1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Put red litmus paper in all test tubes. The solution which turns red litmus to blue will be a basic solution.
Put this blue litmus paper in the remaining two test-tubes. The solution which turns blue litmus paper to red is acidic.
The solution which has no any colour change is neutral (distilled water).
1. Why should curd and sour substances not be kept in brass and copper vessels?
Curd and sour substances contain acids which can react with the metals such as brass and copper to form poisonous metal compounds.
2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Hydrogen (H2) gas is liberated.
Illustration: Take some zinc granules in a test tube. Add 5 mL dilute hydrochloric acid slowly. Zinc reacts with hydrochloric acid and hydrogen gas is evolved.
Test for H2 gas: When H2 gas is 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.
3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
As the end product is calcium chloride (CaCl2) and the gas formed is carbon dioxide, the metal compound A is calcium carbonate (CaCO3). So the reaction is
CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
1. Why do HCl, HNO3, etc show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
H+ ions in aqueous solution are responsible for acidic character. HCl, HNO3, etc. give H+ ions in water. But alcohol and glucose do not give H+ ion in water.
2. Why does an aqueous solution of an acid conduct electricity?
It is due to the presence of charged particles (ions).
3. Why does dry HCl gas not change the colour of the dry litmus paper?
Dry HCl gas does not give H+ ions. Therefore, it does not change the colour of dry litmus paper. HCl gives H+ ions only in presence of water.
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
If water is added to concentrated acid, large amount of heat is evolved at once. As a result, some water changes to steam explosively. It can splash the acid on face or clothes resulting in acid burns.
5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
On dilution, number of hydronium ions per volume decreases. Thus concentration decreases.
6. How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide?
The concentration of hydroxide ions increases but only up to a limit. After this, concentration becomes constant.
1. You have two solutions A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
pH value less than 7 is acidic and more than 7 is basic. Solution A has more H+ ion concentration. So it is acidic and solution B is basic.
2. What effect does the concentration of H+ (aq) ions have on the nature of the solution?
More the concentration of H+ ions, higher the acidic nature of the solution.
3. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Basic solutions have H+ (aq) ions. But these are less than OH– ions that is responsible for basic nature.
4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
Under the acidic condition (low pH) of the soil.
1. What is the common name of the compound CaOCl2?
2. Name the substance which on treatment with chlorine yields bleaching powder.
Slaked lime [Ca(OH)2].
3. Name the sodium compound which is used for softening hard water.
4. What will happen if a solution of sodium hydrogen carbonate is heated. Give the equation of the reaction involved?
Sodium hydrogen carbonate (NaHCO3) on heating gives sodium carbonate (Na2CO3) and CO2 is evolved.
5. Write an equation to show the reaction between plaster of Paris and water.
CaSO4. ½ H2O +1½ H2O → CaSO4.2H2O
(Plaster of Paris) (Gypsum)