# Acids Bases and Salts

## Class 10 NCERT Science

### NCERT

1   You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

##### Solution :

If the colour of red litmus paper gets changed to blue, then it is a base and if there is no colour change, then it is either acidic or neutral. Thus, basic solution can be easily identified. Let us mark the three test tubes as A, B, and C. A drop of the solution in A is put on the red litmus paper. Same is repeated with solution B and C. If either of them changes colour to blue, then it is basic. Therefore, out of three, one is eliminated. Out of the remaining two, any one can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then the nature of the drops of the mixtures is checked. If the colour of red litmus turns blue, then the second solution is neutral and if there is no change in colour, then the second solution is acidic.This is because acidic and basic solutions neutralize each other. Hence, we can distinguish between the three types of solutions.

2   Why should curd and sour substances not be kept in brass and copper vessels?

##### Solution :

Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.

3   Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

$\text{Result:}$ $\\$ $HCl$dissociates into $H^+$ and $Cl^-$ ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, the glucose solution does not dissociate into ions. Therefore, it does not conduct electricity.$\\$ $\text{Conclusion:}$$\\ From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids.\\ That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids. 26 Why does distilled water not conduct electricity, whereas rain water does? ##### Solution : Distilled water is a pure form of water and is devoid of any ionic species. Therefore, it does not conduct electricity. Rain water, being an impure form of water, contains many ionic species such as acids and therefore it conducts electricity. 27 Why do acids not show acidic behaviour in the absence of water? ##### Solution : Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions from an acid occurs in the presence of water only. It is the hydrogen ions that are responsible for the acidic behaviour. 28 Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1,11, 7 and 9, respectively. Which solution is\\ (a) neutral?\\ (b) strongly alkaline?\\ (c) strongly acidic?\\ (d) weakly acidic?\\ (e) weakly alkaline?\\ Arrange the pH in increasing order of hydrogen-ion concentration. ##### Solution : (a) Neutral \to Solution D with pH 7$$\\$ (b) Strongly alkaline $\to$ Solution $C$ with $pH 11$$\\ (c) Strongly acidic \to Solution B with pH 1$$\\$ (d) Weakly acidic $\to$ Solution $A$ with $pH 4$$\\ (e) Weakly alkaline \to Solution E with pH 9$$\\$ The $pH$ can be arranged in the increasing order of the concentration of hydrogen ions as: $11 < 9 < 7 < 4 < 1$$\\$

29   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 $(CH_3COOH)$ is added to test tube $B$. In which test tube will the fizzing occur more vigorously and why?

##### Solution :

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 $CH_3COOH$ and therefore produces hydrogen gas at a faster speed due to which fizzing occurs.

30   Fresh milk has a $pH$ of $6$. How do you think the $pH$ will change as it turns into curd?Explain your answer.

##### Solution :

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 $pH.$

31   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?

##### Solution :

(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.

32   Plaster of Paris should be stored in a moisture-proof container. Explain why?

##### Solution :

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.$\\$ $\underset{\text{Plaster of paris}}{CaSO_4.\dfrac{1}{2}H_2O}+\underset{\text{water}}{1\dfrac{1}{2}H_2)}\to \underset{\text{Gypsum(Hard solid)}}{CaSO_4.2H_2O}$

33   What is a neutralization reaction? Give two examples.

##### Solution :

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)$\underset{\text{Base}}{NaOH}+\underset{\text{Acid}}{HCL}\to \underset{\text{Salt}}{NaCl}+\underset{\text{Water}}{H_2O}$ $\\$(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.$\\$ $Mg(OH)_2+2HCl\to MgCl_2+2H_2O$ $\\$

34   Give two important uses of washing soda and baking soda.

##### Solution :

Two important uses 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 $CO_2$ that makes bread or cake fluffy.$\\$ (b) It is used in soda-acid fire extinguishers.