Acids, Bases And Salts

• We will first mark the three test tubes, A, B and C.
• Now, we will divide the red litmus paper into three parts and put a drop from each test tube on the individual red litmus paper.
• The test tube which has the base will turn the red litmus blue.
• Now, both the acid and distilled water (which is neutral) will have no effect on the litmus paper so in both cases the red litmus will remain red.
• To differentiate between the two, mix both the test tubes with the solution we have identified as the base and put a drop from these two solutions on red litmus again.
• The solution which will have the acid + base won't change the colour of the litmus because acid and base neutralise each other.
• The solution which will have the base _ distilled water will change the colour of red litmus to slightly blue.
• In this way, we can identify the three solutions.

Now, we know that the solution that turned the red litmus blue is the base. In this case, let's assume it is the solution in test tube B.



Now, to find out which solution is the acid and which one is the distilled water, we continue the experiment in the following way.

Curd and other sour substances contain acids. When these substances are kept in brass and copper vessels, the acid reacts with metal and liberated hydrogen gas and other harmful substances which may spoil these substances.

Hydrogen gas is usually liberated when an acid reacts with a metal.

For example: When zinc granules react with dilute sulphuric acid, then hydrogen gas is liberated and zinc sulphate solution is formed.

The evolved hydrogen gas can be tested by taking a burning candle near gas filled bubbles. If candle burns with pop sound, it confirms the evolution of hydrogen gas.

• Carbon dioxide is formed when dilute hydrochloric acid reacts with a metal carbonate (or metal hydrogen carbonate) producing effervescence.
• Hence, During the chemical reaction, the evolved gas is carbon dioxide.
• Since, one of the compounds formed during this reaction is calcium chloride(as given in question), it means the metal compound is calcium carbonate.

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + CO₂ (g) + H₂O (I)

Acids form hydrogen ions [H⁺ (aq) ions] in its aqueous solution which are responsible for their acidic characters. HCl and HN0₃ form hydrogen ions in aqueous solution hence, they show acidic characters in aqueous solution. While solutions of compounds like alcohol and glucose do not form hydrogen ions in the solution, hence, they do not show acidic character.

The presence of hydrogen ions (H⁺) or hydronium ions (H₃O⁺) in an aqueous solution of acid are responsible for conducting electricity. For example; hydrochloric acid (HCl) dissociates in aqueous solution and gives hydrogen ions [H ⁺(aq)] and chloride ions [Cl^-(aq)] which carry electric current. So, due to the presence of these ions, an aqueous solution of hydrochloric acid conducts electricity.

H₂O(l)+HCl(l) → H₃O⁺(aq) +Cl^-(aq)

• The colour of litmus paper changes only in the presence of ions like hydrogen (H⁺) or hydronium (H₃O⁺) ions.
• HCl can produce these ions only in the form of an aqueous solution.
• Hence dry HCl gas does not change the colour of dry litmus paper.

The process of dissolving an acid in water is a highly exothermic in which a large amount of heat is evolved. Care should be taken while mixing a concentrated acid with water. The acid must always be added slowly to water because evolved heat is easily absorbed by the large amount of water. If water is added to concentrated acid, the heat generated may cause the mixture to splash out and cause burns.

When the solution of an acid is diluted, the concentration of hydronium ions (H₃O⁺) per unit volume decreases.

The concentration of hydroxide ions (OH^-) would increase, when excess of base is dissolved in a solution of sodium hydroxide.

The pH value indicates the acidic and basic nature of a solution. The pH value of less than 7 indicates an acidic solution while greater than 7 indicates a basic solution. The pH value of a neutral solution is 7. Thus, in this case, the solution A with pH=6 is acidic and has more hydrogen ion concentration than the solution B (pH=8) which is basic.

Higher the concentration of H⁺(aq) ions in a solution, more acidic the solution will be.

The basic solution also has hydrogen ions [H⁺ (aq) ions]. However, they are basic because the concentration of hydrogen ions in them is less than the concentration of hydroxide ions [OH^- ions].

Quicklime, slaked lime and chalk are basic in nature. When they react with acids, neutralization reaction takes place. If the soil is too acidic and not good for cultivation, then to neutralize the acidity of soil, the farmer would treat the soil with these substances.

The common name of compound CaOCl₂ is bleaching powder.

Bleaching powder is produced by the action of chlorine on dry slaked lime.

Sodium carbonate (Washing soda) is used for softening hard water.

When a solution of sodium hydrogen carbonate is heated, it undergoes decomposition reaction to form sodium carbonate and carbon dioxide gas is liberated. The equation of the reaction involved is:

The solution which turns red litmus to blue is basic in nature. The pH value of base solution is greater than 7.

A solution reacts with crushed egg-shells to give a gas that turns lime water milky contains hydrochloric acid.

According to the Question, 10ml of NaOH neutralises 8ml Solution of HCl

∴ 1 ml of NaOH neutralises =

Therefore 20 ml of NaOH will neutralise=



16 ml of HCl solution will be required to neutralize the 20 ml of NaOH.

Antacids are a group of mild bases that have no toxic effects on the body. Being basic in nature, antacid reacts with excess of acid in the stomach and neutralizes it. This gives relief to the person concerned.

(a) H₂SO₄ (aq) + Zn (s) → ZnSO₄ (aq) + H₂ (g)

(b) 2HCl ₍aq₎ + Mg_(s)→ MgCl_{2 (aq)} + H_2(g)

(c) 3H₂SO₄ (aq) + 2Al (s) → Al₂(SO₄)₃ (aq) + 3H₂ (g)

(d) 6HCl (aq) +2Fe (s) → 2FeCl₃(aq) +3H₂(g)

(i) Take solution of dilute hydrochloric solution and glucose.

(ii) Two nails are fitted on a cork, and are placed it in a 100mL beaker.

(iii) Then nails are connected to the two terminals of a 6-volt battery through a bulb and a switch as shown in the figure.

(iv) Dilute HCl is poured in the beaker and current is switched on.

(v) Same experiment is performed with glucose solution.

(vi) It will be observed that the bulb glows in HCl solution and does not glow in the glucose solution.

HCl dissociate into H + (aq) and Cl -(aq) ions which are responsible for conducting electricity. On the other hand, the glucose solution does not dissociate into ions. Therefore, it does not conduct electricity.

That’s why, though compounds like glucose and alcohol contain hydrogen, they are not categorized as acids.

Distilled water is a pure form of water which does not contain ions, hence distilled water does not conduct electricity. Rain water, being an impure form of water, contains many ionic compounds like acids, hence it conducts electricity.

Acids form hydrogen ions [H⁺ (aq) ions] in its aqueous solution which are responsible for their acidic characters. In the absence of water, acids do not produce hydrogen ions and hence do not show acidic behavior.

(a) (i) The pH of neutral solution is 7. Thus, the neutral solution is D with pH=7.

(ii) The pH value of greater than 7 indicates the basic solution. Higher pH value represents strong alkaline solution. Thus, strong alkaline solution is C with pH=11.

(iii) The pH value of less than 7 indicates the acidic solution. Lower pH value represents a strong acidic solution. Thus, a strong acidic solution is B with pH=1.

(iv) weakly acidic: A (pH=4);

(v) weakly alkaline: E (pH=9)

(b) pH scale quantifies the concentration of hydrogen ion in a solution. The pH value increases with a decrease in hydrogen ion concentration. The solution having pH 11 will have the minimum hydrogen ion concentration than the solution having pH 1. Thus, the arrangement of solutions in the increasing order of their hydrogen ion concentrations as follows:

C < E <D <A <B
(pH 11) < (pH 9) <(pH 7)< (pH 4)< (pH 1)

When acid reacts with magnesium metal, hydrogen gas is produced which causes fizzing. Hydrochloric acid is a much stronger acid than acetic acid thus having greater number of hydrogen ions in it. Hence, during the chemical reaction with magnesium metal, hydrochloric acid will produce more hydrogen gas and the fizzing will be more vigorous in test-tube A.

The pH of fresh milk is 6. As milk turns into curd, lactic acid is formed due to which it become more acidic. Therefore, the pH of milk will reduce.

As the milk turns into curd, its pH will fall below 6 (it will become more acidic). This is because an acid (lactic acid) is produced when milk turns into curd.

(a) Since fresh milk is slightly acidic in nature, the milkman adds little baking soda to make the milk slightly alkaline. This will prevent the milk from getting sour due to lactic acid formation.

(b) For setting into a curd, the lactic acid, formed during curdling, needs to first neutralize the alkali present in the milk. Hence, the slightly alkaline milk takes longer time to set as curd.

Plaster of Paris should be stored in a moisture-proof container because it absorbs moisture (or water) to form a hard substance called gypsum which makes Plaster of Paris useless.

The reaction between an acid and a base to give a salt and water is known as neutralization reaction. That is:

Acid + Base → Salt + Water

For example:

(i) Sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl) to form sodium chloride (NaCl) and water:

(ii) Magnesium hydroxide reacts with hydrochloric acid to form magnesium chloride and water:

(a) Uses of Washing Soda:

(i) It is used for removing permanent hardness of water.

(ii) It is used in the manufacture of sodium compounds such as borax.

(b) Uses of Baking Soda:

(i) It is used in making baking powder. Baking powder is a mixture of baking soda and a mild edible acid called tartaric acid. When baking powder is heated or mixed in water, it releases CO₂ that makes cakes or bread fluffy.

(ii) It is used in soda-acid fire extinguishers.