I. Conceptual Understanding
1. 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 Hydrogen ion concentration. (2 Marks)
A.
* Increasing order of Hydrogen ion concentration is pH = 11, 9, 7, 4, 1 it means solutions C, E, D, A and B.
2. What is neutralization reaction? Give two examples. (2 Marks)
A. * The reaction of an acid with a base to give a salt and water is known as a neutralization reaction.
3. What happens when an acid or base is mixed with water? (2 Marks)
A. * Mixing an acid or base with water result 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.
4. Why does tooth decay start when pH of mouth is lower than 5.5. (4 Marks)
A. * Tooth decay starts when the pH of the mouth is lower than 5.5.
* Tooth enamel, made of Calcium phosphate is the hardest substance in the body.
* It does not dissolve in water, but is corroded 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.
Preventions:
* The best way to prevent this is to clean the mouth after eating the food.
* Using tooth pastes, which are generally basic neutralize the excess acid and prevent tooth decay.
5. Dry Hydrogen chloride gas does not turn blue litmus to red whereas Hydrochloric acid does. Why? (2 Marks)
A. * Dry Hydrogen gas does not dissociate into ions in the absence of water. So it does not turn blue litmus to red.
* Hydrochloric acid dissociates into H+ and Cl- ions in the presence of water.
* So Hydrochloric acid turns blue litmus to red.
HCl + H2O H3O+ + Cl-
II. ASKING QUESTIONS AND MAKING HYPOTHESIS
6. Why does not distilled water conduct electricity?
A. * Distilled water is purified and does not contain any impurities, so it is unable to conduct electricity.
* Water molecules on their own have no charge and as a result they cannot swap electrons.
* Without swapping of electrons, electricity is unable to travel through distilled water.
7. Why pure Acetic acid does not turn blue litmus to red? (1 Mark)
A. Acetic acid is a weak acid. Further it is a hydrocarbon. As such it does not turn blue litmus to red.
8. A milkman adds a very small amount of baking soda to fresh milk. (2 Marks)
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.
A. a) By adding small amount of baking soda, the milkman shifts the pH of the fresh milk from 6 to slightly alkaline so that he can keep it for longer time as the milk in alkaline condition.
b) Due to adding of baking soda, this milk becomes slightly basic because of which acids produced in this milk, due to bacterial actions are neutralized.
Hence this milk takes longer time to set as curd.
9. Plaster of Paris should be stored in moisture- proof container explain. Why? (2 Marks)
A. * Plaster of Paris should be stored in a moisture-proof container as it absorbs water from moisture and turn into hard substance (Gypsum) as shown in the following reaction.
QUESTIONS AND ANSWERS GIVEN IN THE LESSON
THINK AND DISCUSS
1. What is litmus solution? Mention other indicators.
A. * Litmus solution is a dye extracted from lichen, a plant belonging to the division of Thallophyta and is used as indicator.
* In neutral solution litmus colour is purple.
* Coloured petals of some flowers such as Hydrangea, Petunia and Geranium are also used as indicators.
2. Is the substance present in antacid tablet acidic or basic. (AS-1) (1 Mark)
A. * Sodium hydrogen carbonate is one of the substances present in antacid tablet.
* This substance is an alkali.
3. What type of reaction takes place in stomach when an antacid tablet is consumed? (AS-7) (2 Marks)
A. * Our stomach produces Hydrochloric acid which helps in digestion of the food without harming the stomach.
* During in digestion the stomach produces too much acid and this causes pain and irritation.
* To get rid of this pain, people use bases called antacids.
* The antacids neutralize the excess of acid in the stomach.
* So this is a neutralization reaction.
4. You are provided with three test tubes containing distilled water, an acid and a base solution respectively. If you are given only blue litmus paper, how do you identify the contents of each test tube. (AS-3) (4 Marks)
A. * Let A, B and C be the three test tubes.
* Put the blue litmus paper in the solution of test tube A. If it turns red then the solution is acid. Otherwise it may be a base or distilled water.
* If the litmus paper is turned red when put in the solution of A, now put that red litmus paper in the solution of B. If this litmus paper turns blue then this solution is base. Otherwise it is distilled water.
* If the solution in B is base, then C is distilled water, If B is not a base, then C is a base and B is distilled water.
5. Which gas is usually liberated when an acid reacts with a metal? How will you test for the presence of this gas. (AS-3) (2 Marks)
A. * Hydrogen gas is evolved when an acid reacts with a metal.
* When a burning candle or match stick is brought near the gas, it burns with a pop sound indicating the gas as Hydrogen.
6. A compound of calcium reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle, turns lime water milky. Write balanced equation for the reaction if one of the compounds formed is Calcium chloride. (AS-1) (2 Marks)
A. * As the gas evolved extinguishes a burning candle, turns lime water milky, it is Carbon dioxide.
* The chemical equation is
Calcium carbonate + dilute Hydrochloric acid Calcium chloride + Carbondioxide + Water
CaCO3 + 2 HCl → CaCl2 + CO2 + H2O
7. Why do HCl, HNO3 ... etc., show acidic characters in aqueous solutions, while solutions of compounds like alcohol and glucose do not show acidic character? (AS-1) (2 Marks)
A. * The acidity of HCl, HNO3 etc., is attributed to the H+ ions produced by them in aqueous solutions.
* In solutions of compounds like alcohol and glucose H+ ions are not produced. So they do not show acidic character.
8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid. (AS-3) (2 Marks)
A. * The process of dissolving an acid or a base in water is an exothermic process.
* Care must be taken while mixing concentrated Nitric acid or Sulphuric acid with water.
* If water is added to 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.
* So the acid must always be added slowly to water with constant stirring.
9. What will happen if the pH value in our body increases? (2 Marks) (AS-2)
A. * Human blood pH should be slightly alkaline (7.35 - 7.45). A pH below 7.0 means that the body is acidic making it to prone to ailments, fatigue and disease.
* When the pH is body tissues are overly acidic a state called 'acidosis' occurs.
* Acidosis is one of the basic causes of many diseases as it creates a burden on the liver, kidney, digestive tract, heart and affects every cell in the body.
* A healthy body stores large alkaline reserves to meet emergency demands if too many acid -producing foods are consumed.
* However those normal reserves can be depleted balanced body chemistry is of utmost importance for the maintenance of health and corrections of disease.
* When the body is in an alkaline state of 7.0 or above, diseases cannot gain a foot hold.
10. Why do living organism have narrow pH range? (2 Marks) (AS-7)
A. * Living organism have narrow pH range.
* In humans a pH change can kill our normal flora (the resident bacteria in our bodies).
* If the bacteria are not there we would be more susceptible to infections leading to a disease state because our host defences would be compromised.
* Finally the change in pH leads to irregular biological processes like indigestion, stomach pain etc.,
* So living organism should maintain a narrow pH range.
ADDITIONAL QUESTIONS AND ANSWERS
1. Conceptual understanding
1. Mention few natural acid-base indicators: (2 Marks) (AS-1)
A. * There are many natural materials like litmus, extract of red cabbage, turmeric solution and extracts of coloured petals of some flowers contain dye molecules which are weak acids or bases.
* These can be used as acid-base indicators to detect the nature of the solution for acidity or basicity.
* These are natural acid-base indicators.
2. Write the names of two synthetic indicators. (1 Mark) (AS-1)
A. * Indicators such as methyl orange and phenolphthalein that can be used to test for acids and bases.
* These are synthetic indicators.
3. What are olfactor indicators? (1 Mark) (AS-1)
A. Olfactory indicators are substances whose odour changes in acidic or basic media.
4. Give an example of reaction acids and bases with metals. (2 Marks) (AS-1)
A. Acids and Bases react with metals and release Hydrogen
e.g.: 1) 2 HCl (aq.) + Zn(S) → ZnCl2 (aq.) + H2 (g)
Acid + Metal → Salt + Hydrogen
2) 2 NaOH + Zn → Na2ZnO2 + H2
(Sodium Zincate)
Base + Metal → Salt + Hydrogen
5. Write the Chemical equations for the reaction of Carbonates with acids. (2 Marks) (AS-1)
A. * Sodium carbonate reacts with Hydrochloric acid and release Carbon dioxide along with the formation of Sodium chloride salt.
* Na2CO3 (s) + 2 HCl (aq) → 2 NaCl (aq) + H2O (l) + CO2 (g)
6. What are the products formed when an acid reacts with a metal hydrogen carbonate. (2 Marks) (AS-1)
A. * When Sodium hydrogen carbonate reacts with Hydrochloric acid carbon dioxide is released and a salt sodium chloride is formed.
NaHCO3 (s) + HCl (aq) → NaCl (aq) + H2O (l) + CO2 (g)
7. What is the action of acids with metallic oxides. Give an example. (2 Marks) (AS-1)
A. * All dilute mineral acids (H2SO4, HCl, HNO3 ....... etc,) react with all metallic oxides to form their respective metallic salts and water.
* Metallic oxide + Acid (dil.) → Metallic salt + Water
ZnO + 2 HCl → ZnCl2 + H2O
Zinc oxide Dil. Hydrochloric acid (Zinc chloride) (Water)
8. What is the chemical formula for table salt. (1 Mark) (AS-1)
A. * The Chemical formula for table salt is NaCl.
* It is Sodium chloride.
9. What is the significance of 10 H2O in the formula Na2CO3.10 H2O. (1 Mark) (AS-1)
A. * Na2CO3 .10 H2O is Hydrated sodium carbonate.
* This contains 10 water molecules in one formula unit of Sodium Carbonate.
* Due to these 10 water molecules 'Sodium Carbonate' is not wet.
10. What do acids have in common. (2 Marks) (AS-1)
A. * We know that acids generate Hydrogen gas on reacting with metals.
* So Hydrogen seem to be common element to all acids.
11. Which are responsible for the acidic property of a substance? (1 Mark) (AS-1)
A. Acids produce H+ ions (Hydrogen ions) in solution which are responsible for their acidic property.
12. What do bases have in Common.
A. OH- ions (Hydroxide ions) is common in all bases.
13. How is bleaching powder prepared? Write its formula. (2 Marks) (AS -1)
A. * Bleaching powder is produced by the action of Chlorine on dry slaked lime (Ca(OH)2).
Ca(OH)2 + Cl2→ CaOCl2 + H2O
* The formula for bleaching powder is CaOCl2
14. What is baking powder? (1 Mark) (AS-1)
A. Baking powder is a mixture of baking soda and a mild acid tartaric acid.
15. What is a neutralization reaction? (2 Marks) (AS-1)
A. * The reaction of an acid with a base to give a salt and water is known as a neutralization reaction.
* It can be written as:
Base + Acid → Salt + Water
16. Write the general reaction between a metal oxide and an acid. (1 Mark) (AS-1)
A. Metal Oxide + Acid → Salt + Water
17. How can you conclude that metal oxides are basic in nature. (2 Marks) (AS-1)
A. * Both metallic oxides and metal hydrides give salt and water when they react with an acid.
* Thus we can conclude that metal oxides are basic in nature like metal hydroxides.
18. What is the reaction between Carbon dioxide and Calcium hydroxide. (Lime water). (2 Marks) (AS-1)
A. * Calcium hydroxide (lime water), which is a base, reacts with Carbon dioxide to produce salt and water.
* Ca(OH)2 (aq) + CO2 (g) → CaCO3 ↓ + H2O (l)
(white precipitate)
19. How can you conclude that Carbon dioxide which is a non metallic oxide is acidic in nature? (2 Marks) (AS-1)
A. * Calcium hydroxide which is a base reacts with carbon dioxide to produce salt and water.
* This reaction is similar to the reaction between a base and an acid.
* Thus we can conclude that carbon dioxide which is a non metallic oxide is acidic in nature.
* In general all non metal oxides are acidic in nature.
20. How can you conclude that dry HCl gas is not an acid? (1 Mark) (AS-1)
A. * Dry HCl gas is not an acid because with it, there is no change in colour of dry litmus paper.
* HCl aqueous solution is an acid because wet litmus paper turned into red.
21. Write the chemical equation of dissociation of HCl in water. (2 Marks) (AS-1)
A. * The dissociation of HCl in water is shown below
HCl + H2O → H3O+ + Cl- (aq)
* Hydrogen ions can not exist as bare ions. So we represent H+ as Hydronium ion H3O+
H+ + H2O → H3O+
22. What happens when a base is dissolved in water. Give two examples. (4 Marks) (AS-1)
A. * When a base is dissolved in water the following dissociation takes place.
a) NaOH (s) 
Na+(aq) + OH-(aq)
b) KOH (s) K+ (aq) + OH- (aq)
c) Mg (OH)2 (s) Mg2+(aq) + 2 OH- (aq)
* On dissolving bases in water produces hydroxide (OH-)ions.
23. What are alkalis. Name two alkalis. (2 Marks) (AS-1)
A. * Bases which are soluble in water are called alkalis.
* All bases do not dissolve in water. Be(OH)2 is slightly soluble in water.
* NaOH, KOH which are soluble in water are alkalis.
24. Why is acetic acid a weak acid? (1 Mark) (AS-1)
A. * Acetic acid (CH3COOH) in its diluted stage gives fewer H3O+ ions.
* So it is a weak acid.
25. What is water of crystallization? (1 Mark) (AS-1)
A. Water of crystallization is the fixed number of water molecules present in one formula unit of salt.
26. What is a universal indicator? How is it useful. (2 Marks) (AS-1)
A. * The universal indicator can also be used to know the strength of acid or base.
* Universal indicator is a mixture of several indicators.
* The universal indicator shows different colours at different concentrations of Hydrogen ions in a solution.
27. What is dilution? (2 Marks) (AS-1)
A. * Mixing an acid or base with water result 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.
28. What is a pH scale. How does this value vary from acids to bases. (4 Marks) (AS-1)
A. * A scale for measuring Hydrogen ion concentration in a solution is called pH scale. (The 'p' in pH stands for 'Potenz', in German 'Potenz' means power)
* pH value of a solution is simply a number which indicates the acidic or basic nature of a solution.
* The pH of neutral solutions is 7.
* Values less than 7 on the pH scale represent an acidic solution.
* As the pH value increases from 7 to 14, it represents a decrease in H3O+ ion concentration or an increase in OH- ion concentration in the solution.
* pH value of a solution above '7' represents a basic solution.
29. What is rock salt? How is it mined? (2 Marks) (AS-1)
A. * Deposites of solid salt are also found in several parts of the world.
* These deposits of large crystals are often brown due to impurities.
* This is called rock salt.
* Beds of rock salt were formed when seas of bygone ages dried up.
* Rock salt is mined like coal.
30. Write the chemical formula for the following substances: (Each 1 Mark) (AS-1)
a) Sodium Chloride b) Acetic acid
c) Gypsum d) Bleaching Powder
e) Baking Soda f) Washing Soda
g) Plaster of Paris h) Slaked lime
i) Sodium hydroxide
A. a) Sodium Chloride: NaCl
b) Acetic acid: CH3COOH
c) Gypsum: CaSO4.2 H2O
d) Bleaching Powder: CaOCl2
e) Baking Soda: NaHCO3
f) Washing Soda: Na2CO3 . 10 H2O
g) Plaster of Paris: CaSO4 . 
H2O
h) Slaked lime: Ca(OH)2
i) Sodium hydroxide: NaOH
31. a) If someone in the family is suffering from a problem of acidity, which of the following would you suggest as a remedy? Lemon juice, vinegar or baking soda solution.
b) Which property do you think of while suggesting the remedy?
A. a) Baking Soda solution is suggested.
b) Baking Soda can neutralize the acidity.
32. How can you decide the nature of the salt in a neutralization reaction? (2 Marks) (AS-1)
b * The nature of the salt is decided from a knowledge of the strengths of the acid and base reacting in the neutralization reaction.
* Following reactions reveal the nature of the Salt.
i) Strong acid + Strong base → Neutral salt
ii) Strong acid + Weak base → Acidic salt
iii) Weak acid + Strong base → Basic salt
iv) Weak acid + Weak base → Salt
Its nature depends upon the relative strength of acid and base in the reaction.
33. Describe the preparation of washing soda. What are its uses. (4 Marks) (AS-1)
A. * Baking Soda is obtained from Sodium Chloride. The reaction is
NaCl + H2O + CO2 + NH3→ NH4Cl + NaHCO3
* On heating baking soda (NaHCO3)Sodium Carbonate is obtained.
2 NaHCO3
Na2CO3 + H2O + CO2
* Recrystallization of Sodium Carbonate gives washing soda.
Na2CO3 + 10 H2O → Na2CO3 .10 H2O
Uses:
i) Sodium carbonate (washing soda) is used in glass, soap and paper industries.
ii) It is used in the manufacture of Sodium compounds such as Borax.
iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.
iv) It is used for removing permanent hardness of water.
34. How is bleaching powder prepared? Write its uses. (4 Marks) (AS-1)
A. Preparation of Bleaching Powder:
* Chlorine is produced during the electrolysis of aqueous Sodium Chloride (Brine).
* This chlorine gas is used for the manufacture of bleaching powder.
* Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2]. Bleaching powder is represented by formula CaOCl2, though the actual composition is quite complex.
* Ca(OH)2 + Cl2→ CaOCl2 + H2O
Uses of Bleaching Powder:
* It is used for bleaching cotton and linen in the textile industry for bleaching wood pulp in paper industry and for bleaching washed clothes in laundry.
* Used as an oxidizing agent in many chemical industries.
* Used for disinfecting drinking water to make it free of germs.
* Used as a reagent in the preparation of chloroform.
35. Describe the preparation of Baking soda. What are its uses. (4 Marks) (AS-1)
A. * The chemical name of the compound is Sodium hydrogen carbonate (NaHCO3). It is prepared as follows:
* NaCl + H2O + CO2 + NH3→ NH4Cl + NaHCO3
Uses:
* Baking powder is a mixture of baking soda and a mild edible acid such as tartaric acid.
* Baking soda is sometimes added for faster cooking.
* It is also used in soda-acid fire extinguishers.
* It acts as mild antiseptic.
* Sodium hydrogen carbonate is also an ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.
* Baking Powder is used in the preparation of bread or cake to rise making them soft and spongy.
36. Write about acids and bases. (2 Marks) (AS-1)
A. * Acids are sour to taste and turn Blue litmus to red.
* Bases are soapy to touch and turn Red litmus to Blue.
II. ASKING QUESTIONS AND MAKING HYPOTHESIS
36. Why Pickles and Sour substances are not stored in Brass and Copper vessels? (2 Marks) (AS-2)
A. * Pickles and Sour substances consists of acids which react with Copper or brass vessels to liberate Hydrogen gas and form soluble salts.
* These Salts are poisonous to animal life when taken internally, so they are not stored in those vessels.
37. What happens when excess Carbondioxide is passed through lime water? (2 Marks) (AS-2)
A. * When Carbondioxide is passed through lime water it turns milky white due to the formation of Calcium Carbonate.
* Ca(OH)2 (aq) + CO2 (g) → CaCO3 ↓ + H2O (l)
lime water (White precipitate)
* On passing excess of Carbon dioxide, the following reaction takes place.
CaCO3 (s) + H2O (l) + CO2 (g) → Ca(HCO3)2 (aq)
(Soluble in water)
38. Why are pickles never stored in Tin Cans? (2 Marks) (AS-2)
A. * Pickles, Jams and Jellies are never stored in Tin Cans as all of them contain organic acids.
* These acids react with the metals to form harmful salts.
* Further these acids corrode the container.
* So these are stored in glass vessels or plastic vessels.
39. Why Copper and brass cooking vessels are coated with tin metal (Kalai)?(4 Marks) (AS-2)
A. * When food is cooked in these vessels without coating of Tin metal (Kalai), the organic acids present in the food materials react with Copper and corrode it.
* Moreover the Copper salts formed by acids are poisonous.
* The Kalai protects the vessels from the action of acids for a while and hence prevents poisoning.
* However now a days many people are using stainless steel cooking vessels. These are not affected by the acids present in the food materials.
40. Are plants and animals pH sensitive? Explain. (2 Marks) (AS-2)
A. * Living organisms can survive only in a narrow range of pH change.
* When pH of rain water is less than 5.6, it is called 'acid rain'.
* When acid rain flows in to the rivers, it lowers the pH of the river water, the survival of aquatic life in such rivers becomes difficult.
41. Do you KNow → (1)
Litmus solution is a dye extracted from lichen, a plant belonging to the division of 'Thallophyta' and is used as indicator. In neutral solution litmus colour is purple. Coloured petals of some flowers such as Hydrangea, Petunia and Geranium are also used as indicators.
42. Do you Know? → (2)
Salt - A symbol of freedom struggle: You know common salt as a substance which enhances the taste of food; it also has played a remarkable role in motivating the people towards freedom struggle. The tax levied by the British government on common food substance (salt), for both the poor and the rich, made them to become unite in the freedom struggle.
You must have heard about Mahatma Gandhi's Dandi March and about 'Salt Satyagraha' in the struggle for freedom of India.
Writer : C.V. SARVESWARA SARMA
