A teacher provided acetic acid, water, lemon juice, an aqueous solution of sodium hydrogen carbonate and sodium hydroxide to students in the school laboratory to determine the pH values of these substances using pH papers. One of the students reported the pH values of the given substances as 3 , 12 , 4 , 8 and 14 respectively. Which one of these values is not correct? Write its correct value stating the reason.

The pH is a measure of how acidic/basic water is. The range goes from 0 - 14 , 7 being neutral. The pH of less than 7 indicates acidity, whereas a pH of greater than 7 indicates a base. The value of pH for water is not correct. The correct value of pH of water is 7 because it has an almost equal concentration of H + and OH - due to which it is neutral.

How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water which can be removed by it?

The chemical formula for washing soda is Na 2 CO 3 . 10 H 2 O . It is sodium carbonate with ten molecules of water of crystallization hence its chemical name is sodium carbonate decahydrate. It is formed by the recrystallization of sodium carbonate. Na 2 CO 3 sodium carbonate + 10 H 2 O → Na 2 CO 3 . 10 H 2 O washing soda Washing soda is a salt that is a resultant of the neutralization reaction between NaOH , a strong base and H 2 CO 3 , a weak acid. Therefore, it is a basic salt. Washing soda can remove the permanent as well as the temporary hardness of water. The carbonate ions from sodium carbonate react with calcium and magnesium ions present in the hard water and form an insoluble precipitate which turns the water soft.

Baking soda is a mixture of:

Sodium hydrogen carbonate and tartaric acid

Sodium carbonate and acetic acid

Sodium carbonate and tartaric acid

Sodium hydrogen carbonate and acetic acid

How is sodium hydroxide manufactured in industries? Name the process. In this process, a gas X is formed as a by-product. This gas reacts with lime water to give a compound Y, which is used as a bleaching agent in the chemical industry. Identify X and Y and write the chemical equation of the reaction involved.

Sodium hydroxide is manufactured by the electrolysis of a concentrated aqueous solution of sodium chloride. In electrolysis of aqueous sodium chloride solution gives chlorine gas and sodium hydroxide. 2 NaCl aq + 2 H 2 O l → Electrolysis 2 NaOH aq + Cl 2 g + H 2 g Sodium chloride water Sodium hydroxide Chlorine Hydrogen ( Brine ) ( Caustic soda ) Sodium hydroxide is industrially produced using the electrolytic chloralkali process. Gas X is chlorine gas and compound Y is calcium oxychloride (Bleaching powder). Cl 2 g + Ca OH 2 → CaOCl 2 + H 2 O Chlorine gas ( X ) Lime water Calcium Water oxychloride ( Y )

Dry hydrogen chloride gas does not change the colour of dry litmus paper. Why ?

Dry hydrogen chloride gas does not change the colour of dry litmus paper because only in an aqueous solution can acids dissociate into ions. Dry hydrogen chloride does not contain any hydrogen ions, so it does not show acidic behaviour and thus does not change the colour of dry litmus paper.

While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

While diluting an acid a lot of heat is liberated during the reaction as it is an exothermic reaction. While diluting an acid, the acid is preferred to be added to water rather than the water being added to the acid. Adding water to a concentrated acid releases a large amount of heat, which can cause an explosion and acid burns on the skin, clothing, and other body parts. So when a concentrated acid is added to water, heat is easily absorbed by a large amount of water. Hence, it is recommended that the acid should be added to water and not water to the acid.

The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses.

The salt commonly used to make tasty and crispy pakoras is baking soda which is chemically known as sodium hydrogen carbonate. Formation of NaHCO 3 : NaCl + H 2 O + CO 2 + NH 3 → NH 4 Cl + NaHCO 3 Two uses of Baking Soda are: It is used in the fire extinguisher. It is also used as an ingredient of antacid.