Viva Vice: pH of Samples | Lab Manuals for Class 10 PDF Download

Q.1. What is the pH of pure water at 25°C (298 K)?

The pH of pure water at 25°C is equal to 7.

Q.2. On opening the soda water bottle the dissolved CO2 comes out, would the pH of the  solution increase or decrease as the gas comes out ? Explain your answer.

The pH of the solution will increase as the gas comes out and the solution becomes less acidic.

Q.3. Solid sodium bicarbonate was placed on a strip of pH paper. What colour would you observe on a strip of pH paper?

The colour on a strip of pH paper would not change. (Since, an acid or base shows their nature in aqueous state only and not in solid or pure state, hence solid sodium bicarbonate does not affect the colour of pH paper).

Q.4. A student was given four unknown colourless samples labelled A, B, C and D and asked to test their pH using pH paper. He observed that the colour of pH paper turned to light green, dark red, light orange and dark blue with samples A, B, C and D respectively. Which sample is most acidic?

Sample B

Q.5. Four students A, B, C and D measured the pH values of water, lemon juice and sodium bicarbonate solution. What is the correct decreasing order of the pH values?

The correct decreasing order of pH values is lemon juice < water < sodium bicarbonate.

Q.6. Which will be more acidic and why?
(i) A solution with pH value of 6.0
(ii) A solution with pH value of 5.0

[H⁺] in solution with pH 5.0 = 10^-5 M
[H⁺] in solution with pH 6.0 = 10^-6 M
As 10^-5 > 10-6 hence, solution with pH 5 is more acidic.

Q.7. Two solutions A and B were found to have pH value of 6 and 8 respectively. What inference can be drawn from this?

pH value is 6, it means solution A is an acid. pH value is 8, it means solution B is base.

Q.8. Reena place one drop of each of hydrochloric acid, sodium hydroxide and lemon juice on a strip of red litmus paper one by one, in which case would you observe change of colour of litmus paper?

(NaOH) sodium hydroxide.

Q.9. During an experiment a student accidently spilled hydrochloric acid on his hand. His teacher applied dilute solution of another chemical substance which relieved him from the burning sensation. What could possibly that dilute solution?

The applied solution could be a basic solution like Mg(OH)2. It reacts with acid to form neutral salt.

Q.10. Choose strong acid and strong base from the following: CH₃COOH, NH₄OH, KOH, HCI

Strong acid - HCI, Strong base - KOH

Q.11. A student test a sample of drinking water and reports its pH as 6 at room temperature, which kind of compound do you think may have been added to water?

Acidic compound like bleaching powder.
Bleaching powder in water liberates chlorine which makes its pH 6 (i.e. the solution becomes acidic).

Q.12. A student observed that the colour of pH paper changes to green when she dipped it in water. She added a few drops of concentrated hydrochloric acid to the water. What change in colour she would observe on pH paper?

The colour of pH paper would turn to light red.
(Reason: The colour of pH paper is green in neutral medium while it is red in acidic medium).

Q.13. What effect does the concentration of H⁺ (aq) have on the acidic nature of the solution?

Higher the concentration of H⁺ ions in a solution, more acidic is the solution.

Q.14. How is the concentration of hydronium ions (H₃O⁺) affected, when a solution of an acid is diluted?

On dilution of an acid, the concentration of hydronium ions (H₃O⁺) per unit volume decreases.

Q.15. What according to you should be the pH of dil. HCI and dil. NaOH solution? Observe and explain your findings.

pH of dilute HCI is found to be 2 and pH of dilute NaOH is 14. Solutions with pH = 0 to 3 are strongly acidic, so dilute HCI is a strong acid. And the solution with pH = 12 to 14 are strongly basic, so dilute NaOH solution is a strong base.

Q.16. What do you mean by pH?

The pH is the measure of the acidic (or basic) power of a solution. It is a scale for measuring hydrogen ion concentration in a solution. Higher the hydronium ion concentration in the solution, lower is their pH values.