Chapter 02 Acids, Bases and Salts Exercise
EXERCISES
1. A solution turns red litmus blue, its $pH$ is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Show Answer
Answer
(d) Bases turn red litmus blue and acids turn blue litmus red. Basic solution has a pH value more than 7 . Since the solution turns red litmus blue, its $pH$ is likely to be 10 .
2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a) $NaCl$
(b) $HCl$
(c) $LiCl$
(d) $KCl$
Show Answer
Answer
(b) The solution contains $HCl$.
3. $10 mL$ 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 $HCl$ solution (the same solution as before) required to neutralise it will be
(a) $4 mL$
(b) $8 mL$
(c) $12 mL$
(d) $16 mL$
Show Answer
Answer
(d) $16 mL$ of $HCl$ solution will be required.
4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Show Answer
Answer
(c) Antacid is used for treating indigestion.
5. 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.
Show Answer
Answer
(a) Sulphuric acid + Zinc $\rightarrow$ Zinc sulphate + Hydrogen
$H_2 SO _{4(a q)}+Zn _{(s)} \longrightarrow ZnSO _{4(a q)}+H _{2(g)}$
(b) Hydrochloric acid + Magnesium $\rightarrow$ Magnesium chloride + Hydrogen
$2 HCl _{(a q)}+Mg _{(s)} \longrightarrow MgCl _{2(a q)}+H _{2(g)}$
(c) Sulphuric acid + Aluminium $\rightarrow$ Aluminium sulphate + Hydrogen
$3 H_2 SO _{4(a q)}+2 Al _{(s)} \longrightarrow Al_2(SO_4) _{3(a q)}+3 H _{2(g)}$
(d) Hydrochloric acid + Iron $\rightarrow$ Ferric chloride + Hydrogen
$ 6 HCl _{(x q)}+2 Fe _{(s)} \longrightarrow 2 FeCl _{3(a q)}+3 H _{2(s)} $
6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Show Answer
Answer
Two nails are fitted on a cork and are kept it in a $100 mL$ beaker. The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch. Some dilute $HCl$ is poured in the beaker and the current is switched on. The same experiment is then performed with glucose solution and alcohol solution.
7. Why does distilled water not conduct electricity, whereas rain water does?
Show Answer
Answer
Distilled water is a pure form of water and is devoid of any ionic species. Therefore, it does not conduct electricity. Rain water, being an impure form of water, contains many ionic species such as acids and therefore it conducts electricity.
8. Why do acids not show acidic behaviour in the absence of water?
Show Answer
Answer
Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions from an acid occurs in the presence of water only. It is the hydrogen ions that are responsible for the acidic behaviour.
9. Five solutions $A, B, C, D$ and $E$ when tested with universal indicator showed $pH$ as $4,1,11,7$ and 9 , respectively. Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the $pH$ in increasing order of hydrogen-ion concentration.
Show Answer
Answer
(a) Neutral $\rightarrow$ Solution D with $pH 7$
(b) Strongly alkaline $\rightarrow$ Solution C with $pH 11$
(c) Strongly acidic $\rightarrow$ Solution B with $pH 1$
(d) Weakly acidic $\rightarrow$ Solution A with $pH 4$
(e) Weakly alkaline $\rightarrow$ Solution E with $pH 9$
The $pH$ can be arranged in the increasing order of the concentration of hydrogen ions as: $11<9<$ $7<4<1$
10. 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 $(CH_3 COOH)$ is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Show Answer
Answer
The fizzing will occur strongly in test tube A, in which hydrochloric acid ( $HCl$) is added. This is because $HCl$ is a stronger acid than $CH 3 COOH$ and therefore produces hydrogen gas at a faster speed due to which fizzing occurs.
11. Fresh milk has a $pH$ of 6 . How do you think the $pH$ will change as it turns into curd? Explain your answer.
Show Answer
Answer
The pH of milk is 6 . As it changes to curd, the $pH$ will reduce because curd is acidic in nature. The acids present in it decrease the $pH$.
12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the $pH$ of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Show Answer
Answer
(a) The milkman shifts the $pH$ of the fresh milk from 6 to slightly alkaline because in alkaline condition, milk does not set as curd easily.
(b) Since this milk is slightly basic than usual milk, acids produced to set the curd are neutralized by the base. Therefore, it takes a longer time for the curd to set.
13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Show Answer
Answer
Plaster of Paris (POP) should be stored in a moisture-proof container because Plaster of Paris, a powdery mass, absorbs water (moisture) to form a hard solid known as gypsum.
$ \begin{aligned} & \underset{\text{POP}}{CaSO_4 \cdot \dfrac{1}{2} H_2 O+1 \dfrac{1}{2} H_2 O} \longrightarrow \underset{\text{Gypsum(Hard solid)}}{CaSO_4 \cdot 2 H_2 O} \\ \end{aligned} $
14. What is a neutralisation reaction? Give two examples.
Show Answer
Answer
A reaction in which an acid and base react with each other to give a salt and water is termed as neutralization reaction. In this reaction, energy is evolved in the form of heat.
For example:
(i)
$ \begin{aligned} & NaOH+HCl \longrightarrow NaCl+\underset{2}{H_2 O} \\ & \text{ (Base) } \quad \text{ (Acid) } \end{aligned} $
(ii) During indigestion (caused due to the production of excess of hydrochloric acid in the stomach), we administer an antacid (generally milk of magnesia, $Mg(OH)_2$, which is basic in nature). The antacid neutralizes the excess of acids and thus gives relief from indigestion.
$ Mg(OH)_2+2 HCl \to MgCl_2+2 H_2 O $
15. Give two important uses of washing soda and baking soda.
Show Answer
Answer
Two important used of washing soda and baking soda are as follows:
(1) Washing soda:
(a) It is used in glass, soap, and paper industries.
(b) It is used to remove permanent hardness of water.
(2) Baking soda:
(a) It is used as baking powder. Baking powder is a mixture of baking soda and a mild acid known as tartaric acid. When it is heated or mixed in water, it releases CO2 that makes bread or cake fluffy.
(b) It is used in soda-acid fire extinguishers.
