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UNIT No. 6 ACIDS, BASES/ALKALIS AND SALTS
Science Grade 8 Unit 6 Notes and Assessments
Q 1. Why compounds are divided into different groups?
Ans: More than three million compounds are known to scientists. These compounds are divided into different groups to make their studies easier.
Q 2. What is the meaning of acid? /Define acid. /What are the acids?
Ans: The word acid is derived from the Latin word ‘acidus’ which means sour. A group of compounds that have a sour taste is called acid.
Compounds that produce hydrogen (H+) in their aqueous solution are called acids.
Q 3. Give some examples of acids.
Ans: Hydrochloric acid, Lactic acid, Nitric acid, Sulphuric acid, Tartaric acid.
Q 4. What are organic acids?
Ans: Some acids that occur in plants and animals are known as organic acids.
For example Formic acid, Lactic acid, Tartaric acid, and Malic acid.
What are mineral acids?
Answer Q 5. Acids that are obtained from minerals are called mineral acids or inorganic acids.
For example Sulphuric acid, Nitric acid, Phosphoric acid, and Hydrochloric acid.
Q 6. What are the sources of the following acids?
7. What are the general properties of acids?
Ans: Common/general properties of all acids:
- All acids have a sour taste.
- All acids turn the blue litmus solution red.
- All acids turn methyl orange solution red.
- Strong acids burn skin and destroy animal tissues.
- Their aqueous solutions are good conductors of electricity.
- Acids react with reactive metals (Mg, Zn) to form salt and hydrogen.
- Acids react with metal carbonates to liberate CO2.viii. Acids react with bases to form salt and water.
8. What are the products, when acids react with reactive metals?
Ans: Acids react with reactive metals (Mg, Zn) to form salt and hydrogen for example,
- Mg + 2HCl → MgCl2 + H2
- Zn +H2SO4 → ZnSO4 + H2
9. Which gas is obtained from the reaction of acids and metal carbonates or metal hydrogen carbonates?
Ans: Carbon dioxide (CO2) is obtained from the reaction of acids and metal carbonates or metal hydrogen carbonates.
10. What is neutralization?
Ans: Acids react with bases to form salt and water. This process is called neutralization.
H2SO4 + 2NaOH → Na2SO4 + 2H2O
11. What is the identification test for carbon dioxide? / How can you identify carbon dioxide?
Ans: Carbon dioxide turns lime water milky. This is a test for the identification of carbon dioxide gas.
12. What are the uses of hydrochloric acid (HCl)?
Uses of hydrochloric acid
- It is used for cleaning rust from the surface of metals.
- Many people use it for the purification of common salt (NaCl).
- we use hydrochloric acid to make Aqua Regia.
- It helps to digest food in our stomach.
- It is used to make glucose from starch.
13. What are the uses of nitric acid (HNO3)?
- We use nitric acid to make ammonium nitrate fertilizer.
- We use it in the manufacture of dyes, plastic and artificial silk.
- It is used in the manufacture of explosives.
14. Write some uses of sulphuric acid (H2SO4).
The King of Chemicals
The uses of sulphuric acid are so large that it is known as the king of chemicals.
- Many countries use it in petroleum refining.
- It is used in the paper, leather and textile industries.
- Its use in lead batteries is very common.
- It is used in the manufacture of fertilizers.
- The use of sulphuric acid in paints, soaps and detergents is very common.
- It is used in artificial silk and drugs.
15. Which acid is called the king of chemicals?
Ans: Sulphuric acid (H2SO4) is called the king of chemicals.
16. Which fertilizers are made of sulphuric acid?
- Ammonium phosphate ii. Calcium superphosphate
- Calcium ammonium phosphate.
17. Write uses of acetic acid.
Ans: It is used in the preparation of pickles.
It is used in the manufacture of synthetic fibre.
18. What are the bases?
Ans: Compounds that produce hydroxide ions (OH-) in their aqueous solutions are bases. They have properties that are contrary to acids.
19. What are alkalis? What is meant by alkalis? What is its source?
Ans: The bases which are soluble in water are called alkalis. The word alkali has been taken from the Arabic word “Qali” which means “from ashes”. Alkalis are obtained from the ashes of plants.
Q 20. Give some examples of alkalis.
|Sodium hydroxide||NaOH||Potassium hydroxide||KOH|
|Calcium hydroxide||Ca(OH)2||Ammonium hydroxide||NH4OH|
Q 21. What are the general properties of alkalis or basis?
- Aqueous solutions of bases have a soapy touch.
- Bases turn red litmus blue.
- Bases turn colorless phenolphthalein pink.
- They turn methyl orange-yellow.
- They turn turmeric paper brown.
- Aqueous solutions of bases are good conductors of electricity.
- Bases react with acids and form salts and water. This reaction is called neutralization.
- Alkalis react with fats to form soap.
- Alkalis when heated with ammonium salts, it produces ammonia gas.
22. How can you identify ammonia gas?
Ans: Ammonia gas has a pungent smell. Ammonia turns moist red litmus paper blue.
23. Write some common uses of sodium hydroxide (NaOH).
Ans: Sodium hydroxide is largely used in:
- Textile industries
- Plastic Industries
- Making soap.
- Petroleum refining v. Making rayon
- Manufacture of paper pulp. Vii. Medicines
24. Write some common uses of calcium hydroxide Ca(OH)2.
Ans: Calcium hydroxide is called slaked lime. It is used:
- To make bleaching powder.
- As a dressing material for acid burns.
- As a water softener.
- For neutralizing acidity present in the soil.
- In making lime sulphur sprays are used as fungicides.
25. What are the uses of ammonium hydroxide (NH4OH)?
Ammonium hydroxide is used:
- To remove grease from window panes.
- To remove ink spots from clothes.
- As a reagent in laboratories.
- For the treatment of bees’ stings.
26. Is soda water acidic or basic?
Ans: Soda water is acidic due to carbon dioxide.
27. Which alkali is commonly used to open a drain?
Ans: Sodium hydro oxide (NaOH) is commonly used to open a drain.
28. What are salts?
Ans: Salt is an ionic compound formed by the neutralization of an acid and a base.
29. Write down the chemical equation showing the reaction of ammonia and water.
NH3 + H2O → NH4OH
30. What are the properties of salts?
Properties of salts
- Salts exist in a solid state.
- They have high melting and boiling points.
- Generally, they are soluble in water. However, salts like calcium carbonate, lead chloride and cadmium sulphate are insoluble in water.
- Aqueous solutions of metal salts or their molten forms conduct electricity.
- Carbonates and bicarbonates react with acids to liberate carbon dioxide gas.
31. What is hydrolysis?
Ans: The chemical reaction of water with salt produces acid and a base, this reaction is called hydrolysis.
32. Describe the role of salts in the human body.
- Salts of sodium and potassium are needed for the proper functioning of muscles and the nervous system.
- Salts of calcium are responsible for the strength of bones.
- Salt of iodine is used for the treatment of goiter.
- Potash alum is used to coagulate the blood coming out of a wound.
33. Which salt is useful for the treatment of goitre?
Ans: Salt of iodine is used for the treatment of goitre.
34. What are the uses of potash alum?
Ans: Potash alum is used to make clots of the blood coming out of a wound.
It is also used for the purification of water.
35. What are the uses of salts in daily life?
- Salt is used to preserve fish and pickles.
- We use common salt for seasoning food.
- Baking soda is used in bread and cake making.
- Washing soda is used for washing clothes.
- Sodium potassium tartrate is used as a laxative.
36. Which salt is used to make bread and cake?
Ans: Baking soda
37. What are the uses of salts in industries?
- Sodium chloride (common salt) is used to make washing soda, caustic soda, chlorine and hydrogen chloride.
- Sodium carbonate is used for softening hard water.
- It is also used to make glass and soap.
- Potassium nitrate is used to prepare gunpowder and fireworks. It is also used in fertilizers.
38. What is meant by pH scale?
Ans: The scale which is used to measure the strength of the acidic or alkaline solution is known as the pH scale.
39. Write down the chemical equation showing the reaction of ammonia and water.
Ans: NH3+ H2O → NH4OH
40. What is the effect of dilute HCl on the colours of the following? i. Methyl orange ii. Phenolphthalein iii. Blue litmus
- Methyl orange-red
- Phenolphthalein colourless
- Turn Blue litmus red
What is a base? Write down the names and formulae of four bases.
- Compounds that produce hydroxide ions (OH-) in their aqueous solutions are bases. They have properties that are contrary to acids.
- Sodium hydroxide NaOH ii. Potassium hydroxide KOH
- Calcium hydroxide Ca(OH)2
- Ammonium hydroxide NH4OH
- Magnesium hydroxide Mg(OH)2
42. What is the action of caustic soda (NaOH) on the colour of the following?
- Red litmus.
- Methyl orange
i. Red litmus turns blue.
ii. Phenolphthalein turns pink.
iii. Methyl orange yellow.
43. What happens when magnesium reacts with dilute HCl?
Ans: Mg + 2HCl → H2+ MgCl2
The acid reacts with the metal to form hydrogen and salt.
44. What happens when sodium hydrogen carbonate reacts with dilute H2SO4?
NaHCO3 + H2SO4 → Na2SO4 + H2O + CO2
Acids react with metal hydrogen carbonate to form a salt, water and carbon dioxide.
45. What happens when copper oxide reacts with dilute sulphuric acid?
CuO + H2SO4 → CuSO4 + H2O
46. What happens when sodium reacts with chlorine?
Ans: Sodium reacts with chlorine and sodium chloride (table salt)
Na +Cl → NaCl
47. Why are the aqueous solutions of NaHCO3 and Na2CO3 basic in nature?
Ans: NaHCO3 and Na2CO3 make strong alkalis in water. Their solution becomes basic.
48. How does the soil become acidic?
Ans: Carbonic acid and nitric acid cause acid rain and due to the acid rain soil become acidic.
49. What is a pH Meter?
Ans: The instrument which is used to measure the exact pH of the solutions is called a pH meter. It shows pH on the digital display in the form of digits.
50. What are the indicators?
Ans: An indicator is a substance that shows different colours in acidic and basic solutions. Litmus, turmeric, methyl orange, phenolphthalein, china rose and red cabbages are examples of indicators.
51. Which are the natural indicators?
Ans: Red cabbage, turmeric, China rose and litmus are natural indicators.
52. Name a salt that can reduce the acidity in our stomach?
Ans: sodium bicarbonate (NaHCO3) is used to reduce the acidity of the stomach. It is called soda mint. Milk of magnesia Mg(OH)2 can reduce the acidity of the stomach
53. Sulphuric acid (H2SO4) molecule can give two protons in water whereas hydrochloric acid (HCl) molecule can give only one proton. Does that mean sulphuric acid is twice as strong an acid as hydrochloric acid?
Ans: Yes, sulphuric acid (H2SO4) is twice as strong as hydrochloric acid (HCl)
Both hydrochloric acid (HCl) and sulfuric acids (H2SO4) are strong minerals (inorganic) acids. But, there are some important differences between the two.HCl is a monobasic acid, which means its molecule has only one ionizable hydrogen atom. H2SO4 is a dibasic acid, as it has two ionizable hydrogen atoms in its molecule.
54. How is the litmus solution prepared?
Ans: In this process, the wood pulp is converted to paper, the paper is infused with the lichen solution, and the paper is dried up and packed.
55. What happens when a salt-like copper sulphate reacts with water?
CuSO4 + H2O → H2SO4 + CuO
56. Indicate in front of each salt the acid and the base which have been used to produce them.
|Name of salt||Acid||Base|
|Potassium hydrogen sulphate||Sulphuric acid||Potassium hydroxide|
|Calcium acetate||Acetic acid||Calcium Hydroxide|
|Magnesium nitrate||Nitric acid||Magnesium hydroxide|
|Ammonium oxalate||Oxalic acid||Ammonium hydroxide|
|Sodium potassium tartrate||Potassium Bitartrate||Sodium carbonate|
|Ferric chloride||Hydrochloric acid||Ferric hydroxide|