which causes a threat to the survival of aquatic life.

pH of Soil : Plants require a specific range of pH for their healthy growth. If pH of soil of any particular place is less or more than normal than the farmers add suitable fertilizers to it.

• Our body functions between the range of 7.0 to 7.8 living organisms can survive only in the narrow range of pH change.

Tooth decay and pH : Bacteria present in the mouth produce acids by degredation of sugar and food particles remaining in themouth.Using toothpaste which is generally basic can neutralise the excess acid and prevent tooth
decay.

• Bee sting or Nettle sting contains methanoic acid which causes pain and irritation. When we use a weak base like baking soda on it we get relief.

• Neutral Salts : Strong Acid + Strong base

pH value is 7 eg. NaCl, CaSO4

• Acidic Salts : Strong Acid + weak base

pH value is less than 7 eq. NH4Cl, NH4 NO3

• Basic Salts : Strong base + weak acid

pH value is more than 7 eg. CaCO3, CH3 COONa

Chemicals from Common Salt

• Sodium chloride is called as common salt used in our food. It is derived from seawater.

• Rock Salt is the brown coloured large crystals. This s mined like coal.

• Common Salt is an important raw material for manymaterials of daily use such as.

Sodiumhydroxide

Washing Soda

Bleaching Power.

Sodium Hydroxide

• Preparation : Prepared by the method called chlor-alkali Called chlor-alkali because we get chlorine and a base in this.

2NaCl(aq) + 2H2O(l) ⇒ 2NaOH(aq) + Cl2(g) + H2(g)

Bleaching Power

• Preparation : Ca(OH)2 + Cl2 ⇒ CaOCl2 + H2O

calcium hydroxide chlorine bleaching water powder uses in textile, factories and laundry, used as disinfectant\n\n

Baking Soda

• Common name – Sodium Hydrogen Carbonate

• Preparation : NaCl + H2O + CO2 + NH3 ⇒ NH4Cl + NaHCO3

Sodium chloride+ Water+ Carbondioxide+ Ammonia ⇒ Sodium hydrogen carbonate\n

• On heating NaHCO3 produces : NaHCO3 ⇒ Na2CO3 + H2O + CO2

• CO2 produced causes dough to rise and make cakes, pastries spongy.

• Uses : In household, ingredients of antacid, In making baking power

• On heating baking powder produces

NaHCO3 + H+ ⇒ CO2 + H2O + Sodium Salt of acid

Washing Soda

• Preparation : Recrystallisation of sodium carbonate
Na2CO3 + 10H2O ⇒ Na2CO3. 10H2O

• Uses

Used in glass, soap and paper industry
Cleaning agent for domestic purposes.
Removal of hardness of water.
Manufacturere of borax.

Water of crystallization

Fixed no. of water molecules present in one formula unit of a salt.

• On heating copper sulphate crystals water droplets appear, formula of hydrated copper sulphate – CuSO4. 5H2O.

• gypsum also contains water of crystallisation.

• Formula of gypsum – CaSO4.2H2O

• On heating gypsum at 373k it becomes CaSO4.½H2O is plaster of paris.

• Plaster of Paris is used as plaster for fractured bones.

• When plaster of Paris is mixed with water it changes to gypsum.
CaSO4.½ H2O + 1½ H2O ⇒ CaSO4.2H2O\n

• Uses of plaster of Paris :Making toys, decorative material and smooth surfaces.

NCERT Solution

Question 1:

Consider this situation:

You are given three test tubes. The three test tubes contain distilled water, acidic solution and basic solution respectively. There is only red litmus paper available in order to identify what is there in each test tube. How will you find out what is in each of the test tubes?

Solution:

Using the red litmus paper, we can identify the content in each of the test tube. This can be done by noticing the color change of the red litmus paper.

  • If the red litmus paper changes to blue color it means that, the solution is a basic solution.
  • If we put the changed litmus paper into another solution to see if it changes to red again. If it does change that means that solution will be an acidic solution.
  • The solution that has no change in any of the litmus paper will be the neutral solution therefore that will be the distilled water.

Question 2:

What is the reason behind curd or any other sour substance not to be kept in copper or brass vessels?

Solution:

Curd and sour food substances contain acids; these acidic substances have a possibility of reacting with the metal. If this reaction takes place then it can cause food poisoning and damage people’s health.

Question 3:

When acids react with metal, which gas is liberated?

Solution:

If an acid reacts with any metal, a salt and hydrogen gas will be formed.

Metal + Acid -> Salt + Hydrogen gas

Question 4:

The reaction of metal A and dilute hydrochloric acid produces a fizz. This gas that is a product of the reaction can extinguish candles. Explain the reaction.

Solution:

The gas that can extinguish a burning candle is carbon dioxide is formed by the reaction of dilute hydrochloric acid reaction with metal carbonate. When a metal compound ‘A’ reacts with dilute hydrochloric acid, it produces some fizz or effervescence. The compounds formed in this reaction is calcium chloride and it shows that the metal is calcium carbonate. By this, we can state that the compound A is calcium carbonate. Calcium carbonate reacts with the dilute hydrochloric acid to form calcium + carbon dioxide + water. Therefore this product that is formed can extinguish a burning candle.

Question 5:

HCl, HNO3 are compounds that show acidic character in aqueous solutions and on the other hand compounds such as alcohol/glucose don’t show any acidic character. Why is that?

Solution:

Acids are substances that can dissociate on the dissolving of water, which results in production of hydrogen ions. Some acids show acidic character as they dissociate in the aqueous solution which results in the production of hydrogen ions (acids like HCl, HNO3).

Compounds similar to glucose or alcohol also do contain hydrogen element but they do not show signs of acidic nature. This is due to the fact that the 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.

Question 6:

What is the reason to an acid aqueous solution conducting electricity?

Solution:

Due to charged particles, there is electricity that is conducted in aqueous solutions. The charged particles are called ions and they help conduct electricity.

Question 7:

Why does dry litmus paper not change color in presence of dry HCl?

Solution:

Because, HCl does not have hydrogen ions present. Therefore does not show any acidic behavior.

Question 8:

When diluting acid, why is it important to add acid in the water and not water in the acid?

Solution:

  • Diluting is a process that involves adding concentrated acid to the water. The concentrated water will be added gradually by stirring.
  • There is heat that is evolved slowly and gradually so that concentrated acid is added because diluting acid easily absorbs large quantity of water.
  • If it happens in the other way by adding water to diluted acid the amount of heat evolved is all don’t at once and not gradually. This may cause a splash and could end up in acid burn.

Question 9:

When an acid is diluted, how does it affect the concentration of the hydronium ions H3O+?

Solution:

When dilution happens and the concentrated solution is mixed with water, it results in the decrease of hydronium ions per unit volume.

Question 10:

How does excess base dissolved in the water affect the concentration of the hydroxide ions?

Solution:

The solution of base is diluted when mixed with more water, therefore the concentration of hydroxide ions will decrease per unit volume.

Question 11: 

You are given two solutions called E, J their pH is 6 and 8 respectively.

Answer the following:

  1. Which of the two solutions have more hydrogen ion concentration?
  2. Which is acidic and which is basic?

Solution:

In order to find the hydrogen ion concentration we can use the rule that states, “The pH of any solution is inversely proportional to the hydrogen ion concentration”. Therefore, it means that the solution that has lower pH number will have the higher hydrogen ion concentration. So it means solution E will have the higher hydrogen ion concentration. In addition, solution J will be basic and E will be acidic.

Question 12:

What is the effect of concentration of the hydrogen ions have on the nature of a solution?

Solution:

When acids are added to water, they produce hydrogen ions in water therefore the concentration of the hydrogen ions will increase in water. Since the solution will have more hydrogen ions, it will definitely be acidic in nature.

Question 13:

Does basic solutions have hydrogen ions? If they do then why are they basic in nature?

Solution:

Basic solutions do not have hydrogen ions. Since the solution has excess of hydroxide ions, the basic solution does not have hydrogen ions.

Question 14:

In what condition does a farmer need to treat the soil on his field with quick lime or slaked lime or chalk?

Solution:

If the soil that is on the field were too acidic then he would have to treat the soil with quicklime. If the soil is, too acidic it means that it will be having a low ph. Therefor the farmer would need to add lime or even slaked lime so that his soil is not too acidic.

Question 15:

Write down the common name for the compound CaOCl2:

Solution:

It is called bleaching powder.

Question 16:

Give names of substances after the treatment with chlorine will give bleaching powder:

Solution:

Calcium hydroxide

Question 17:

What is the name of the sodium compound that is used to softening hard water?

Solution:

Sodium carbonate

Question 18:

Write the reaction for the heating of sodium hydro carbonate

Solution:

2NaHCO3 ——- heat——–> Na2CO3 + H2O + CO2

Question 20:

Why rainwater conducts electricity and distilled water doesn’t?

Solution:

  • Distilled water does not contain any ionic compounds in it.
  • Whereas rainwater has a lot, more compounds.
  • Rainwater has dissolved acidic gas such as carbon dioxide from the air and that forms carbonic acid. This means that it has hydrogen ions and carbonate ions. Therefore, with the presence of acids, rainwater can conduct electricity.

Question 21:

Why is that acids don’t have their acidic behavior in the absence of water?

Solution:

The acidic behavior from acids are because of the presence of hydrogen ions. Hydrogen ions can only be produced in the presence of water and therefore water is definitely needed if acids are to show their acidic behavior.

Question 22:

Find out which solution is which with respect to their pH and arrange them in an increasing order of hydrogen ion concentration:

SolutionpH
Neutral – A7
Strongly alkaline – B11
Strongly acidic – C1
Weekly acidic – D4
Weekly alkaline – E9

Solution:

In increasing order of hydrogen ion concentration:

pH 11(B) -> pH 9(E) -> pH 7(A) -> pH 4(D) -> pH 1 (B)

Question 23:

There are two test tubes A and B. In test tube, A HCl is added and in test tube, B  is added. In both the test tubes, magnesium ribbons are kept in both the test tubes. So, find out and explain in which test tube there will be more fizzing give reasons.

Solution:

HCl is a strong acid where acetic is a weaker acid. The reason why fizzing occurs is because of the evolution of the hydrogen gas by reacting with the acid on the magnesium ribbon. Since HCl is a very strong acid there is a lot of liberation of hydrogen gas from test tube A. therefore, more fizzing take place in test tube A.

Question 24:

Fresh milk from cows have pH of 6. So how does the pH change when this milk is turned to curd?

Solution:

The formation of lactic acid is what turns the milk to curd therefore it will result in the pH changing below 6.

Question 25:

Why should plaster of paris be stored in moisture proof container?

Solution:

Moisture can affect it by slowing the setting of the plaster because of hydration. Which will end up making it useless.

Question 26:

What is meaning of neutralization reaction? Give examples:

Solution:

The reaction of acid + base gives a product of salt + water, which is considered a neutralization reaction.

Examples:

NaOH + HCL -> NaCl + H2O

Mg(OH)+ H2CO3 -> MgCO3 + 2H2O

Question 27:

What are two important purposes of washing soda and baking soda?

Solution:

Washing sodaBaking soda
1.       It is used as an electrolyte1.       It can be used to test the garden soil for acidity. If bubbles are developed then the soil Is too acidic
2.       It can be used domestically as water softener for laundry.2.       If used on washing car then it will remove dead bug bodies without damaging the color or the paint on the car.

Important Questions

1.How will you test a gas which is liberated when hydrochloric acid reacts with an active metal ?

Ans. The gas produced when hydrochloric acid reacts with an active metal is hydrogen gas. The gas is passed through soap solution when soap bubbles are formed. Hydrogen gas being lighter than air. these bubbles rise up. When a burning candle is brought near soap bubbles containing hydrogen gas, a pop sound is heard due to the combustion of hydrogen to form water.

2H2 + O2 – 2H2O

2. Why does the flow of acid rain water into a river make the survival of aquatic life in the river difficult ?

Ans. When acid rain water flows into the river, it makes the river water acidic. Animals need a pH of 7 – 7.8 for survival. When acid rain water flows into the river, the pH of river water becomes less than 7. This makes the survival of aquatic life in the river difficult.

3. At what pH in the mouth is tooth decay faster and why ?

Ans. At pH lower than 5.5 in the mouth, tooth decay becomes faster because the acid produced reacts with the enamel of the teeth and corrodes it.

4. Write balanced chemical equations for the reactions taking place when dry blue crystals of copper sulphate are dropped into concentrated sulphuric acid.

Ans. Concentrated sulphuric acid is a powerful dehydrating agent. It takes out water of crystallisation from the crystals of copper sulphate leaving behind anhydrous white powder of CuSO4.

CuSO4.5H2O – CuSO4 + 5H2O

5. A compound which is prepared from gypsum has the property of hardening when mixed with proper quantity of water. Identify the compound and write its chemical formula.

Ans. The compound is Plaster of paris, CaSO4. 1/2 H2O

6. Name the acid present in (i) nettle sting (ii) curd.

Ans. The acid present are as under :

(i) Nettle sting : Methanoic acid   (ii) Curd : Lactic acid

7. Name a salt which does not contain water of crystallisation.

Ans. Sodium hydrogencarbonate (NaHCO3) does not contain water of crystallisation.

8. What happens when nitric acid is added to egg shell ?

Ans. Egg shell contains calcium carbonate. The following reaction will take place :

CaCO3 + 2HNO3 – Ca(NO3)2 + CO2 + H2O

9. How will you distinguish between baking powder and washing soda by heating ?

Ans. ON heating baking powder, CO2 gas is liberated when turns lime water milky.

2NaHCO3 – Na2CO3 + H2O + CO2

No gas is liberated on heating washing soda.

Na2CO3.10H2O – Na2CO3 + 10H2O

10. What is baking powder ? How does it make the cake soft and spongy ?

Ans. Baking powder is a mixture of sodium hydrogencarbonate and a mild edible acid like tartaric acid. When baking powder is heated or mixed with water, the following reaction takes place :

NaHCO3 + H+ – CO2 + H2O + Sodium salt of acid

11. What is the chemical formula for Plaster of Paris ? How is it prepared ? State the common and chemical names of the compound formed when Plaster of Paris is mixed with water.

Ans. The chemical formula for Plaster of Paris is CaSO4.1/2H2O. It is prepared by heating gypsum at 373 K as shown by the chemical equation :

CaSO4.2H2O – CaSO4.1/2H2O +3/2H2O

WHen Plaster of Paris is mixed with water, gypsum (common name) is formed again. Chemically it is known as calcium sulphate dihydrate.

CaSO4.1/2H2O + 3/2H2O – CaSO4.2H2O

12. How is bleaching powder prepared ? why does bleaching powder

(i) smell strongly of chlorine ?

(ii) not dissolve completely in water ?

Ans. Bleaching powder is prepared by the action of Cl2 on dry slaked lime.

Ca(OH)2 + Cl2 – CaOCl2 + H2O

Bleaching powder smells strongly of chlorine because it is loosely bound to slaked lime. Carbon dioxide present in the air combines with bleaching powder liberating chlorine.

CaOCl2 + CO2 – CaCO3 + Cl2

Bleaching powder reacts with water as under :

CaOCl2 + H2O – Ca(OH)2 + Cl2

Ca(OH)2 + CO2 – CaCO3 + H2O

Calcium carbonate is an insoluble substance. Hence, bleaching powder does not dissolve in water completely.

13. A knife, which is used to cut a fruit, was immediately dipped into water containing drops of blue litmus solution. If the colour of the solution is changed is red, what inference can be drown about the nature of the fruit and why ?

Ans. The fruit is a citrus fruits because fruits contain an acid. The acid sticking to the knife turns blue litmus into red.

14. You might have seen lemon or tamarind juice being used to clean tarnished surface of copper vessels. Explain why these sour substances are effective in cleaning the vessels.

Ans. Lemon juice contains citric acid while tamarind contains tartaric acid. These acids act as cleansing agents. They react with the layer of copper carbonate on the surface of vessels to form soluble salts which are washed away by water.

15. Why should curd and other sour foodstuffs not to be kept in brass of copper vessels ?

Ans. Curd and other sour foodstuffs contain acids which react with copper metal to form poisonous salts, which may cause food poisoning. Hence it is not advisable to keep curd and sour foodstuffs in brass or copper vessels.

16. How will you prove that the given salt is a carbonate of metal ?

Ans. Take about 1 g of the salt in a test tube. Add about 1 mL of dilute H2SO4. Pass vapours produced through lime water taken in another test tube. If the lime water turns milky, the salt is a carbonate of a metal.

17.  What are antacids ? Name two compounds which are used as antacids.

Ans. Antacids are mild bases which are used to neutralise the effect of excess acid produced in the stomach as a result of indigestion.

18. Dry HCl gas does not change the colour of dry blue litmus paper. Give reason.

Ans. Dry HCl gas does not contain H+ ions. These ions are produced when HCl gas comes into contact with water.

HCl + H2O – H3O+ + OH-

The H3O+ ions or H+(aq) react with blue litmus and the colour is changed to red.

19. Though the compounds such as glucose and alcohol have hydrogen atoms in their molecule, yet they are not categorised as acids. Why ?

Ans. Substances which furnish H+ ions on dissolving in water act as acids. Glucose and alcohol do not furnish H+ ions in water. Hence they are not categorised as acids.

20. Classify the following salts into acidic, basic or neutral :

Potassium sulphate, ammonium chloride, sodium carbonate, sodium chloride.

Ans. Acidic salts : Ammonium chloride

Basic salts : Sodium carbonate

Neutral salts : Potassium sulphate, sodium chloride

21. In one of the industrial processes used for manufacture of sodium hydroxide, a gas X is formed as by- product. The gas X reacts with dry slaked lime to give a compound Y which is used as a bleaching agent in chemical industry. Identify X and Y.

Ans. During the manufacture of sodium hydroxide by electrolysis of sodium chloride, chlorine gas is formed as a by-product. It reacts with dry slaked lime, Ca(OH)2 to form bleaching powder CaOCl2, which is used as a bleaching agent in chemical industry.

22. A dry pellet of a common base B, when kept in open, absorbs moisture and turns sticky. The compound is also a by-product of CHlor-alkali process. Identify B. What type of reaction occurs when B is treated with an acidic oxide ?

Ans. Sodium hydroxide is produced as a by-product in chlor-alkali process, along with Cl2 and H2. Pallet of NaOH when kept in open absorbs moisture and turns sticky.

2NaOH + CO2 – Na2CO3 + H2O

23. Explain the effect on concentration of hydronium ion [H3O]+ and pH of an acidic solution on dilution.

Ans. Concentration of hydronium ion [H3O]+ decreases on dilution of an acidic solution because the same number of [H3O]+ ions are present in a larger volume.

pH of the acidic solution increases on dilution because the concentration of H+ ions decreases. Lower the H+ ion concentration higher the pH value.

24. What is meant by water of crystallisation ? How would you show that copper sulphate crystals contain water of crystallisation ?

Ans. Water which is present in the crystals of a compounds is called water of crystallisation . It is defined as the fixed number of water molecules present in one formula unit of a salt.

Copper sulphate crystals are blue in colour. On heating solid copper sulphate crystals, they lose water of crystallization and become white. On adding a few drops of water to the white powder, blue colour of copper sulphate is restored. This shows that copper sulphate crystals contain water of crystallisation.

25. While constructing a house, a builder selects marble flooring and marble table top for the kitchen where vinegar and juices of lemon tamarind etc, are more often used for cooking. Will you agree to this selection and why ?

Ans. No, this selection is wrong.

Vinegar and tamarind etc contains acid. Acid react with marble flooring and marble table top. Marble is calcium carbonate which reacts with acids.