Exercises
Q1. 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?
Answer:
Using only red litmus paper: (i) Dip the red litmus paper into each test tube one by one. (ii) The solution that turns the red litmus paper blue is the basic solution. (iii) For the remaining two test tubes, dip the now blue litmus paper (from the basic solution) into each. The solution that turns the blue litmus paper red is the acidic solution. (iv) The solution that does not change the colour of either red or blue litmus paper is distilled water (neutral).
Q2. Why should curd and sour substances not be kept in brass and copper vessels?
Answer:
Curd and other sour substances contain acids (like lactic acid). Brass and copper vessels can react with these acids. The reaction produces toxic salts (like copper sulphate or zinc salts) and hydrogen gas, which can contaminate the food and cause food poisoning. Therefore, they should not be stored in such metal vessels.
Q3. 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?
Answer:
Hydrogen gas (H2) is usually liberated when an acid reacts with a metal. Example: Zinc (Zn) reacts with dilute sulphuric acid (H2SO4) to produce zinc sulphate (ZnSO4) and hydrogen gas (H2). The reaction is: Zn(s) + H2SO4(aq) ? ZnSO4(aq) + H2(g). Test for hydrogen gas: Bring a burning candle or a burning splinter near the gas. Hydrogen gas burns with a 'pop' sound, confirming its presence.
Q4. 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.
Answer:
The gas that extinguishes a burning candle is carbon dioxide (CO2). Effervescence indicates the release of gas. Since one product is calcium chloride (CaCl2), compound A must be a carbonate or hydrogencarbonate of calcium. Therefore, compound A is calcium carbonate (CaCO3), also known as marble, chalk, or limestone. The balanced chemical equation is: CaCO3(s) + 2HCl(aq) ? CaCl2(aq) + H2O(l) + CO2(g)