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Practice each question, then open the answer or worked solution.
A solution turns red litmus blue, its pH is likely to be
a) 1 (b) 4 (c) 5 (d) 10
Solution: The answer is 10 because litmus paper turns blue when the solution reacts with a basic solution (PH more than 7). Hence, 10 is the answer.
A solution reacts with crushed eggshells to give a gas that turns lime-water milky. The solution contains
a) NaCl (b) HCl (c) LiCl (d) KCl
Solution: The answer is HCl.
Eggshells contain calcium carbonate, which on reaction with HCl, liberates CO2 gas, which turns lime water into milky.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Solution: Since 10 ml of NaOH requires 8 mL of HCL, 20 ml of NaOH requires 8 x 2 = 16mL of HCl. Hence, the answer is option (d) 16mL.
Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Solution: Indigestion is due to the excess production of acid in the stomach. Medicines used to treat indigestion is called Antacid.
Write word equations and then balanced equations for the reaction taking place when
(a) Dilute sulphuric acid reacts with zinc granules.
(b) Dilute hydrochloric acid reacts with magnesium ribbon.
(c) Dilute sulphuric acid reacts with aluminium powder.
(d) Dilute hydrochloric acid reacts with iron filings.
(a) Dilute sulphuric acid reacts with zinc granules.
=> Dilute sulphuric acid + zinc → Zinc Sulphate + Hydrogen Gas
=> H2SO4(aq) + Zn → ZnSO4(aq) + H2(g)
(b) Dilute hydrochloric acid reacts with magnesium ribbon.
=> Dilute Hydrochloric + Magnesium → Magnesium Chloride + Hydrogen Gas
=> 2HCl(aq) + Mg → MgCl2(aq) + H2(g)
(c) Dilute sulphuric acid reacts with aluminium powder.
=> Dilute Sulphuric Acid + Aluminium → Aluminium Sulphate + Hydrogen Gas
=> 3H2SO4(aq) + 2Al(s) → Al2(SO4)3(aq) + 3H2(g)
(d) Dilute hydrochloric acid reacts with iron filings.
=> Dilute Hydrochloric Acid + Iron → Ferrous Chloride + Hydrogen Gas
=> 6HCl(aq) + 3Fe(s) → 3FeCl2(aq) + 3H2(g)
Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Solution: Insert two nails into the wooden or rubber cork and place them on a beaker, as shown in the figure. Connect the iron nail to a bulb, a 6-volt battery and a wire connected to the switch. Pour some alcohol or glucose so as to dip the nails in glucose or alcohol. Turn the switch on, and you see the bulb not glowing despite of connection to the switch. Now empty the beaker and add the HCL solution. This time, the bulb glows. This proves acid can conduct electricity, but alcohol and glucose do not conduct electricity.
Why does distilled water not conduct electricity, whereas rain water does?
Why do acids not show acidic behaviour in the absence of water?
Solution: The acidic behaviour of acids is because of the presence of hydrogen ions. Hydrogen ions can only be produced in the presence of water, and therefore, water is definitely needed if acids are to show their acidic behaviour.
Five solutions, A, B, C, D and E, when tested with a universal indicator, showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is
(e) Weakly alkaline?
Solution: In increasing order of hydrogen ion concentration,
pH 11(C) < pH 9(E) < pH 7 (D) < pH 4 (A) < pH 1 (B)
PH11 – Strongly alkaline
pH9 – Weakly alkaline
PH7 – Neutral
pH4 – Weakly acidic
pH1 – Strongly acidic
Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are the same. In which test tube will the fizzing occur more vigorously and why?
Solution: HCl is a strong acid, whereas acetic is a weaker acid. Fizzing occurs because of the production of the hydrogen gas obtained due to the reaction of the acid on the magnesium ribbon. Since HCl is a very strong acid, there is a lot of liberation of hydrogen gas from test tube A. Therefore, more fizzing takes place in test tube A.