PHYSICAL SCIENCE LESSON-3 ACID BASES AND SALT

 PHYSICAL SCIENCE

LESSON-3

ACID BASES AND SALT

Reflections on concepts

Question 1.
An acid or a base is mixed with water. Is this process exothermic or endothermic one?
Answer:
When acid or base is mixed with water heat is liberated. So, this process is exothermic.

Question 2.
Distilled water does not conduct electricity. Why?
Answer:

1. Water consists of H₃O⁺ and OH^– ions.
2. In distilled water, the concentration of both H₃O⁺ and OH^– is same. Hence they do not form as ions, so distilled water can be treated as a neutral solution.
3. As there is no flow of ions, distilled water does not conduct electricity.
4. On the other hand, while falling to earth through the atmosphere rainwater dissolves some acidic gases like CO₂, SO₂, and N₂O and forms acids such as H₂CO₃, H₂SO₃, HNO₃.
5. These acids produce Ions. Due to presence of Ions, rainwater conducts electricity.

Question 3.
Draw a neat diagram for showing acid solution In water conducting electricity.
Answer:

Question 4.
Why the flow of acid rain Into a river make the survival of aquatic lite in a river difficult?
Answer:

• When pH value of rainwater is less than 5.6, it is called acid rain.
• When acid rain flows into the rivers, it lowers the pH of the river water.
• Living organisms can survive only in a narrow range of pli change.
• So flow of acid rainwater into rivers Is dangerous to the survival of aquatic “ life. These acids have free ions that conduct electric current.

 

Question 5.
What Is baking powder? How does It make the cake soft and spongy?
Answer:
Baking powder s a mixture of baking soda and mild edible acid such as tartaric acid.

1. The chemical name of baking powder is sodium hydrogen carbonate (NaHCO₃).
2. It is added to the dough while making cake because on heating It Is converted to sodium carbonate with release of carbon dioxide.
   NaHCO₃ + H⁺ → CO₂ + H₂O + Sodium salt of acid
3. While baking the cake the CO₂ released by baking powder causes the cake or bread to rise making them soft and spongy.

Question 6.
What is a neutralization reaction? Give two examples.
Answer:
Neutralization reaction: The reaction between an acid and a base to give a salt and water is known as a neutralization reaction.

Examples:
(i) When diluted NaOH solution reacts with HCl solution, NaCl (Sodium chloride) salt and water are formed.
NaOH (aq) + HCl (aq) → NaCl (aq) +H₂O

(ii) When diluted KOH solution reacts with Nitric acid solution, KNO₃ (Potassium nitrate) salt and water are formed.
KOH (aq) + HNO₃ (aq) → KNO₃ (aq) + H₂O

Question 7.
Dry hydrogen chloride gas does not turn blue litmus to red whereas aqueous hydrogen chloric acid does. Why?
Answer:

1. Dry hydrogen chloride gas does not dissociate in the absence of water. Hence it does not turn blue litmus to red. So dry hydrogen chloride gas is not acidic.
2. The HCl (hydrochloric acid) dissociates in the presence of water. Hence it turns blue litmus to red. So. HC (Hydrochloric acid) is acidic.
   The dissociation wIll be as follows:
   HCl + H₂O → H₃O⁺ + Cl^–

Question 8.
Give two important uses for each of washing soda and baking soda.
Answer:
Uses of Washing soda (Na₂CO₃):

• Washing soda (sodium carbonate) is used in glass, soap and paper industries.
• It is used In the manufacture of sodium compounds such as borax.
• Sodium carbonate can be used as a cleaning agent for domestic purposes.
• It is used for removing permanent hardness of water.

Uses of Baking soda (NaHCO₃):

1. Baking soda (Sodium hydrogen carbonate) is used for faster cooking.
2. Baking powder (a mixture of baking soda and a mild acid) is used in preparation of cakes.
3. Sodium hydrogen carbonate Is also an ingredient in antacids.
4. It is also used in soda-acid, fire extinguishers.
5. It acts as a mild antiseptic.

Application Of Concepts

Question 1.
Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9 respectively. Classify the solutions given below.
(a) neutral
(b) strongly alkaline
(c) strongly acidic
(d) weakly acidic
(e) weakly alkaline

Type

Required pH

Given pH

Solution

a) Neutral

7

7

D

b) Strongly alkalIne

10-14

11

C

c) Strongly Acidic

0-3

1

B

d) Weakly AcIdic

4-7

4

A

e) Weakly Alkaline

7-9

9

E

Order of pH is In increasing order of hydrogen Ion co4centratIon, higher the hydrogen ion concentration lower is the pH value.
11<9< 7<4< 1
C<E<D<A<B

 

Question 2.
Why does tooth decay start when the pH of mouth is lower than 5.5?
(or)
What value of pH in the mouth leads to tooth decay? Why?
Answer:

1. Tooth decay starts when the pH of the mouth is lower than 5.5.
2. Tooth enamel, made up of calcium phosphate is the hardest substance in the body.
3. It does not dissolve ¡n water but is corroded when the pH in the mouth is below 5.5 due to acids.
4. Bacteria present In the mouth produce free acids whose pH is lower than 5.5, due to degradation of sugar and food particles present in the mouth. Due to this the pH of the mouth falls.

Question 3.
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 6to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Answer:
(a) There is a shift In p’ of fresh milk from 6 to slightly alkaline when a little of baking soda is added to It is because the W ion concentration has been decreased and so the nature of solution has become slightly alkaline.

(b) Lactic acid which was formed initially, reduce the basic nature of the baking soda. Then more lactic acid is needed to convert milk Into curd. That is why it takes time to produce more lactic acid and hence the milk take a long time to set as curd.

Question 4.
Plaster of Parts should be stored in a moisture-proof container. Explain why.
Answer:

• Plaster of Paris means calcium sulphate hemihydrate (CaSO₄. ½H₂O).
• Plaster of París on mixing with water, sets into hard solid mass due to the formation of gypsum (CaSO₄. 2H₂O)

Because of the above reason Plaster of Pans should be stored in a moisture-proof container so that it won’t turn into gypsum.

Question 5.
Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid is added to test tube A, while acetic acid Is added to test tube B. The valances and concentration of both the acids Is same. In which test tube will the fizzing occur more vigorously and why?
Answer:

1. The fizzing occur more vigorously In test tube A.
2. Hydrochloric acid contains more H₃O⁺ ions and it is a strong acid. Hence the reaction will be faster in test tube A.
3. Acetic acid contains fewer H₃O⁺ ions and it is a weak acid. Hence the reaction will be slower.

 

Multiple choices questions

Question 1.
The colour of methyl orange indicator in acidic medium is ………………….. ( )
(A) yellow
(B) green
(C) orange
(D) red
Answer:
(D) red

Question 2.
The colour of phenolphthalein Indicator in basic solution is ………………….. ( )
(A) yellow
(B) green
(C) pink
(D) orange
Answer:
(C) pink

Question 3.
Colour of methyl orange In alkali conditions ……………………………. ( )
(A) orange
(B) yellow
(C) red
(D) blue
Answer:
(B) yellow

Question 4.
A solution turns red litmus blue, its pH is likely to be …………………………….. ( )
(A) 1
(B) 4
(C) 5
(D) 10
Answer:
(D) 10

Question 5.
One of the following solutions reacts with crushed egg shells to give a gas that turns lime-water milky, the solution is of …………………………….. ( )
(A) NaCl
(B) HCl
(C) LiCl
(D) KCl
Answer:
(B) HCl

 

Question 6.
If a base dissolves in water, by what name is it better known’ ……………………….. ( )
(A) neutral
(B) base
(C) acid
(D) alkali
Answer:
(D) alkali

Question 7.
Which of the following substances when mixed together will produce table salt? ( )
(A) Sodium thiosulphate and sulphur dioxide
(B) Hydrochloric acid and sodium hydroxide
(C) Chlorine and oxygen
(D) Nitric acid and sodium hydrogen carbonate
Answer:
(B) Hydrochloric acid and sodium hydroxide

Question 8.
What colour would hydrochloric acid (pH=1) turn universal indicator? ()
(A) orange
(B) purple
(C) yellow
(D) red
Answer:
(D) red

Question 9.
Which one of the following types of medicines is used for treating Indigestion? ()
(A) antibiotic
(B) analgesic
(C) antacid
(D) antiseptic
Answer:
(C) antacid

Question 10.
What gas is produced when magnesium is made to react with hydrochloric acid? ()
(A) hydrogen
(b) oxygen
(C) carbon dioxide
(D) no gas is produced
Answer:
(A) hydrogen

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