Acid bases and salts
1.Two solutions A and B have pH values of 5 and 8 respectively. Which solution will be basic in nature?
Solution B
2. If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done?
[NCERT Exemplar]
The hand should be washed immediately with plenty of water and a paste of sodium hydrogencarbonate (a base) must be applied to neutralise the acid.
3. If someone is suffering from the problem of acidity after overeating; which of the following would you suggest as a remedy?
Lemon juice, Baking soda solution or Vinegar.
Baking soda solution because it is basic in nature and hence neutralises the excess acid present in the stomach.
4. Arrange the following in the increasing order of acidic strength.
Acetic acid, water and hydrochloric acid
Water < acetic acid < hydrochloric acid.
5. Why does tooth decay start when the pH of mouth is lower than 5.5?
Tooth starts decaying when the pH of our mouth is lower than 5.5. This is because below this pH value, the medium of the mouth becomes more acidic due to which tooth enamel corrodes at a faster rate.
6. Two solutions X and Y have pH values of 3.0 and 9.5 respectively. Which of these will turn litmus solution from blue to red and which will turn phenolphthalein from colourless to pink?
X will turn blue litmus to red because it is acidic in nature having pH value less than 7. Y will turn phenolphthalein from colourless to pink because it is basic in nature having pH value greater than 7.
7. Why aqueous ammonia solution considered a base although ammonia does not contain hydroxyl (OH–) ion?
Ammonia dissolves in water as follows:
NH3+H2O→NH+4+OH–
The hydroxyl ions produced in the solution are responsible for basic character of ammonia.
8. A knife, which is used to cut a fruit, was immediately dipped into water containing drops of blue litmus solution. The colour of the solution changes to red, what is the nature of the fruit?
The fruit is acidic as it turns blue litmus solution to red.
9. Explain why an aqueous solution of ammonium chloride is acidic in nature.
Ammonium chloride (NH4Cl) is the salt of a strong acid, hydrochloric acid (HCl), and a weak base ammonium hydroxide (NH4OH), so an aqueous solution of ammonium chloride is acidic in nature.
10. What are the products formed when an acid reacts with a base?[CCE 2014]
Acid reacts with a base to form salt and water.
11. Fresh milk has a pH of 6. When it changes into curd (yogurt), will its pH value increase or decrease? Why?
When milk changes into curd (yogurt), its pH value decreases. This is because during curd formation, lactic acid is produced which makes it acidic.
12. A white chemical compound becomes hard on mixing proper quantity of water. It is also used in surgery to maintain joints in a fixed position. Name the chemical compound.
Plaster of Paris (Calcium sulphate hemihydrate).
13. Which one of these has a higher concentration of H+ ions?
1M HCl or 1M CH3COOH
1 M HCl has a higher concentration of H+ ions because it is a stronger acid than CH3COOH.
14. In addition to sodium hydrogencarbonate, baking powder contains a substance ‘X’. Name the substance ‘X’.
Tartaric acid.
15. What is the commercial name of calcium sulphate hemihydrate?
Plaster of Paris.
16. Name the substance obtained by the action of chlorine on dry slaked lime.
Bleaching powder.
17. How many molecules of water of crystallisation are there in
12
washing soda crystals?
10.
18. Why does the milkiness disappear on passing excess of carbon dioxide to lime water?
When carbon dioxide is passed in excess, calcium carbonate (which appears milky) gets converted into calcium hydrogencarbonate which is soluble. Hence, the milkiness disappears.
CaCO3(s)+H2O(l)+CO2(g)→Ca(HCO3)2(aq)(Solubleinwater)
19. An aqueous solution turns red litmus solution blue. Excess addition of which solution would reverse the change—ammonium hydroxide solution or hydrochloric acid?
Hydrochloric acid because adding excess acid to the base would turn blue litmus solution red.
20. Plaster of Paris should be stored in moisture proof container. Explain why.
Plaster of Paris reacts with moisture and gets hardened due to formation of gypsum so it is stored in moisture proof container.
21. Why is the electrolysis of a concentrated solution of sodium chloride known as chlor-alkali process?
It is because of the products formed: chlor for chlorine and alkali for sodium hydroxide.