# NCERT Solutions

## Class 10 Science

###### acid bases and salts

Q1) A solution turns red litmus blue, its pH is likely to be

• (a) 1
• (b) 4
• (c) 5
• (d) 10

(d) 10, as the solution is basic.
Note : Red litmus paper turns blue in a base.

Q2) 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

(b) HCl,Egg shells contains calcium carbonate, which on reaction with HCl liberates CO2 gas which turn lime water to milky.

Q3)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

(d) 16mL 10 ml of NaOH requires 8 ml of HCl for neutralization 20 ml of NaOH will require 8 divided by 10×20 = 16 ml of HCl

Q4)Which one of the following types of medicines is used for treating indigestion ?

• (a) Antibiotic
• (b) Analgesic
• (c) Antacid
• (d) Antiseptic

(C) , Antacid

Q5)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 filing

(a) Zinc + dilute sulphuric acid ? Zinc sulphate + Hydrogen

$$Zn(s) + H_{2}SO_{4}(aq)$$

$$\downarrow$$

$$ZnSO_{4}(aq) + H_{2}(g)$$

(b) Magnesium ribbon + dil. Hydrochloric acid ? Magnesium chloride + Hydrogen

$$Mg(s) + 2HCl(aq)$$

$$\downarrow$$

$$MgCl_{2} + H_{2}(g)$$

(c)Aluminium powder + dil. Sulphuric acid ? Aluminium sulphate + Hydrogen

$$2Al(s) + 3H_{2}SO_{4}(aq)$$

$$\downarrow$$

$$Al_{2}(SO_{4})_{3} (aq) + 3H_{2}(g)$$

(d)Iron filings + Dilute hydrochloric acid ? Ferric chloride + Hydrogen

$$2Fe(s) + 6HCl(aq)$$

$$\downarrow$$

$$2FeCl_{3}(aq) + 3H_{2}(g)$$

Q6)Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

The nails are fitted in a cork and are kept in a 100 ml beaker. Then these are connected to the two terminals of a 6 volt battery along with a bulb and a switch. Now some dilute HCl is poured in the beaker and current is allowed to pass through the circuit. The same experiment is carried out using glucose and alcohol solutions separately in place of HCl.

Observations :
The bulb glows in HCl solution only. The reason is that HCl solution dissociates into H+ and Cl– ions which help to conduct electricity while glucose and alcohol solutions do not dissociate into ions and so, do not conduct electricity. That is why we find the bulb glowing in HCl solution only. Conclusions The above activity proves that all acids contain hydrogen but not all compounds containing hydrogen are acids. Due to this reason compounds such as alcohols and glucose although contain hydrogen, but they are not categorized as acids. Q7)Why does distilled water not conduct electricity, whereas rain water does ?

Distilled water cannot conduct electricity because it does not contain ions while rain water conducts electricity as it contains ions due presence of dissolved salts in it. Distilled water which is a pure form of water, is neither acidic nor basic in nature. So distilled water does not dissociate into ions. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.Distilled water cannot conduct electricity because it does not contain ions while rain water conducts electricity as it contains ions due presence of dissolved salts in it. Distilled water which is a pure form of water, is neither acidic nor basic in nature. So distilled water does not dissociate into ions. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.
Rain water being an impure form of water, contains many ionic species. These ions are responsible for electrical conductivity of rain water.

Q8)Why do acids not show acidic behaviour in the absence of water ?

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.

Q9) 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 :

1. D
2. C
3. B
4. A
5. E
Increasing order of hydrogen ion concentration
11 < 9 < 7 < 4 < 1
i.e., C < E < D < A < B

Q10)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. In which test tube will the fizzing occur more vigorously and why ?

###### Solution :

Acetic acid (CH3COOH) is a weak acid whereas hydrochloric acid (HCl) is a strong acid. Fizzing occurs in the test tube due to the evolution of hydrogen gas by the action of acid on magnesium ribbon. Since hydrochloric acid is a strong acid a large amount of hydrogen gas is liberated in the test tube A. So fizzing occurs more vigorously in test tube A

Q11)Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain.

###### Solution :

Fresh milk has a pH 6, during curdling, the lactobacillus keeps growing and it breaks down the lactose (the natural sugar in milk) into lactic acid which acidifies the milk even further. Therefore, the pH of the milk will drop (decrease). As the more acidic the substance the lower the pH.

Q12)A milkman adds a very small amount of baking soda to fresh milk.

1. A milkman adds a very small amount of baking soda to fresh milk.
2. Why does this milk take a long time to set as curd ?

###### Solution :

1. Milk is made slightly alkaline so that it may not get sour easily due to the formation of lactic acid in it.
2. The alkaline milk takes a longer time to set into curd because the lactic acid being formed has to first neutralise the alkali present in it.

Q13)Plaster of Paris should be stored in a moisture proof container. Explain why?

###### Solution :

Plaster of Paris should be stored in a moisture proof container because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration. This will make the plaster of Paris useless after sometime.

Q14)What is a neutralisation reaction ? Give two examples.

###### Solution :

The reaction between an acid and a base to form salt and water is called a neutralisation reaction. we will look at an example as Sulphuric Acid and Potassium Hydroxide neutralize each other in the following reaction: $$H_{2}SO{4} + 2KOH \rightarrow K_{2}SO_{4} + 2H_{2}O$$ In a neutralization reaction between an acid and a base the typical outcome is a salt formed by the positive ion from the base and the negative ion from the acid. In this case the positive potassium ion and polyatomic sulphate to form salt potassium sulphate. The positive ion from acid and negative hydroxide ion from base forms water. $$H^{+} + OH^{-} \rightarrow H_{2}O$$

Q15)Give two important uses of washing soda and baking soda.

###### Solution :

Uses of Washing soda :

1. Washing soda is used in glass, soap and paper industries
2. It is used for removing permanent hardness of water.
3. Baking soda is used as an antacid in medicines to remove acidity of the stomach.
4. Baking soda is used for making baking powder (used in making cakes, bread, etc.).