Here in this blog post, we provide updated Acids Bases and Salts Important Questions Class 10 Science for the academic year of 2021 2022.
 Fresh milk has a pH of 6. Explain why the pH changes as it turns into curd. (2 Marks)
Answer: The pH 6 of milk is due to the casein (milk protein) which is a phosphoprotein and therefore milk is slightly acidic.
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) as it gets converted into curd.Curd and other sour foodstuffs contain acids, which can react with the metal of the vessel to form poisonous metal compounds which can cause food poisoning and damage our health.
 Compounds such as alcohols and glucose contain Hydrogen but are not categorized as acids. Describe an activity to prove it. (4 Marks)
Answer: Prepare solutions of glucose, alcohol, Hydrochloric acid and sulphuric acid etc., Connect two different coloured electrical wires to graphite rods separately in a 100 ml beaker as shown in the figure.
- Connect free ends of the wire to 230 Volts AC plug and complete the circuit by connecting a bulb to one of the wires.
- Now pour some dilute HCl in the beaker and switch on the current.
- We observe that the bulb glows.
- Repeat activity with dilute Sulphuric acid and glucose and alcohol solutions.
- We will notice that the bulb glows only in acid solutions but not in glucose and alcohol solutions.
- Glowing of bulb indicates that there is flow of electric current through the solution.
- Acid solutions have ions and the moment of these ions in solution helps for flow of electric current through the solution.
- The positive ion (cation) present in HCl solution is H+. This suggests that acids produce Hydrogen ions H+ in solution, which are responsible for their acidic properties.
- In glucose and alcohol solution the bulb did not glow indicating the absence of H+ ions in these solutions.
So glucose and alcohol are not categorized as acids.
 Why does distilled water not conduct electricity, whereas rainwater does ?
Distilled water does not conduct electricity because it does not contain any ionic compound (like acids, bases or salts) dissolved in it.
Rainwater, while falling to the earth through the atmosphere, dissolves an acidic gas carbon dioxide from the air and forms carbonic acid (H2CO3). Carbonic acid provides hydrogen ions, H+ (aq) and carbonate ions, CO(aq)32- to rainwater. Hence, due to the presence of carbonic acid which provides ions to rainwater, the rainwater conducts electricity.
 Why do acids not show acidic behaviour in the absence of water ?
The acidic behaviour of many acids is due to the presence of hydrogen ions, [H+ (aq) ions], in them. Only in the presence of water the acid produces hydrogen ions [H+ (aq) ions] . So in the absence of water, an acid will not form hydrogen ions and hence will not show its acidic behaviour.
 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
(b) Strongly alkaline
(c) Strongly acidic
(d) Weakly acidic
(e) Weakly alkaline
Arrange the pH in increasing order of hydrogen ion concentration.
Increasing order of hydrogen ion concentration
11 < 9 < 7 < 4 < 1
i. e., C < E < D < A < B
 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 ?
Fizzing will occur more vigorously in test tube A. Hydrochloric acid (HCl) is a strong acid whereas acetic acid (CH3COOH) is a weak acid. Being strong acid, the hydrochloric acid solution contains a much greater amount of hydrogen ions in it due to which the fizzing will occur more vigorously in test tube A (containing hydrochloric acid). The fizzing is due to the evolution of hydrogen gas which is formed by the action of acid on the magnesium metal of magnesium ribbon. Magnesium ribbon + dil. Hydrochloric acid → Magnesium chloride + Hydrogen
Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)
 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 ?
(a) Milk is made slightly alkaline so that it might not get 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.
 Plaster of Paris should be stored in a moisture proof container. Explain why?
Plaster of paris is calcium sulphate hemihydrate. Its chemical formula is: CaSO4.1/2H2O. Plaster of Paris is prepared by heating gypsum to a temperature at 373K and it loses 3/4th of its water of crystallisation and forms Plaster of Paris.
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
- It is used for healing the fracture of bones.
- It is used as a fireproofing material
The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
 Give two important uses of washing soda and baking soda.
Uses of washing soda ,NaCO3:
(i) Washing soda is used in glass, soap and paper industries.
(ii) It is used for removing permanent hardness of water.
Uses of baking soda NaHCO3.10H2O:
(i) Baking soda is used as an antacid in medicines to remove acidity of the stomach.
(ii) Baking soda is used for making baking powder[NaHCO3 + edible acide like tartaric aide (used in making cakes, bread, etc.).
Reaction Based Problem
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) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
(b) Magnesium ribbon + dil. Hydrochloric acid → Magnesium chloride + Hydrogen
Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)
(c) Aluminium powder + dil. Sulphuric acid > Aluminium sulphate + Hydrogen
2Al (s) + 3H2SO4 (aq) → Al2 (SO4)3 (aq) + 3H2 (g)
(d) Iron filings + Dilute hydrochloric acid > Ferric chloride + Hydrogen
2Fe (s) + 6HCl (aq) → 2FeCl3 (aq) + 3H2 (g)
- A solution turns red litmus blue, its pH is likely to be
- A solution reacts with crushed-egg shells to give a gas that turns lime water milky. The solution contains
(b) Egg shells are made up of calcium carbonate, which on reaction with HCl liberates CO2 gas. C02 gas turns lime water (Ca(OH)2) to milky because it will form the calcium carbonate again, which is a white precipitate.
- 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HC1. If we take 20 mL of the same solution of NaOH, the amount of HC1 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) 16 mL
- Which one of the following types of medicines is used for treating indigestion?