Acids, Bases And Salts-Class-VII

 

AN INTRODUCTION TO ACIDS, BASES AND SALTS

Nature has provided us with many elements which combine together in various proportions to form compounds. You have studied in your previous class that materials can be classified on the basis of properties, such as appearance, solubility, combustibility, etc. Similarly, compounds can be grouped on the basis of their test (sweet, sour or salty) or their reactions with other substances.

 

Ask your mother to give you some vinegar, jaggery, common salt, raw lemon, tamarind and baking powder from the kitchen. Now taste each of these and note the differences in their taste. Classify these substances as sweet, sour, bitter, salty or tasteless.

 

You have classified the substances in the previous activity on the basis of taste. Let us revisit the compounds tested in Activity-1:

(a) Vinegar, raw lemon and tamarind taste sour. These compounds are known as acids. The word acid comes from the Latin word acere which means 'sour'.

(b) Baking powder is bitter in taste. The compounds that are bitte to taste and soapy to touch are known as bases. Take some washing soda and feel it. It is soapy to touch, hence, it is a base. Do not taste washing soda.

(c) Common salt is salty to taste. It is neither sour nor bitter. This shows that it has neither the characteristics of an acid nor of a base. Such compounds are known as salts. They are formed when acids and bases react with each other.

Let us first start with understanding acids and bases.

 

ACIDS

In late 1800s, Swedish scientist Svante Arrhenius proposed that water can dissolve many compounds by dissociating them into ions. He suggested that acids contain hydrogen that dissolves in water to give the positive hydrogen ion.

All the properties of acids are due to the presence of these positively charged hydrogen ions (H').

 

Depending on the source, acids are classified as:

(a) Mineral acids: Acids which are prepared artificially frorn minerals found in the earth are called mineral acids. For example, sulphuric acid, nitric acid, hydrochloric acid, etc. These are also called inorganic acids.

(b) Organic acids: Acids which occur naturally and are found in plants and animals are called organic acids, for example, citric acid, acetic acid, lactic acid, etc. A bee-sting venom contains formic acid, an organic acid.

Several fruits like oranges, lemons, grapes, etc., are sour in taste because they contain acids. The sour taste of lemon is due to the presence of citric acid, while that of vinegar is due to acetic acid. Given in Table-1 are some important natural acids and their sources.

 

Acids found in food materials
Food material	Acid present
Milk, curd	Lactic acid
Proteins	Amino acid
Tea	Tannic acid
Vinegar	Acetic acid
Apple	Maleic acid
Tamarind, grapes	Tartaric acid
Citrus fruits	Citric acid
Cold drinks (carbonated)	Carbonic acid

 

Acids can also be classified on the basis of their strength:

(a) Strong acids: Acids which dissociate (separate) completely in water to give a large number of hydrogen ions (H*) are called strong acids, for example, sulphuric acid, hydrochloric acid, nitric acid, etc.

(b) Weak acids: The acids which do not dissociate completely in water, and furnish a small number of hydrogen ions are called weak acids, for example, acetic acid, carbonic acid, formic acid, etc.

 

On the basis of quantity of water in an acid, they can be classified as:

(a) Concentrated acids: Acids which contain lesser quantity of water dissolved in them are called concentrated acids. For example, sulphuric acid is usually found in concentrated form.

(b) Dilute acids: Acids which are dissolved in larger quantity of water are called dilute acids. An acid can be diluted by adding more water to its aqueous solution. Dilute acids contain lesser hydrogen ions in their solution due to the addition of more water. This decreases the acidity and thereby their strength.

 

PROPERTIES OF ACIDS

Various characteristics of acid are as follows:

HUNGRY BRAINS

(a) Acids are sour to taste.

(b) They are soluble in water.

(c) They have different effects on different indicators. They turn blue litmus red and methyl orange pink. They have no effect on phenolphthalein.

(d) They are good conductors of electricity in solutions.

(e) Acids are generally colourless.

(f) Strong acids may also cause blisters on the skin and hence, must be handled carefully. Strong acids are corrosive and can destroy materials like wood, paper, cloth, iron, aluminium, etc. That is why they are stored in containers or bottles of glass.

 

Caution! Never add water to an acid for dilution as it may spill out and burn your skin. Instead, add acid very slowly to water while constantly stirring the solution.

 

USES OF ACIDS

Acids find extensive use in our day to day life. Some of their uses are discussed below.

Fire extinguishers:

Acids reacts with carbonates to form carbon dioxide and water. This property of acids finds application in fire extinguishers (use of sulphuric acid to produce foam of carbon dioxide gas).

The common fire extinguisher (foam spray type) which you may have seen at homes, offices, schools, lifts, etc., is based on the property of carbon dioxide to extinguish fire. Let us see how it works.

A fire extinguisher is a metal cylinder with a nozzle. Inside the cylinder are chambers. There is sulphuric acid in the inner chamber and sodium carbonate (commonly known as washing soda) in the outer chamber. When the cylinder is inverted and the nozzle is pressed, sulphuric acid reacts with sodium carbonate to produce carbon dioxide.

The carbon dioxide gas coming out from the nozzle is directed at the fire to extinguish it. Carbon dioxide being heavier than air settles on the fire, cuts off the supply of air, and extinguishes the fire.

 

Acidity: Acidity is a common ailment faced by many people. It is caused due to excessive secretion of hydrochloric acid in the stomach. Hydrochloric acid helps in the digestion of food by providing an acidic medium to stomach. However, excess acid may get secreted due to tension or worry, overeating, or eating spicy food. At times, an empty stomach can lead to acidity as the acid produced is not utilised by food. This excess acid causes damage to the walls of stomach resulting in ulcers (wounds). Acidity can be reduced by taking chilled milk or antacid tablets. Antacids are bases and they neutralise the effect of acid.

 

Some common uses of acids
1	The most important acid is sulphuric acid (H₂SO₄) which is used for manufacturing fertilisers such as superphosphate.
2	Acids are used in the batteries of cars, trucks and invertors.
3	Acids are used in making drugs, paints, dyes, detergents, plastics, etc.
4	Textile, paper and leather industries also use large amounts of acids for dyeing purposes.
5	Acids are used to remove salt deposits or scales from steam boilers. This process of removing scales is known as descaling.
6	Acids are widely used in the purification of common salt.
7	Acids are required in the purification of noble metals like gold, silver, etc.
8	Vinegar is prepared from acetic acid solution in water which is used in pickles and preservation of other food items.
9	Acetic acid is also used for making baking powder.

Bases

Bases are those compounds that give negative hydroxide/hydroxyl ions (OH) in water.

Usually, bases are classified as:

(a) Strong bases: Bases which dissociate completely in water are called strong bases, for example, sodium hydroxide and potassium hydroxide. These are corrosive in nature and can burn the skin.

(b) Weak bases: Bases which do not dissociate completely or dissociate to a small extent in water are called weak bases. For example, zinc hydroxide, copper hydroxide, magnesium hydroxide, etc.

 

Some common bases
Common name	Chemical name	Formula
Quick lime	Calcium oxide	CaO
Slaked lime/lime water	Calcium hydroxide	Ca(OH)2
Caustic soda	Sodium hydroxide	NaOH
Caustic potash	Potassium hydroxide	KOH
Washing soda	Sodium carbonate	Na2CO2
Milk of magnesia/antacid	Magnesium hydroxide	Mg(OH),
Baking soda carbonate	Sodium hydrogen	NaHCO,

 

PROPERTIES OF BASES

Physical characteristics of bases

(a) All bases are bitter to taste, for example, antacids.

(b) They have different effects on different indicators-they turn red litmus to blue, phenolphthalein to pink and methyl orange to yellow.

(c) Basic solutions are slippery to touch, like soap.

(d) Bases are good conductors of electricity in solutions.

(e) Strong bases are highly corrosive and cause blisters when they come in contact with skin.

 

All bases are not soluble in water. Water soluble bases are called alkalis, for example, sodium hydroxide, calcium hydroxide, etc. All alkalis are bases but all bases are not alkalis. Bases that are insoluble in water are called alkaline bases.

 

USES OF BASES

Many bases find extensive use in our day-to-day life, in laboratories, and also in industries. Some bases are also useful for agriculture. Soil which is too acidic for healthy growth of plants is neutralised by adding calcium oxide (CaO, quicklime). Powdered calcium carbonate (CaCO,, limestone) is also added to the soil for the same purpose.

 

Uses of bases
1	Bases are used in white wash solutions.
2	These are used to reduce the acidity of soil.
3	Calcium hydroxide (slaked lime) is used to prepare bleaching powder which is used for brightening of white clothes.
4	Lime water is used as laboratory reagent.
5	Bases are used for manufacturing soap, medicines, fibres, plaster, mortar, etc.
6	These are mainly used to prepare washing soda and for softening hard water.
7	These are used in paper making.
8	These are also used in making fertilisers and synthetic fibres.
9	These are also used as cleaning agent and ink remover.
10	Some bases are used as antacids to relieve acidity.

 

TESTING ACIDS AND BASES

There are many ways of testing a substance to find out whether it is an acid or a base.

Tasting: Acids are sour while bases are bitter to taste. Different acids are present in different food materials. Some fruits taste sour as they contain acids. However, we can't taste everything.

 

Touching: Acids burn on touch, while bases are soapy to touch. Weak acids are safe to handle but strong acids can cause skin burns. Nitric acid and hydrochloric acid are strong acids.

A solution of baking soda and water feels soapy to touch as baking soda is a base. Strong bases such as sodium hydroxide and potassium hydroxide are used in manufacturing of soap and washing soda.

Caution: Both acids and bases, when strongly concentrated, can cause damage to the skin as they are corrosive.

 

Using an indicator: An indicator is a substance which gives specific results with acids and bases. It changes its colour when added to an acid or a base and gives distinctive colours with both. Acid-base indicators can be natural or synthetic. Examples of natural indicators are extracts obtained from red cabbage, China rose and lichens. Some synthetic indicators are phenolphthalein and methyl orange.

 

Litmus solution/paper: Litmus is the most commonly used acid-base indicator. It is a purple-coloured dye obtained from lichens. Litmus solution is made by adding this dye to distilled water. This purple solution turns red when added to an acid and blue when added to a base. Litmus solution is also available as red litmus and blue litmus solution. Blue and red litmus papers are easily available and generally used in laboratories. An acid turns blue litmus red, while red litmus remains unchanged. On the other hand, a base turns red litmus blue, but blue litmus remains unchanged.

 

Neutral substances

The acidity of any solution is caused by the presence of hydrogen ions, while the basicity of a solution depends on the presence of hydroxide or hydroxyl ions. What if a substance has neither acidic nor basic characteristics? Such a substance will not bring about any change in the colour of indicators and is called a neutral substance. A few examples are given below:

 

Neutral solids: Some neutral solids are sodium chloride, i.e., common salt, sugar and glucose.

Neutral liquids: Water is a neutral liquid as are kerosene, ether, mustard oil, benzene, petrol, etc.

Neutral gases: Some neutral gases are oxygen, nitrogen and hydrogen.

 

SALTS

Salt is a compound made up of two ions; positive ion or cation (acid part) and negative ion or anion (base part).A metal salt is formed when a metal replaces the hydrogen of an acid.

The product of the reaction between an acid and a base is referred to as a salt. This refers not only to the compound known as common salt or table salt (NaCl), but also to the compound formed by the neutralisation of acids with bases.

The process in which an acid completely reacts with a base or vice versa to form salt and water is called neutralisation. It is an exothermic reaction. Common salt can be prepared by neutralisation.

 

PROPERTIES OF SALTS

The common properties of salts formed by the neutralisation of an acid with a base are:

1. Salts are mostly soluble in water.

2. They help in the conduction of electricity.

3. Some salts have a tendency to absorb moisture from nature, i.e., they are hygroscopic.

4. Most salts have high melting and boiling temperatures.

 

USES OF SALTS

Different salts have different properties, which can be used for various purposes.

Our body also requires many salts, like salts of chloride, salts of sulphate, bicarbonates, phosphates of calcium, magnesium, sodium, potassium etc.

We see several neutralisation reactions in our everyday life. Some of them are discussed below:

 

Uses of salts
1	Common salt is an essential part of our food.
2	Salts are used in preparing hydrochloric acid, caustic soda, baking soda, washing soda, chlorine, etc.
3	These are used in manufacturing soap.
4	These are used as a preservative in pickles, meat, fish, etc.
5	Fire extinguishers use washing soda to produce CO, gas.
6	Salts are also used to prepare cement and glass.
7	A salt called potash alum (phitkari) is used in the purification of water.
8	Another salt called Plaster of Paris (POP) is used in hospitals for plastering and setting broken bones.
9	POP is also used for making statues.
10	A salt of silver is used in developing the photographic films.

 

NEUTRALISATION IN EVERYDAY LIFE

Indigestion due to Acidity: You are aware that the medium for digestion of food is acidic in the stomach. Sometimes indigestion can occur due to excessive acid production by the stomach. The medicine to treat this common condition is a base (Magnesium hydroxide). It works by neutralising the excessive acid.

 

Protection of Teeth: How does tooth decay take place? Bacteria present in the mouth work on sugar left on our teeth producing acid. This acid built-up results in tooth decay. Tooth pastes contain bases which help to neutralise the acid produced and protect our teeth.

 

Neutralising Soil: Sometimes soil becomes too acidic due to mismanagement of fertilisers. Adding bases like lime (calcium oxide or calcium hydroxide) the soil can be neutralised and made fit for the next crop. If the soil is too basic then manure is added to it before sowing. Manure produces acid neutralising the soil.

 

Insect Bites: Stings of bees and ants are acidic, i.e., when they sting, an acid is injected into our skin. The acidic effect can be neutralized by applying moist baking soda (a base) or calamine solution (a base). On the other hand, the sting of a wasp is basic in nature. It can be neutralized by applying vinegar on it.

 

 

 

What are ions- Charged atoms or a group of charged atoms are called ions. The ions which have positive. charge (+ve) are called positive jons or cations and the ones with a negative charge (-ve) are called negative ions or anions. Acids are compounds that produce positive hydrogen ions (H) in water, while bases are compounds that produce negative hydroxyl ions (OH) in water.

Aqueous solution is a solution in which the solvent is water.

Pure rain water is slightly acidic due to the presence of a weak acid, carbonic acid (H,CO₂). The acid forms when carbon dioxide in air combines with water vapour in the air.

Indicators-A chemical substance that is used to identify whether a compound is acidic or basic by showing a colour change is called an indicator.

In cultivated fields, the demand of minerals in the soil becomes heavy. This deprives the soil of its natural minerals. Therefore, to fulfill the mineral requirements of cultivated crops, minerals need to be added in the form of fertilisers.

Acids: The substances which furnish hydrogen ions in water.

Bases: Substances which dissociate in water to give hydroxyl ions.

Salts: Compounds formed when acids and bases react. They are neither sour, nor bitter to taste.

Neutral substances: Substances which do not bring about any change in the colour of indicators.

Strong acid: Dissociates completely in water.

Weak acid: Does not dissociate completely in water.

Dilute acid: Acids dissolved in large quantity of water.

Concentrated acid: Acids dissolved in small quantity of water.

Acidity: Ailment caused by excessive secretion of hydrochloric acid in the stomach.

Strong base: Dissociates completely in water.

Weak base: Does not dissociate completely in water.

Neutralisation: The process in which an acid completely reacts with a base to form salt and water.

 

 

SUMMARY

1. Chemical compounds can be grouped as acids, bases and salts.

2. Acids are sour compounds which may be found in nature (organic acids) or made artificially (mineral acids).

3. Bases are bitter compounds which are soapy to touch.

4. Water soluble bases are called alkalis. Water insoluble bases are called alkaline bases.

5. A chemical substance which is used to identify an acid or base by change of colour is called an indicator.

6. Blue litmus turns red in acids and the red litmus turns blue in a base.

7. Substances which do not bring about any change in the colour of indicators are called neutral substances.

8. The reaction between an acid and a base is known as neutralisation reaction. Salt and water are produced in this reaction.

9. A salt is a neutral substance which is formed due to the reaction of an acid with a base or vice-versa.

10. Many salts are required by our body to work 11. There are several neutralisation reactions in our everyday lives.

Exercises

 

 

MULTIPLE CHOICE QUESTIONS:

 

1. The word acid is derived from which Latin word?

a) Acere

b) Acerbus

c) Acidus

d) Acris

Answer: a) Acere

Explanation: The term “acid” comes from the Latin word acere, meaning “sour.”

 

2. Which of the following tastes sour?

a) Common salt

b) Vinegar and lemon

c) Baking powder

d) Sugar

Answer: b) Vinegar and lemon

Explanation: Vinegar, lemon, and tamarind taste sour because they contain acids.

 

3. Which compounds are bitter in taste and soapy to touch?

a) Acids

b) Bases

c) Salts

d) Sugars

Answer: b) Bases

Explanation: Bases taste bitter and feel soapy to touch, e.g., baking powder and washing soda.

 

4. Which substance is salty in taste and neither acidic nor basic?

a) Lemon

b) Common salt

c) Vinegar

d) Baking soda

Answer: b) Common salt

Explanation: Common salt does not show properties of acid or base; it is a neutral salt.

 

5. Salts are formed when—

a) Acids react with acids

b) Bases react with bases

c) Acids react with bases

d) Acids react with water

Answer: c) Acids react with bases

Explanation: Salts result from neutralization between acids and bases.

 

6. Who proposed that acids contain hydrogen ions when dissolved in water?

a) John Dalton

b) Svante Arrhenius

c) Antoine Lavoisier

d) Michael Faraday

Answer: b) Svante Arrhenius

Explanation: Arrhenius suggested acids release hydrogen ions (H⁺) in water.

 

7. Which property of acids is due to the presence of hydrogen ions?

a) Sour taste

b) Conductivity

c) Indicator reactions

d) All of the above

Answer: d) All of the above

Explanation: Acids’ properties like sour taste, conductivity, and indicator effect are due to H⁺ ions.

 

8. Acids prepared artificially from minerals are called—

a) Organic acids

b) Mineral acids

c) Natural acids

d) Weak acids

Answer: b) Mineral acids

Explanation: Mineral acids (e.g., HCl, H₂SO₄, HNO₃) are inorganic acids prepared artificially.

 

9. Acids occurring naturally in plants and animals are—

a) Mineral acids

b) Organic acids

c) Neutral acids

d) Concentrated acids

Answer: b) Organic acids

Explanation: Citric, acetic, lactic, and formic acids are natural organic acids.

 

10. Which acid is present in vinegar?

a) Citric acid

b) Lactic acid

c) Acetic acid

d) Tannic acid

Answer: c) Acetic acid

Explanation: Vinegar owes its sour taste to acetic acid.

 

11. Which acid gives lemon its sour taste?

a) Acetic acid

b) Tartaric acid

c) Citric acid

d) Lactic acid

Answer: c) Citric acid

Explanation: Lemons and other citrus fruits contain citric acid.

 

12. Which organic acid is found in bee-sting venom?

a) Citric acid

b) Acetic acid

c) Formic acid

d) Tannic acid

Answer: c) Formic acid

Explanation: Bee-sting venom contains formic acid.

 

13. Which acid is present in curd and milk?

a) Acetic acid

b) Lactic acid

c) Citric acid

d) Amino acid

Answer: b) Lactic acid

Explanation: Milk and curd contain lactic acid.

 

14. Which acid is present in proteins?

a) Tannic acid

b) Amino acid

c) Acetic acid

d) Citric acid

Answer: b) Amino acid

Explanation: Proteins are made up of amino acids.

 

15. Which acid is present in apples?

a) Acetic acid

b) Maleic acid

c) Tartaric acid

d) Citric acid

Answer: b) Maleic acid

Explanation: Apples contain maleic acid.

 

16. Tamarind and grapes are sour because they contain—

a) Lactic acid

b) Tartaric acid

c) Carbonic acid

d) Amino acid

Answer: b) Tartaric acid

Explanation: Tamarind and grapes are rich in tartaric acid.

 

17. Which acid is present in tea?

a) Acetic acid

b) Tannic acid

c) Carbonic acid

d) Formic acid

Answer: b) Tannic acid

Explanation: Tea contains tannic acid.

 

18. Cold drinks are sour due to—

a) Citric acid

b) Acetic acid

c) Carbonic acid

d) Tartaric acid

Answer: c) Carbonic acid

Explanation: Carbonated drinks contain carbonic acid.

 

19. Which of the following are strong acids?

a) Acetic acid and formic acid

b) Hydrochloric acid and sulphuric acid

c) Carbonic acid and citric acid

d) Lactic acid and tartaric acid

Answer: b) Hydrochloric acid and sulphuric acid

Explanation: Strong acids dissociate completely in water, e.g., HCl, H₂SO₄.

 

20. Which of the following is a weak acid?

a) Nitric acid

b) Hydrochloric acid

c) Acetic acid

d) Sulphuric acid

Answer: c) Acetic acid

Explanation: Acetic acid dissociates partially in water, so it is weak.

 

21. What are concentrated acids?

a) Acids with less water

b) Acids with more water

c) Acids with neutral salts

d) Acids with no ions

Answer: a) Acids with less water

Explanation: Concentrated acids have less water and high acidity.

 

22. What are dilute acids?

a) Acids mixed with bases

b) Acids with more water

c) Acids with salts

d) Acids without H⁺ ions

Answer: b) Acids with more water

Explanation: Dilution reduces acidity by increasing water content.

 

23. How should an acid be diluted safely?

a) Add water to acid

b) Add acid to water slowly

c) Mix both quickly

d) Heat acid and add water

Answer: b) Add acid to water slowly

Explanation: Adding acid to water prevents violent splashing.

 

24. Which property is NOT shown by acids?

a) Sour taste

b) Turn blue litmus red

c) Turn methyl orange pink

d) Turn phenolphthalein pink

Answer: d) Turn phenolphthalein pink

Explanation: Acids do not affect phenolphthalein.

 

25. Which statement is true for acids?

a) They are bitter in taste

b) They are colourless

c) They are poor conductors

d) They turn red litmus blue

Answer: b) They are colourless

Explanation: Most acids are colourless liquids.

 

26. Strong acids must be handled carefully because—

a) They are volatile

b) They are corrosive

c) They are tasteless

d) They are neutral

Answer: b) They are corrosive

Explanation: Strong acids can damage skin, wood, cloth, and metals.

 

27. Which acid is mainly used in fire extinguishers?

a) Acetic acid

b) Hydrochloric acid

c) Sulphuric acid

d) Nitric acid

Answer: c) Sulphuric acid

Explanation: Sulphuric acid reacts with sodium carbonate to release CO₂ in extinguishers.

 

28. In fire extinguishers, sulphuric acid reacts with—

a) Baking soda

b) Sodium carbonate

c) Sodium hydroxide

d) Potassium chloride

Answer: b) Sodium carbonate

Explanation: Reaction of sulphuric acid with sodium carbonate releases CO₂ foam.

 

29. Why does carbon dioxide extinguish fire?

a) It burns oxygen

b) It cuts off air supply

c) It produces heat

d) It is flammable

Answer: b) It cuts off air supply

Explanation: Being heavier than air, CO₂ settles on fire and blocks oxygen.

 

30. Acidity in humans is caused by excess secretion of—

a) Nitric acid

b) Hydrochloric acid

c) Sulphuric acid

d) Carbonic acid

Answer: b) Hydrochloric acid

Explanation: Excess HCl in the stomach leads to acidity and ulcers.

 

31. Which of the following reduces acidity?

a) Hot water

b) Antacid tablets

c) Vinegar

d) More HCl

Answer: b) Antacid tablets

Explanation: Antacids are bases that neutralize excess acid.

 

32. Which everyday drink reduces acidity naturally?

a) Lemon juice

b) Chilled milk

c) Soda

d) Tea

Answer: b) Chilled milk

Explanation: Chilled milk neutralizes excess acid in the stomach.

 

33. The most important industrial acid is—

a) Hydrochloric acid

b) Nitric acid

c) Sulphuric acid

d) Acetic acid

Answer: c) Sulphuric acid

Explanation: Sulphuric acid is called the “King of Chemicals” due to wide use.

 

34. Which acid is used in car batteries?

a) Acetic acid

b) Citric acid

c) Sulphuric acid

d) Hydrochloric acid

Answer: c) Sulphuric acid

Explanation: Lead-acid batteries use sulphuric acid.

 

35. Which acid is used in making vinegar?

a) Citric acid

b) Acetic acid

c) Lactic acid

d) Tartaric acid

Answer: b) Acetic acid

Explanation: Vinegar is a dilute solution of acetic acid.

 

36. Which industry uses acids for dyeing?

a) Textile, paper, leather

b) Steel

c) Cement

d) Dairy

Answer: a) Textile, paper, leather

Explanation: These industries use acids extensively for dyeing.

 

37. What is descaling?

a) Making salts from acids

b) Removing salt deposits from boilers using acids

c) Neutralizing bases

d) Purifying water

Answer: b) Removing salt deposits from boilers using acids

Explanation: Acids dissolve scales in steam boilers.

 

38. Acids are used in the purification of which metals?

a) Aluminium and iron

b) Silver and gold

c) Copper and zinc

d) Tin and lead

Answer: b) Silver and gold

Explanation: Acids are essential for purification of noble metals.

 

39. Which acid is used in baking powder preparation?

a) Citric acid

b) Acetic acid

c) Tartaric acid

d) Carbonic acid

Answer: b) Acetic acid

Explanation: Acetic acid is used in preparing baking powder.

 

40. Which property of acids is used in medicines like antacids?

a) Sour taste

b) Neutralization with bases

c) Conductivity

d) Corrosiveness

Answer: b) Neutralization with bases

Explanation: Antacids neutralize excess HCl in stomach.

 

41. Which of these acids is colourless and corrosive?

a) Vinegar

b) Sulphuric acid

c) Tannic acid

d) Citric acid

Answer: b) Sulphuric acid

Explanation: Concentrated sulphuric acid is colourless and highly corrosive.

 

42. Why are strong acids stored in glass containers?

a) Glass is transparent

b) Glass does not react with acids

c) Acids evaporate in plastic

d) Glass is cheaper

Answer: b) Glass does not react with acids

Explanation: Strong acids corrode metals and other materials but not glass.

 

43. What is the effect of acids on blue litmus?

a) No change

b) Turns blue darker

c) Turns red

d) Turns pink

Answer: c) Turns red

Explanation: Acids change blue litmus to red.

 

44. What is the effect of acids on methyl orange?

a) Yellow to red

b) Yellow to pink

c) Red to yellow

d) No change

Answer: b) Yellow to pink

Explanation: Methyl orange turns pink in acidic solutions.

 

 

45. What is the effect of acids on phenolphthalein?

a) Red

b) Blue

c) No effect

d) Pink

Answer: c) No effect

Explanation: Phenolphthalein remains colourless in acidic solutions.

 

46. Which property makes acids good conductors in solution?

a) Presence of water

b) Hydrogen ions (H⁺)

c) Oxygen ions

d) Sour taste

Answer: b) Hydrogen ions (H⁺)

Explanation: Acids dissociate into H⁺ ions, conducting electricity.

 

47. Which acid causes ulcers when secreted excessively?

a) Citric acid

b) Hydrochloric acid

c) Acetic acid

d) Lactic acid

Answer: b) Hydrochloric acid

Explanation: Excess HCl damages stomach lining, leading to ulcers.

 

48. Which acid is used in fertilizer manufacturing?

a) Nitric acid

b) Sulphuric acid

c) Hydrochloric acid

d) Carbonic acid

Answer: b) Sulphuric acid

Explanation: H₂SO₄ is used in producing superphosphate fertilizers.

 

49. Which acid is used in making plastics and detergents?

a) Acetic acid

b) Hydrochloric acid

c) Sulphuric acid

d) Citric acid

Answer: c) Sulphuric acid

Explanation: Sulphuric acid is widely used in industries for plastics and detergents.

 

50. Why are acids important in food preservation?

a) They neutralize bases

b) They add colour

c) They prevent microbial growth

d) They produce electricity

Answer: c) They prevent microbial growth

Explanation: Acetic acid (vinegar) is used in pickles and food preservation.

 

 

51. Bases are compounds that give which ions in water?

a) Hydrogen ions (H⁺)

b) Hydroxide ions (OH⁻)

c) Oxygen ions (O²⁻)

d) Carbonate ions (CO₃²⁻)

Answer: b) Hydroxide ions (OH⁻)

Explanation: Bases dissociate in water to give hydroxide (OH⁻) ions.

 

52. Which of the following is a strong base?

a) Sodium hydroxide

b) Zinc hydroxide

c) Copper hydroxide

d) Magnesium hydroxide

Answer: a) Sodium hydroxide

Explanation: Strong bases like NaOH and KOH dissociate completely in water.

 

53. Which of these is a weak base?

a) Sodium hydroxide

b) Potassium hydroxide

c) Magnesium hydroxide

d) Calcium hydroxide

Answer: c) Magnesium hydroxide

Explanation: Weak bases do not dissociate fully in water, e.g., Mg(OH)₂.

 

54. Quick lime is chemically known as—

a) Calcium hydroxide

b) Calcium oxide

c) Sodium carbonate

d) Potassium hydroxide

Answer: b) Calcium oxide

Explanation: Quick lime = CaO, used for neutralising acidic soils.

 

55. Slaked lime is—

a) Calcium oxide

b) Calcium carbonate

c) Calcium hydroxide

d) Sodium hydroxide

Answer: c) Calcium hydroxide

Explanation: Ca(OH)₂ is slaked lime, also called lime water.

 

56. Caustic soda is—

a) Na₂CO₃

b) NaOH

c) KOH

d) Mg(OH)₂

Answer: b) NaOH

Explanation: Sodium hydroxide is called caustic soda.

 

57. Washing soda’s chemical formula is—

a) Na₂CO₃

b) NaHCO₃

c) CaCO₃

d) NaOH

Answer: a) Na₂CO₃

Explanation: Washing soda = sodium carbonate (Na₂CO₃).

 

58. Which base is used as an antacid?

a) Sodium hydroxide

b) Magnesium hydroxide

c) Calcium hydroxide

d) Potassium hydroxide

Answer: b) Magnesium hydroxide

Explanation: Mg(OH)₂, also called milk of magnesia, neutralises stomach acid.

 

59. Which base is called caustic potash?

a) NaOH

b) KOH

c) Ca(OH)₂

d) Mg(OH)₂

Answer: b) KOH

Explanation: Potassium hydroxide = caustic potash.

 

60. Which of the following bases is used in whitewash?

a) NaOH

b) Ca(OH)₂

c) Mg(OH)₂

d) KOH

Answer: b) Ca(OH)₂

Explanation: Calcium hydroxide is used in whitewashing walls.

 

61. Which of these is NOT a property of bases?

a) Bitter taste

b) Slippery touch

c) Turn blue litmus red

d) Conduct electricity

Answer: c) Turn blue litmus red

Explanation: Bases turn red litmus blue, not the other way round.

 

62. Which indicator turns pink in bases?

a) Litmus

b) Phenolphthalein

c) Methyl orange

d) China rose

Answer: b) Phenolphthalein

Explanation: Phenolphthalein turns pink in alkaline solutions.

 

63. What colour does methyl orange give in a base?

a) Pink

b) Yellow

c) Red

d) Blue

Answer: b) Yellow

Explanation: Bases turn methyl orange yellow.

 

64. What are water-soluble bases called?

a) Alkaline bases

b) Alkalis

c) Neutral bases

d) Salts

Answer: b) Alkalis

Explanation: Water-soluble bases are alkalis, e.g., NaOH, Ca(OH)₂.

 

65. Which statement is correct?

a) All bases are alkalis

b) All alkalis are bases

c) Bases and alkalis are same

d) None

Answer: b) All alkalis are bases

Explanation: All alkalis are bases, but not all bases are water-soluble.

 

66. Soil that is too acidic is neutralised with—

a) Sodium hydroxide

b) Calcium oxide

c) Vinegar

d) Baking soda

Answer: b) Calcium oxide

Explanation: Quicklime (CaO) neutralises acidic soil.

 

67. Bleaching powder is prepared using—

a) Sodium hydroxide

b) Calcium hydroxide

c) Potassium hydroxide

d) Magnesium hydroxide

Answer: b) Calcium hydroxide

Explanation: Ca(OH)₂ is used to make bleaching powder.

 

68. Bases are used in manufacturing—

a) Soap

b) Medicines

c) Fibres

d) All of the above

Answer: d) All of the above

Explanation: Bases are used for soap, medicines, fibres, plaster, mortar, etc.

 

69. Which natural indicator is obtained from lichens?

a) China rose

b) Litmus

c) Turmeric

d) Red cabbage

Answer: b) Litmus

Explanation: Litmus is a purple dye from lichens.

 

70. Litmus turns red in acids and ____ in bases.

a) Pink

b) Blue

c) Yellow

d) Green

Answer: b) Blue

Explanation: Bases turn red litmus paper to blue.

 

71. Neutral substances show—

a) Red in acids

b) Blue in bases

c) No colour change in indicators

d) Sour taste

Answer: c) No colour change in indicators

Explanation: Neutral substances do not affect litmus or other indicators.

 

72. Which of these is a neutral solid?

a) Common salt

b) NaOH

c) Lemon juice

d) Baking soda

Answer: a) Common salt

Explanation: Substances like NaCl, sugar, glucose are neutral solids.

 

73. Which of these is a neutral liquid?

a) Vinegar

b) Water

c) Lemon juice

d) NaOH solution

Answer: b) Water

Explanation: Pure water is neutral.

 

74. Which of these is a neutral gas?

a) Oxygen

b) Carbon dioxide

c) Chlorine

d) Ammonia

Answer: a) Oxygen

Explanation: Neutral gases include oxygen, nitrogen, hydrogen.

 

75. A salt is formed when—

a) Two acids react

b) Two bases react

c) Acid reacts with base

d) Salt reacts with water

Answer: c) Acid reacts with base

Explanation: Salt is formed during neutralisation.

 

76. Neutralisation produces—

a) Salt only

b) Water only

c) Salt + water

d) Acid + base

Answer: c) Salt + water

Explanation: Neutralisation: acid + base → salt + water.

 

77. Neutralisation is—

a) Endothermic

b) Exothermic

c) Neutral

d) Neither

Answer: b) Exothermic

Explanation: Heat is released during neutralisation.

 

78. Which property is NOT true of salts?

a) Mostly soluble in water

b) Conduct electricity

c) Hygroscopic

d) All salts are sour

Answer: d) All salts are sour

Explanation: Salts are neutral, not sour.

 

79. Potash alum (phitkari) is used in—

a) Food preservation

b) Water purification

c) Photography

d) Cement

Answer: b) Water purification

Explanation: Alum is used in purifying drinking water.

 

80. Which salt is used in POP (Plaster of Paris)?

a) CaSO₄·½H₂O

b) NaCl

c) Na₂CO₃

d) K₂SO₄

Answer: a) CaSO₄·½H₂O

Explanation: Plaster of Paris is calcium sulphate hemihydrate.

 

81. Which salt is essential in our diet?

a) Potash alum

b) Common salt

c) POP

d) Silver salts

Answer: b) Common salt

Explanation: NaCl is essential for body functioning.

 

82. Silver salts are used in—

a) Fire extinguishers

b) Photography

c) Glass making

d) Cement

Answer: b) Photography

Explanation: Silver salts are used to develop photographic films.

 

83. Washing soda in fire extinguishers produces—

a) Oxygen

b) Hydrogen

c) Carbon dioxide

d) Nitrogen

Answer: c) Carbon dioxide

Explanation: Na₂CO₃ reacts with acid to release CO₂, extinguishing fire.

 

84. Indigestion is caused by—

a) Excess base in stomach

b) Excess acid in stomach

c) Salt imbalance

d) Lack of acid

Answer: b) Excess acid in stomach

Explanation: Too much HCl secretion causes acidity.

 

85. Which base acts as an antacid for acidity?

a) NaOH

b) Mg(OH)₂

c) KOH

d) Ca(OH)₂

Answer: b) Mg(OH)₂

Explanation: Milk of magnesia neutralises stomach acid.

 

86. Toothpaste contains—

a) Acids

b) Bases

c) Salts

d) Sugars

Answer: b) Bases

Explanation: Bases in toothpaste neutralise acid and prevent decay.

 

87. Which neutralises acidic soil?

a) Vinegar

b) Lime

c) Sugar

d) Ammonia

Answer: b) Lime

Explanation: Lime (CaO or Ca(OH)₂) is added to neutralise acidic soil.

 

88. Bee sting injects—

a) Acid

b) Base

c) Salt

d) Neutral liquid

Answer: a) Acid

Explanation: Bee stings inject formic acid.

 

89. Bee sting can be neutralised by—

a) Vinegar

b) Moist baking soda

c) Lemon juice

d) Acid solution

Answer: b) Moist baking soda

Explanation: Bases neutralise acidic sting.

 

90. Wasp sting is—

a) Acidic

b) Basic

c) Neutral

d) Salty

Answer: b) Basic

Explanation: Wasp stings inject alkali.

 

91. Wasp sting can be neutralised by—

a) Vinegar

b) Baking soda

c) Salt solution

d) Mg(OH)₂

Answer: a) Vinegar

Explanation: Acid (vinegar) neutralises basic sting.

 

92. Which of the following is hygroscopic?

a) Salts

b) Acids

c) Bases

d) Neutral substances

Answer: a) Salts

Explanation: Many salts absorb moisture from air.

 

93. Pure rainwater is slightly acidic due to—

a) Nitric acid

b) Sulphuric acid

c) Carbonic acid

d) Hydrochloric acid

Answer: c) Carbonic acid

Explanation: CO₂ dissolves in water to form weak carbonic acid.

 

94. Which neutral substance is a liquid?

a) Sugar

b) Glucose

c) Water

d) Oxygen

Answer: c) Water

Explanation: Water is a neutral liquid.

 

95. Which salt is used for making cement and glass?

a) NaCl

b) CaSO₄

c) Na₂CO₃

d) K₂SO₄

Answer: c) Na₂CO₃

Explanation: Washing soda is used in cement and glass industry.

 

96. Neutralisation in everyday life includes—

a) Tooth decay prevention

b) Soil treatment

c) Insect sting treatment

d) All of these

Answer: d) All of these

Explanation: Neutralisation is widely applied in daily life.

 

97. Which solution feels soapy to touch?

a) Baking soda solution

b) Vinegar

c) Lemon juice

d) Sugar water

Answer: a) Baking soda solution

Explanation: Baking soda solution is basic and feels slippery.

 

98. Which salts are required by our body?

a) Chlorides

b) Sulphates

c) Bicarbonates & phosphates

d) All of the above

Answer: d) All of the above

Explanation: The body needs various salts like chloride, sulphate, phosphate, etc.

 

99. Which reaction is exothermic?

a) Acid with acid

b) Base with base

c) Neutralisation

d) Salt with water

Answer: c) Neutralisation

Explanation: Heat is released in neutralisation.

 

100. Which acid-base indicator is made from lichens?

a) Phenolphthalein

b) Methyl orange

c) Litmus

d) China rose

Answer: c) Litmus

Explanation: Litmus is extracted from lichens.