Introduction to Acid and Base | Class 8 | Science | Chapter 12 | Maharashtra State Board
Explore the fascinating world of acids and bases in this lesson. Understand their properties, uses, reactions, and indicators with practical examples. Learn how acids and bases interact in neutralization reactions, the importance of natural and laboratory indicators, and their applications in everyday life.
Questions & Answers
1. Identify the following solutions, whether they are acid or base.
Solution | Change in Indicator Litmus | Change in Indicator Phenolpht-halein | Change in Indicator Methyl orange | Acid/Base |
1. | -- | No change | -- | -- |
2. | -- | -- | Orange colour turns red | -- |
3. | Red litmus turns blue | -- | -- | -- |
Ans.
Solution | Change in Indicator Litmus | Change in Indicator Phenolpht-halein | Change in Indicator Methyl orange | Acid/Base |
1. | Blue litmus turns red | No change | The orange colour turns red | Acid |
2. | Blue litmus turns red | No change | Orange colour turns red | Acid |
3. | Red litmus turns blue | Colourless to pink | The orange turns yellow | Base |
2. Write chemical names from given formulae.
H2SO4 , Ca(OH)2 , HCl, NaOH, KOH, NH4OH
Ans.
H2SO4 = Sulphuric acid
Ca(OH)2 = Calcium hydroxide
HCl = Hydrochloric acid
NaOH = Sodium hydroxide
KOH = Potassium hydroxide
NH4OH = Ammonium hydroxide
3. Sulphuric acid has highest importance in chemical Industry. Why?
Ans.
Sulphuric acid has highest importance in the chemical industry because it is used in the manufacturing of fertilizers like ammonium sulphate and superphosphate of lime. It is used in the manufacturing of hydrochloric acid, nitric acid, phosphoric acid, ether, plastics and metal sulphates. It is also used in the manufacturing of dyes, drugs, perfumes, disinfectants and glue.
4. Give answers.
a. Which acid is used for getting chloride salt?
Ans.
The acid used for getting chloride salt is HCl because this acid provides chloride ion to form chloride salt.
HCL + NaOH → NaCL + H2O
HCl + Mg(OH)2 → MgCl2 +H2O
b. By squizzing lemon on a piece of rock the gas liberated turned lime water milky. which compound is present in the rock?
Ans.
Compound presents in the rock is either a compound of corbonate or bicarbonate because acids react with carbonate or bicarbonate and liberate CO2. This carbon dioxide turns lime water milky.
c. The label on the bottle of chemical is spoiled. How will you find whether the chemical is acidic or not?
Ans.
1. By use of litmus paper.
if the given solution is acidic, it turns blue litmus to red colour.
If the given solution is basic, it turns red litmus to blue colour.
if the given solution is neutral, there is no effect on red and blue litmus paper.
5. Answer the following questions.
a. Explain the difference between acid and base.
Ans.
Acids | Bases | |
1. | These are sour to taste. | These are bitter to taste. |
2. | Acids are proton donors or electron acceptors. | Bases are proton acceptors or electron donors. |
3. | Acids have a pH value of <7. | Bases have a pH value >7. |
4. | Acid turn blue litmus paper to red. | Bases do not change the colour of blue litmus paper. |
5. | Acids do not change the colour of red litmus paper. | Bases turn red litmus paper to blue. |
6. | With china rose indicator, these give a dark pink colour. | Bases give green colour with china rose indicator. |
7. | Acids do not change the colour of the turmeric indicator. | Bases turn the colour of the termeric indicator to red. |
8. | Acid is a substance that contains hydrogen ion (H+). | Bases are substances that contain hydroxyl ion (OH-). |
9. | e.g. HCl, H2SO4, H3PO4 | e.g. NaOH, KOH, Ca(OH)2 |
b. Why indicator does not get affected by salt?
Ans.
Indicators do not get affected by salts because salts are neutral in nature. Indicators work only with acidic and basic substances.
For example:
Indicator | Acidic substance | Basic substance |
Phenophthalein | Colourless | Turns pink |
Methyl orange | Methyl orange turns red | Methyl orange turns yellow |
c. Which substances are produced by neutralization process ?
Ans.
Salt and water are the two substances produced by the neutralization process. For exmple, when hydrochloric acid reacts with sodium hydroxide, sodium chloride (salt) and water are produced.
d. Which are the industrial uses of acids?
Ans.
Acids which are used in the industry are:
Sulphuric acid
Hydrochloric acid
Nitric acid.
1. Sulphuric acid: Sulphuric acid is used
(a) in the manufacture of chemical fertilizers like ammonium sulphate
(b) for cleaning gold and silver articles and
(c) in car batteries.
2. Hydrochloric acid: Hydrochloric acid is used
(a) to clean toilets and
(b) to obtain glucose from starch and also for producing gelatine.
3. Nitric acid: Nitric acid is used
(a) in the manufacture of perfumes
(b) for engraving on copper, brass or silver
(c) in the manufacture of paints and explosives.
6. Select proper word given in bracket and fill in the blanks.
a. Main constituent of acid is ……………. .
Ans.
Main constituent of acid is H+ ion.
b. Main constituent of alkali is …………….. .
Ans.
Main constituent of alkali is OH– ion.
c. Tartaric acid is a …………… acid.
Ans.
Tartaric acid is a weak acid.
7. Match the pairs.
Group 'A' | Group 'B' | |
1. | Tamrind | a. Acetic acid |
2. | Curd | b. Citric acid |
3. | Lemon | c. Tartatic acid |
4. | Vinegar | d. Lactic acid |
Ans.
Group 'A' | Group 'B' | |
1. | Tamrind | c. Tartatic acid |
2. | Curd | d. Lactic acid |
3. | Lemon | b. Citric acid |
4. | Vinegar | a. Acetic acid |
8. State true or false.
a. Oxides of metals are alkaline in nature.
Ans.
True.
b. Salt is acidic.
Ans.
False. (Salt is neutral)
c. Metal corrodes due to salts.
Ans.
False. (Acids and bases corrode metals)
d. Salts are neutral.
Ans.
True. (Not all salts neutral)
9. Classify the following substances into acidic, basic and neutral group:
HCl, NaCl, MgO, KCl, CaO, H2SO4, HNO3, H2O and Na2CO3.
Ans.
Group | Substances |
Acid | HCl, H2SO4, HNO3 |
Base | CaO, MgO, Na2CO3 |
Neutral | H2O, NaCl, KCl |
E X T R A
1. Fill in the blanks:
1. Acid reacts with metal to form …………. gas.
Ans.
Acid reacts with metal to form hydrogen gas.
2. DNA is an acid present in our body, it decides …………. properties.
Ans.
DNA is an acid present in our body, it decides heredity properties.
3. The chemical formula of milk of magnesia is ………….
Ans.
The chemical formula of milk of magnesia is Mg (OH)2.
4. …………. are used to control hyperacidity.
Ans.
Antacids are used to control hyperacidity.
5. Alkali has …………. taste.
Ans.
Alkali has bitter taste.
6. Proteins are made up of ………….
Ans.
Proteins are made up of amino acids.
7. …………. is used in batteries.
Ans.
Dil. H2SO4 is used in batteries.
2. Rewrite the following statements by selecting the correct options:
1. The colour of phenolphthalein indicator in alkaline solution is ………….
(a) yellow
(b) green
(c) orange
(d) pink
Ans.
The colour of phenolphthalein indicator in alkaline solution is pink.
2. …………. is sour to taste.
(a) An acid
(b) An alkali
(c) A salt
(d) Alcohol
Ans.
An acid is sour to taste.
3. When phenolphthalein is added to NaOH, the colour of the solution will become ……………… .
(a) colourless
(b) red
(c) pink
(d) yellow
Ans.
When phenolphthalein is added to 5 NaOH, the colour of the solution will become pink.
4. When phenolphthalein is added to HCl, the colour of the solution will be ……….. .
(a) red
(b) pink
(c) green
(d) colourless
Ans.
When phenolphthalein is added to HCl, the colour of the solution will be colourless.
5. …………. is a natural indicator.
(a) Phenolphthalein
(b) Methyl orange
(c) Litmus
(d) Methyl red
Ans.
Litmus is a natural indicator.
6. The litmus paper or the litmus solution is obtained from …………. plants.
(a) moss
(b) rose
(c) hibiscus
(d) lichen
Ans.
The litmus paper or the litmus solution is obtained from lichen plants.
7. …………. is not an alkali.
(a) Sodium hydroxide
(b) Potassium hydroxide
(c) Copper hydroxide
(d) Calcium hydroxide
Ans.
Copper hydroxide is not an alkali.
8. …………. is a weak acid.
(a) Hydrochloric acid
(b) Nitric acid
(c) Carbonic acid
(d) Sulphuric acid
Ans.
Carbonic acid is a weak acid.
9. When methyl orange is added to HCl, the colour of the solution will be …………… .
(a) red
(b) pink
(c) yellow
(d) colourless
Ans.
When methyl orange is added to HCl, the colour of the solution will be pink.
10. When blue litmus paper is added to NaOH, the colour of the litmus paper will be ……………. .
(a) blue
(b) red
(c) pink
(d) yellow
Ans.
When blue litmus paper is added to NaOH, the colour of the litmus paper will be blue.
3. State whether the following statements are true or false. If a statement is false, correct it and rewrite:
1. Red cabbage is a natural indicator.
Ans.
True
2. Fats of our body are formed by fatty acids.
Ans.
True.
3. Ammonium hydroxide is used in the production of fertilizers.
Ans.
True.
4. Lime water is a weak acid.
Ans.
False. (Limewater is a weak base)
5. Orange colour of methyl orange turns yellow in acid.
Ans.
False. (Orange colour of methyl orange turns pink in acid)
6. Methyl red turns yellow in alkali.
Ans.
True.
7. Oxalic acid is used in aerated cold drinks.
Ans.
False. (Carbonic acid is used in aerated cold drinks).
4. Find the odd one out and justify:
1. Acetic acid, carbonic acid, hydrochloric acid, nitric acid.
Ans.
Acetic acid. (Others are mineral acids.)
2. Hydrogen chloride, sodium hydroxide, calcium oxide, ammonia.
Ans.
Hydrogen chloride. (Others are bases.)
3. HCl, CH3COOH, H2SO4, HNO3
Ans.
CH3COOH. (Others are strong acids.)
4. NaOH, Ca(OH)2, NH4OH, Ba(OH)2
Ans.
NaOH (Others are weak bases.)
5. H2SO4, H2CO3, HCl, HNO3
Ans.
H2CO3 (Others are strong acids.)
6. Citric acid, formic acid, lactic acid, nitric acid.
Ans.
Nitric acid. (Others are organic acids.)
7. Lime, litmus, phenolphthalein, methyl orange.
Ans.
Lime. (Others are indicators.)
5. Define the following:
1. Acid:
A substance which gives H+ ions in water is called an acid.
2. Alkali:
A substance which gives OH– ions in water is called an alkali.
3. Indicator:
Substances which change their colours in presence of acid or base are called indicators.
4. Neutralization:
The reaction between an acid and an alkali to form the r salt and water is termed as neutralization.
6. Answer the following questions in one sentence each:
1. What are natural acids?
Ans.
Acids present in foodstuffs are called natural acids.
2. State three vegetables from which natural indicators can be prepared.
Ans.
Natural indicators: Red cabbage, radish, tomato.
3. State names of any three strong acids.
Ans.
Hydrochloric acid
Sulphuric acid
Nitric acid.
4. State the names of the acids found in lemon juice, Amla, tamarind water and curd.
Ans.
Lemon juice – Citric acid
Amla – Ascorbic acid
Tamarind water – Tartaric acid
Curd – Lactic acid.
5. State names of any three alkalis.
Ans.
Sodium hydroxide
Potassium hydroxide
Calcium hydroxide.
6. State the names of three indicators.
Ans.
Litmus paper
Methyl orange
Phenolphthalein.
7. State the names of three weak alkalis.
Ans.
Calcium hydroxide
Ammonium hydroxide
Magnesium hydroxide.
8. From which plant is litmus obtained?
Ans.
Litmus is obtained from plants called lichens.
9. Name the antacid which is used to control the hyper acidity.
Ans.
Milk of magnesia [Mg(OH)2] is used to control the hyper acidity.
10. How will you neutralize the excess of acid present in the soil?
Ans.
Lime stone or lime water is mixed in the soil to neutralize the excess of acid.
11. When lemon juice falls on a marble kitchen counter, which is the gas that bubbled out?
Ans.
Carbon dioxide.
7. Answer the following questions:
1. What are indicators?
Ans.
Substances which change their colours in presence of acid or base are called indicators.
2. State two acid-base indicators and mention their colour change.
Ans.
Phenolphthalein and methyl orange are two acid-base indicators. Phenolphthalein is colourless in an acidic solution while it turns pink in a basic solution. Methyl orange gives orange colour with an acidic solution and yellow colour with a basic solution.
8. State the colour change for each of the following solutions with red litmus, blue litmus, phenolphthalein and methyl orange solutions.
Ans.
Solution | Red litmus | Blue litmus | Phenolph-thalein | Methyl Orange | |
1. | Sulphuric acid (H2SO4) | No change | Turns red | Colourless | Orange |
2. | Calcium hydroxide [Ca(OH)2] | Turns blue | No change | Turns pink | Yellow |
3. | Acetic acid (CH3COOH) | No change | Turns red | Clourless | Orange |
4. | Sodium hydroxide (NaOH) | Turns blue | No change | Turns pink | Yellow |
5. | Magnesium hydroxide [Mg(OH)2] | Turns blue | No change | Turns pink | Yellow |
9. Name the acid present in
(1) Orange
(2) Vinegar
(3) Tamarind
(4) Curd
(5) Tomato
(6) Lemon.
Ans.
Substance | Acid |
Orange | Citric acid |
Vinegar | Acetic acid |
Tamrind | Tartaric acid |
Curd | Lactic acid |
Tomato | Oxalic acid |
Lemon | Citric acid |
10. Classify the following into acidic substances, basic substances and salts.
(1) Lime water, (2) soda water, (3) soap water, (4) sea water, (5) sweet limeuice, (6) sugar caneuice, (7) buttermilk, (8) a mixture of ash in water, (9) tamarind water.
Ans.
a. Acidic substances:
(1) Soda water, (2) sweet limeuice, (3) buttermilk, (4) tamarind water.
b. Basic substances:
(1) Lime water, (2) Soap water, (3) a mixture of ash in water.
c. Salts:
(1) Sea water, (2) sugar caneuice.
11. Answer the following.
1. What happens when sour substances like limeuice, tamarind water falls on shahabad stones or kitchen platform? Why?
Ans.
When sour substances like limeuice, tamarind water falls on shahabad stones or kitchen platform, citric acid present in limeuice and tartaric acid in tamarind water reacts with metal carbonates present in shahabad stones or kitchen platform, carbon dioxide gas is evolved resulting in the formation of uneven surface.
2. Collect soil samples from your surroundings and find out whether it is acidic or alkaline or neutral?
Ans.
Soil samples collected from the surroundings are acidic in nature.
3. Which substances are used to clean greenish stains on copper vessels and to shine blackish silver utensils?
Ans.
Tamarind pulp is used to clean greenish stains on copper vessels. To shine blackish silver utensils, ammoniacal solution or limeuice or detergent is used.
4. Why toothpaste is used for brushing teeth?
Ans.
Toothpaste contains fluorides and alkali to neutralize the mouth acid.
Hence, the tooth paste, which is generally alkaline, is used for cleaning the teeth as it can neutralize the excess acid in i the mouth and prevent tooth decay.
5. State the properties of acids.
Ans.
Acids are sour in taste.
Acid molecules contain hydrogen ion (H+) as a main constituent.
Acid reacts with metal to form hydrogen gas.
Acid reacts with carbonates and liberates CO2 gas.
Blue litmus turns red in acid.
6. State the uses of acids.
Ans.
Acids are used in the production s of chemical fertilizers.
Acids are used in the production of explosives, oil purification, medicines, dyes and paints.
Hydrochloric acid is used for the preparation of different types of chloride salts.
Dil. H2SO4 acid is used in the batteries (electric cell).
Dil. HCl is used for sterilization of water.
Acid is used for making of white paper from wood pulp.
7. The iron knife shines better after cutting the sour fruits like lemon, raw mangoes. Why?
Ans.
Acids present in sour fruits clean and dissolves the salts present on the surface of iron knife as a result iron knife shines better after cutting the sour fruits.
8. What is meant by neutralization? Give example.
Ans.
When an acid reacts with an alkali to form a salt and water, it is called neutralization.
12. Give scientific reasons:
1. When we have stomachache, doctors prescribe medicine containing magnesium hydroxide.
Ans.
There is hydrochloric acid in our stomach which helps in the digestion of food.
Sometimes in the stomach, there is more hydrochloric acid than we need.
This excess acid causes acidity and indigestion resulting in stomachache.
When magnesium hydroxide is administered it neutralizes the extra acid in the stomach.
Therefore, magnesium hydroxide is the best cure for stomachache.
So, when we have stomachache, doctors prescribe medicine containing magnesium hydroxide.
2. Quicklime or slaked lime is added to the soil.
Ans.
When chemical fertilizers are added to the soil, the soil becomes acidic.
To reduce its acidity quicklime or slaked lime is added to the soil.
3. Tarnished and stained copper vessels are brought to a shine by using tamarind pulp.
Ans.
In due course, unused copper vesselsare covered with a coating of black copper oxide and other compounds of copper which tarnish the vessel. Tamarind contains tartaric acid. When the vessel is rubbed with tamarind pulp, tartaric acid reacts with copper oxide and removes the layer. Thus, the copper vessel regains its shine.
4. Copper and brass utensils are tinned.
Ans.
Copper and brass reacts with i organic acids present in food materials and poisonous salts are formed. Tin does not react with organic acids present in food materials. Therefore, copper or brass vessels are tinned.
13. Distinguish between acid and alkali. OR Distinguish between the following:
Acid and Alkali:
Ans.
Acid | Alkali | |
1. | Acid has sour taste. | Alkali has bitter taste. |
2. | Acid turns blue litmus red. | Alkali turns red litmus blue. |
3. | An acid is an aqueous solution gives H+ ions. | An alkali in an aqueous solution gives OH+ ions. |
4. | Oxides of non-metals form acids. | Oxides of metals form bases. |
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