Acids, Bases and Salts — ICSE Class 10 Chemistry Important Questions

Answer: (b) $H^{+}$H^+ ions as the only positive ions.

An acid dissociates in water to give hydrogen ions (existing as hydronium ions $H_3O^{+}$H_3O^+) as the only positively charged ions, which is the source of its acidic behaviour.

Answer: (b) 7.

In a neutral solution the concentration of $H^{+}$H^+ ions equals that of $OH^{-}$OH^- ions, giving a pH of exactly 7 at room temperature.

Answer: (b) Acid salt.

It is formed by the partial replacement of the replaceable hydrogen of sulphuric acid ($H_2SO_4$H_2SO_4); one replaceable H atom still remains, so it is an acid salt.

Answer: (d) pH 12.

The higher the pH above 7, the more alkaline the solution and the greater its $OH^{-}$OH^- ion concentration. Of the values given, pH 12 is the most alkaline, so it has the highest $OH^{-}$OH^- concentration.

Answer: (a) Both statements are true and R correctly explains A. $HCl$HCl ionises almost completely to give a high $H^{+}$H^+ concentration (strong), while $CH_3COOH$CH_3COOH ionises only slightly (weak); strength depends on the degree of ionisation.

Strong acid: an acid that ionises almost completely in water to give a high concentration of $H^{+}$H^+ ions; strength refers to the degree of ionisation (e.g. $HCl$HCl, $H_2SO_4$H_2SO_4, $HNO_3$HNO_3).

Concentrated acid: an acid containing a very small amount of water (a large amount of acid per unit volume); concentration refers to the amount of water present, not to ionisation. An acid can be strong yet dilute, or weak yet concentrated.

Neutralisation is the reaction between an acid and a base to form salt and water only, in which the $H^{+}$H^+ ions of the acid combine with the $OH^{-}$OH^- ions of the base to form water.

Ionic equation:

$H^{+} + OH^{-} \rightarrow H_2O$H^+ + OH^- → H_2O

Example: $HCl + NaOH \rightarrow NaCl + H_2O$HCl + NaOH → NaCl + H_2O.

A salt is an ionic compound formed when the replaceable hydrogen of an acid is wholly or partially replaced by a metal or an ammonium ($NH_4^{+}$NH_4^+) radical.

Three types:

1. Normal salt: all the replaceable hydrogen of the acid is replaced, e.g. $NaCl$NaCl (sodium chloride).

2. Acid salt: only part of the replaceable hydrogen is replaced, so some H remains, e.g. $NaHCO_3$NaHCO_3 (sodium hydrogen carbonate).

3. Basic salt: contains hydroxide or oxide in addition to the normal salt, e.g. basic lead nitrate, $Pb(OH)NO_3$Pb(OH)NO_3.

An insoluble salt is prepared by precipitation (double decomposition): mix solutions of two suitable soluble salts, one supplying the cation and the other the anion.

Method: Add lead nitrate solution to dilute sulphuric acid (or sodium sulphate solution). A white precipitate of lead sulphate forms at once. Filter to collect the precipitate, wash it with distilled water to remove soluble impurities, and dry it between filter papers.

Equation:

$Pb(NO_3)_2 + H_2SO_4 \rightarrow PbSO_4\downarrow + 2HNO_3$Pb(NO_3)_2 + H_2SO_4 → PbSO_4 + 2HNO_3

Copper is a salt of the insoluble base copper(II) oxide, so it is prepared by the reaction of the insoluble base (or carbonate) with a dilute acid.

Steps:

1. Warm dilute sulphuric acid and add copper(II) oxide (black) in small amounts, stirring, until a little remains undissolved (this ensures all the acid is used up).

2. Filter to remove the excess undissolved oxide; the blue filtrate is copper sulphate solution.

3. Evaporate the filtrate to the point of crystallisation and allow it to cool slowly; blue crystals of $CuSO_4\cdot 5H_2O$CuSO_4· 5H_2O separate. Filter, wash and dry them between filter papers.

Equation:

$CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$CuO + H_2SO_4 → CuSO_4 + H_2O

(a) The pH of a solution is a number, on a scale of 0 to 14, that indicates the concentration of hydrogen ions and hence the acidity or alkalinity of the solution; a lower pH means a higher $H^{+}$H^+ concentration.

(b) Moving from a strong acid to a strong alkali, the pH rises from about 0-1 (strongly acidic), through 7 (neutral), up to about 13-14 (strongly alkaline).

(c) (i) Strong acid: $HCl$HCl, pH about 1. (ii) Weak acid: $CH_3COOH$CH_3COOH (acetic acid), pH about 4-5. (iii) Strong alkali: $NaOH$NaOH, pH about 13-14.

(d) In water, hydrogen chloride ionises to furnish $H^{+}$H^+ (as $H_3O^{+}$H_3O^+) ions, which make the solution acidic. In toluene, a non-polar solvent, $HCl$HCl does not ionise and remains as molecules, so no $H^{+}$H^+ ions are produced and the solution shows no acidic properties. This proves acidity is due to $H^{+}$H^+ ions.

(i) $NaCl$NaCl — Method C (neutralisation/titration): sodium chloride is a soluble salt of a strong acid and strong alkali.
$NaOH + HCl \rightarrow NaCl + H_2O$NaOH + HCl → NaCl + H_2O

(ii) $FeSO_4$FeSO_4 — Method B (displacement of hydrogen from acid by an active metal):
$Fe + H_2SO_4 \rightarrow FeSO_4 + H_2\uparrow$Fe + H_2SO_4 → FeSO_4 + H_2

(iii) $FeCl_3$FeCl_3 — Method A (direct combination of the elements): iron combines directly with chlorine (an active non-metal) to give the higher chloride.
$2Fe + 3Cl_2 \rightarrow 2FeCl_3$2Fe + 3Cl_2 → 2FeCl_3

(iv) $PbCl_2$PbCl_2 — Method E (precipitation/double decomposition): lead chloride is insoluble.
$Pb(NO_3)_2 + 2HCl \rightarrow PbCl_2\downarrow + 2HNO_3$Pb(NO_3)_2 + 2HCl → PbCl_2 + 2HNO_3

(v) $ZnCO_3$ZnCO_3 — Method E (precipitation/double decomposition): zinc carbonate is insoluble.
$ZnSO_4 + Na_2CO_3 \rightarrow ZnCO_3\downarrow + Na_2SO_4$ZnSO_4 + Na_2CO_3 → ZnCO_3 + Na_2SO_4

(i) Lemon juice (pH 2) is the most acidic. Universal indicator turns red in a strongly acidic solution.

(ii) Baking soda solution (pH 9) and sodium hydroxide solution (pH 13) are alkaline, as both have pH greater than 7.

(iii) Higher pH means lower $H^{+}$H^+ concentration, so increasing $H^{+}$H^+ concentration is: $NaOH\,(13) <$NaOH\,(13) < baking soda$\,(9) <$\,(9) < water$\,(7) <$\,(7) < lemon juice$\,(2)$\,(2).

(iv) The pH of the sodium hydroxide solution decreases (moves towards 7). The $H^{+}$H^+ ions from the acidic lemon juice neutralise the $OH^{-}$OH^- ions of the alkali ($H^{+} + OH^{-} \rightarrow H_2O$H^+ + OH^- → H_2O), reducing the alkalinity and lowering the pH.