Lemon juice contains citric acid and tamarind contains tartaric acid. These acids react with a basic layer of copper carbonate on the surface to form soluble salts which are easily removed and surface shines.

pH decreases.
It is due to the formation of lactic acid in the curd. Since, acids generally have a low pH value, when milk changes to curd its pH will decrease.

(i) Alkali
(ii) Acid
(iii) Neutralization reaction
(iv) Base

B will be basic nature, with pH = 8.

(i) Used in bleaching cotton, Wood pulp, clothes.

(ii) Used as oxidising agent in chemical industries.

(iii) Used as disinfectant to kill germs in drinking water.

The chemical formula of washing soda is \(Na_2CO_3.10H_2O.\)
Washing soda.  On heating, baking soda gives sodium carbonate, carbon dioxide and water.  Consequently, the recrystallisation of sodium carbonate gives washing soda.  The chemical equations involved in the process are:

\(\underset{Baking\ soda}{2NaHCO_3}\rightarrow \underset{Sodium\ carbonate}{Na_2CO_3}+\underset{Carbon\ dioxide}{CO_2}+\underset{Water}{H_2O}\)

\(Na_2CO_3\rightarrow10H_2O\xrightarrow {Recrystallization}\underset{Washing\ soda}{Na_2CO_3.10H_2O}\)
Washing soda is used in glass, soap and paper industries.

The chemical formula of baking powder is sodium hydrogen carbonate \((NaHCO_3).\) while, that of washing soda is sodium carbonate carbonate on heating gives \(CO_2\) gas which will turn lime water milky whereas no such gas is obtained from sodium carbonate.

\(2NaHCO_3\xrightarrow {Heat}Na_2CO_3+H_2O+CO_2\)
\(NA_2CO_3.10H_2O\xrightarrow {Heat}Na_2CO_3+10H_2O\)

(i) Reaction occurs in test tube A
2HCl(aq) + Mg(s) \(\rightarrow\) MgCl₂(aq) + H₂(g)
(ii) Reaction occurs in test tube B
2CH₃COOH (aq) + Mg(s) \(\rightarrow\) (CH₃COO)₂Mg(aq) + H₂(g)
Fizzing occurs in both the test tubes due to the evolution of H₂ gas but it is faster in test tube A because of the greater extent of dissociation of HCI (strong acid) as compared to B as CH₃COOH is not completely dissociated (weak acid).

When sodium hydrogen carbonate is heated, carbon dioxide gas is produced alongwith sodium carbonate.
2NaHCO₃ (s)                        \(\xrightarrow[]{Heat}\) Na₂CO₃(s)            + H₂O(l)  +  CO₂(g) \(\uparrow\)
Sodium hydrogen carbonate          Sodium carbonate  Water        Carbon dioxide

Acidic character of a compound is due to the presence of replaceable H⁺ ions, which are released in aqueous solution and are responsible for acidic properties. As HCI, HNO₃ etc have replaceable H⁺ ions, they show acidic character while glucose and alcohol do not have replaceable H⁺ ions, hence, no acidic character is shown by them.

(i) If H⁺ ion concentration is more, pH will be less and vice versa or pH of a solution is inversely proportional to H⁺ ion concentration or pH 52 log (H⁺).
(ii) This solution is neutral.
(iii) 1MNaOH solution.
(iv) When the pH in the mouth is below 5.5, bacteria present in the mouth produce acids by degradation of sugar and corrode the tooth enamel. It can be prevented by using tooth pastes which are generally basic.
(v) As hydronium [H₃O]⁺ ion.

(i) Universal indicator is a mixture of many different indications (or dyes) which give different colours at different pH values of the entire pH scale. The colour produced by the universal indicator is used to find the pH value of acid or base by matching the colour with the colours on pH colour chart.
(ii) Solution A is acidic and will turn litmus solution from blue to red.
Solution B is basic and will turn phenolphthalein from colourless to pink.
(iii) Green colour will be obtained.

(a) Concentrated sulphuric acid is called the king of chemicals as it is a very strong acid and is highly corrosive.

(b) The three chemical reactions that take place during the conversion of sulfur dioxide to sulphuric acid in the 'Contact Process' are given below.

Step I: Sulphur dioxide is mixed with air and passed over vanadiumpent (V) oxide catalyst heated to \(450^oC\) to form sulphur trioxide.

\(\underset{Sulphur\ dioxide}{2SO_{2(g)}}+\underset{Oxygen}{O_{2(g)}}\underset{\rightleftharpoons }{V_2O_5}\underset{Sulphur\ trioxide}{2SO_{3(g)}}+Heat\)

Step II : Sulphur trioxide is dissolved in concentrated sulphuric acid to form a thick fuming liquid called oleum.

\(\underset{Sulphuric\ acid}{H_2SO_4(l)}+\underset{Sulphur\ trioxide}{SO_{3(g)}}\begin{matrix} V_2O_5 \\ \rightleftharpoons \\ \end{matrix}\underset{Oleum}{H_2S_2O_{7(l)}}\)

Step III: Oleum is diluted with a calculated quantity of water to obtain sulphuric acid.

\(H_2S_2O_{7(l)}+\underset{Water}{H_2O_{(l)}}\rightarrow\underset{Sulphuric\ acid}{2H_2SO_{4(aq)}}\)

(c) If water is added to sulphuric acid then a large amount of heat is evolved.  This changes water to steam, which can splash the acid.  Hence, it is dangerous.

Sodium hydroxide (NaOH) is a commonly used base and is hygroscopic, that is, it absorbs moisture from the atmosphere and becomes sticky.  The acidic oxides react with base to give salt and water. The reaction between NaOH and \(CO_2\) can be given as \(2NaOH+CO_2\rightarrow Na_2CO_3+H_2O\)

(a) Baking soda is sodium hydrogen carbonate.  On heating, it is converted carbonate.  On heating, it is converted into sodium carbonate which is bitter to taste.

\(2NaHCO_3\xrightarrow{Heat}Na_2CO_3+H_2O+CO_2\)

(b) Baking soda can be converted into baking powder by the addition of appropriate acid to it.

(c) The role of tartaric acid is to neutralise sodium carbonate and cake will not taste bitter.

(i) Bulb A and B do not glow but bulb C glows.
(ii) Glucose and alcohol solutions do not conduct electricity as they do not have ions. Dil. HCI contains ions so the flow of ions is responsible for the flow of current
(iii) After replacement, bulb glows in B as NaOH solution contains ions (Na⁺ and OH^- ions).

(i) The acid is to be added slowly in water to prevent the mixture to be splashed. The reaction is highly exothermic, therefore, constant cooling should be done.
(ii) The process is called dilution.

(i) (a) A solution of glucose will not conduct electricity because it does not have ions.
(b) Oil. HCl will conduct electricity because it produces H⁺ ions in water.
(ii) HCI is stronger because it gives rise to more H⁺ ions than acetic acid.
(iii) The strength of the acid decreases.

(i) Increase in the strength of alkali. So the nature of solution will be basic.
(ii) Hydroxide ion or hydroxyl ion or OH^- ion.
(iii) A paper impregnated with the universal indicator is generally used to measure the pH of a solution.

(i) Alkali. e.g., NaOH (Sodium hydroxide).
(ii) Tooth decay starts when the pH of the mouth is lower than 5·5.It can be prevented by using tooth pastes which are generally basic.
(iii) Bee-sting has acid that causes pain and irritation. Baking soda is alkaline, neutralises the acid and gives relief.