Solutions-Class-10-CBSE-NCERT-Science-Chapter-2-Acids, Bases and Salts

• Put the red litmus paper in all the test tubes, turn by turn. The solution which turns red litmus to blue will be a basic solution.
• The blue litmus paper formed here can now be used to test the acidic solution.
• Put the blue litmus paper obtained above in the remaining two test-tubes, turn-by-turn. The solution which turns the blue litmus paper to red will be the acidic solution.
• The solution which has no effect on any litmus paper will be neutral and hence it will be distilled water.

Curd and sour substances should not be kept in brass and copper vessels because these and other sour food-stuffs contain acids which can react with the metal of the vessel to form poisonous metal compounds which can cause food poisoning and affect our health adversely.

(a) Hydrogen (H₂) gas is liberated when an acid reacts with a metal.

(b) Example : Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂ ↑

(c) Test for H₂ gas : H₂ gas is not soluble in water, when passed through soap solution, it gets trapped into bubbles. Bring a burning candle near the soap bubble filled with gas. The soap bubble bursts and hydrogen gas burns with a pop sound.

As the end product is calcium chloride and the gas formed is carbon dioxide, the metal compound ‘A’ must be calcium carbonate. Therefore, the reaction between calcium carbonate and hydrochloric acid is

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

Calcium carbonate                 Calcium chloride

• H⁺ ions in aqueous solution are responsible for acidic character.
• HCl, HNO₃, etc. give H⁺ ions in water while alcohol and glucose do not give H⁺ ion in water. Therefore, alcohol and glucose do not show acidic character.

An acid molecule dissociates in an aqueous solution to produce H⁺ [or H₃O⁺] ions and corresponding anions A⁻. These free ions carry the electrical charge from one place to other hence, conduct electricity.

Dry HCl gas does not give H⁺ ions, so it does not show any acidic property. Hence it does not change the colour of dry litmus paper.

• While diluting an acid it is recommended that the acid should be added to water and not water to the acid because if water is added to concentrated acid to dilute it, then a large amount of heat is evolved at once. This heat changes some of the water to steam explosively which can splash the acid on one’s face or clothes and cause acid burns.
• Water is a strong heat absorber, so we add acid to it slowly and with steady stirring, releasing just a tiny quantity of heat at a time and preparing dilute acid.

An acid dissociates into hydronium ions [H₃O⁺] and anions when dissolved in water. When a solution is diluted, the volume of the solution increases but the number of ions remains the same, so concentration of hydronium ions [[H₃O⁺] per unit volume decreases.

Sodium hydroxide is a strong base. When excess base is dissolved in the solution of NaOH, the concentration of hydroxide ions [OH⁻] per unit volume increases due to dissociation of NaOH as well as the other base in aqueous solution.

NaOH(s)  \(\underrightarrow{H_2O}\)  Na⁺(aq) + OH⁻(aq)

Any other base

M(OH)_x (\underrightarrow{H_2O}\)  M⁺ + xOH⁻

• A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution.
• Since solution A has more hydrogen ion concentration, solution ‘A’ is acidic.
• Solution ‘B’ pH = 8 is basic.

More the concentration of H⁺ ions, higher the acidic nature of the solution.

Basic solutions have H⁺(aq) ions. But these are far less in number than OH^– ions that is responsible for their basic nature.

If the soil is too acidic (having low pH) then it is treated with materials like quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate). The purpose is to neutralise or decrease the acidity of the soil.

Bleaching powder.

Slaked lime Ca(OH)₂.

Sodium carbonate.

Solution of sodium hydrogen carbonate on heating gives sodium carbonate and carbon dioxide gas is evolved.

CaSO₄.\(\frac{1}{2}\)H₂O + 1\(\frac{1}{2}\)H₂O → CaSO₄.2H₂O

(d) 10

Bases turn red litmus blue and acids turn blue litmus red. Basic solution has a pH value more than 7. Since the solution turns red litmus blue, its pH is likely to be 10.

(b) HCl.

(d) 16 mL

(c) Antacid.

(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) +  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)

Alcohols and glucose contain hydrogen but are not categorised as acids because they do not ionise in the solution to produce [H⁺(aq)] ions and hence can't conduct electricity.

Activity : Set the apparatus as shown below and test for the solution of HC, alcohol and glucose.

Observation :

• The bulb glows when dil. HCl was added to beaker.
• The bulb did not glow when alcohol and glucose solution was added to the beaker.

Conclusion :

• Alcohol and glucose cannot form ions in solution and hence, can't conduct electricity.
• HCl solution forms [H⁺(aq)]ions and hence, can conduct electricity suggesting that electric current is carried through the solution by ions formed by them.

• Distilled water does not contain any ions and have no electrolytes. Hence, it does not conduct electricity.
• But in rain water, a small amount of electrolyte is present in the form of acids.
• These acids are produced due to dissolution of acidic oxides (e.g. SO₂, NO₂) in rain water and make it a better conductor of electricity.

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. It is the hydrogen ions that are responsible for the acidic behaviour.

(a) Neutral → Solution D with pH 7

(b) Strongly alkaline → Solution C with pH 11

(c) Strongly acidic → Solution B with pH 1

(d) Weakly acidic → Solution A with pH 4

(e) Weakly alkaline → Solution E with pH 9

The pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1

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.

pH of milk falls below 6 as it turns into curd due to the formation of lactic acid during this process. Lactic acid present in it reduces its pH value.

Alkaline medium does not allow milk to turn sour easily.

(a) Milk is made slightly alkaline so that it may 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 (POP) is chemically calcium sulphate hemihydrate (CaSO₄.\(\frac{1}{2}\)H₂O).

When it comes in contact with water it sets into a hard solid mass, called gypsum.

CaSO₄.\(\frac{1}{2}\)H₂O + 1\(\frac{1}{2}\)H₂O → CaSO₄.2H₂O (gypsum)

To prevent this, POP must be stored in moisture-proof containers.

A reaction in which an acid and base react with each other to give a salt and water is termed as neutralization reaction. In this reaction, energy is evolved in the form of heat.

Examples:

(i)

NaOH + HCL → NaCl + H₂O

(ii) During indigestion (caused due to the production of excess of hydrochloric acid in the stomach), we administer an antacid (generally milk of magnesia, Mg(OH)₂ which is basic in nature). The antacid neutralizes the excess of acids and thus gives relief from indigestion.

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Uses of washing soda :

• Washing soda is used in glass, soap and paper industries.
• It is used for removing permanent hardness of water.

Uses of baking soda :

• Baking soda is used as an antacid in medicines to remove acidity of the stomach.
• Baking soda is used for making baking powder (used in making cakes, bread, etc.).