Solutions of Acids, Bases and Salts (Page No - 82) - Chemistry Lakhmir Singh, Class 10 | Extra Documents, Videos & Tests for Class 10 PDF Download

Question 54:
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) Milk is made slightly alkaline so that it may not become sour easily due to the formation of lactic acid in it.
(b) 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.

Question 55:
Which of the following elements would form oxides which would indicate pH values less than seven, using moist pH paper ?
Magnesium, Carbon, Sulphur, Hydrogen , Copper
Solution :
Carbon and Sulphur being non-metals form acidic oxides.

Question 56:
The pH values of five solutions A, B, C, D and E are given below :
A             1
B             5
C             7
D             11
E             13

Which solution is

1. weakly alkaline
2. neutral
3. strongly acidic
4. strongly alkaline, and
5. weakly acidic ?

Solution :

1. Weakly alkaline: D (pH = 11)
2. Neutral : C (pH = 7)
3. Strongly acidic: A (pH = 1)
4. Strongly alkaline: E (pH = 13)
5. Weakly acidic : B (pH = 5)

Question 57:
Potatoes grow well on Anhad’s farm which has soil with a pH of 5.5. Anhad decides to add lot of lime to soil so that he can grow broccoli in the same farm :
(a) Do potatoes grow better in acidic or alkaline soil ?
(b) Does broccoli grow better in acidic or alkaline soil ?
Solution :
(a) Potatoes grow better in acidic soil having pH = 5.5
(b) Broccoli grows better in a lkaline soil since adding a lot of lime to acidic soil will make it basic in nature.

Question 58:
Here are some results of solutions tested with universal indicator paper :
Sulphuric acid : Red
Metal polish : Dark blue
Washing-up liquid : Yellow
Milk of magnesia : Light blue
Oven cleaner : Purple
Car battery acid : Pink
Arrange the solutions in order of their increasing pH values (starting with the one with the lowest pH).
Solution :
Sulphuric acid < car battery acid < washing up liquid < milk of magnesia < metal polish < oven cleaner since:
Red : pH = 1
Pink : pH = 3-4
Yellow: pH = 5-6
Light blue : pH = 9
Dark blue : pH = 10
Purple: pH = 11

Question 59:
Solution A turns universal indicator blue to purple whereas solution B turns universal indicator orange to red.
(a) What will be the action of solution A on litmus ?
(b) What will be action of solution B on litmus ?
(c) Name any two substances which can give solutions like A.
(d) Name any two substances which can give solutions like B.
(e) What sort of reaction takes place when solution A reacts with solution B ?
Solution :
(a) Solution A turns universal indicator blue to purple so it is basic in nature and will turn litmus blue.
(b) Solution B turns universal indicator orange to red so it is acidic in nature and will turn litmus red.
(c) Milk of magnesia and sodium hydroxide solution are bases like solution A.
(d) Lemon juice and hydrochloric acid are acids like solution B.
(e) Neutralisation reaction.

Question 60:
A first-aid manual suggests that vinegar should be used to treat wasp stings and baking soda for bee stings. What does this information tell you about the chemical nature of:
(a) wasp stings ?
(b) bee stings ?
Solution :
(a) Wasp stings are alkaline in nature since they are treated using acids like vinegar.
(b) Bee stings are acidic in nature since they are treated using bases like baking soda.

Question 61:
(a) Explain why the pH in a person’s mouth becomes lower after each meal.
(b) What damage could be caused while the pH is low ?
(c) How could the person change his eating habits to lessen chances of suffering from tooth decay ?
Solution :
(a) T he pH in a person’s mouth becomes lower after each meal because bacteria present in the mouth breaks down the sugar to form acids.
(b) If the pH is low, the tooth starts decaying.
(c) A person can lessen the chances of suffering from tooth decay by changing his eating habits such as eating less of sugary foods like ice-creams, candies, sweets etc.

Question 62:
A group of students measured the pH of some substances they found in their homes. Their results are given in the following table :

(a) What would the students have used to measure the pH ?
(b) Which solution is the most acidic
(c) Which solution is the most alkaline ?
(d) Which solutions are neutral ?
(e) Which solution can be used to treat wasp stings ?
(f) Which solution can be used to treat bee stings ?
Solution :
(a) Universal indicator paper is used to measure the pH.
(b) Lemon juice with pH = 2.5 is the most acidic.
(c) Household ammonia with pH = 12 is the most alkaline.
(d) Salt solution and sugar solution with pH = 7 are neutral.
(e) Vinegar (acid) can be used to treat wasp stings since it injects an alkaline liquid into the skin.
(f) Baking soda can be used to treat bee stings since it injects methanoic acid into the skin.