Video 2-7. Red cabbage pigment as a pH indicator (source).
Many natural dyes that give the color to flower petals are acid-base indicators. You may try vinegar (aqueous acetic acid) and household ammonia on various flowers of different colors. Maybe you will be amazed how the colors change when some flowers are sprayed with ammonia or vinegar. If you decide to try such experiments, use regular white vinegar and regular household ammonia. It is advised to avoid using scented ammonia solutions such as "lemon ammonia" and those containing detergents such as "foaming ammonia". The cheapest no-name brands are usually best suited for acid-base experiments with flowers. Note that although vinegar is a weak acid and ammonia is a weak base, both have a pretty strong smell. Avoid inhaling vapors of vinegar and especially ammonia when doing experiments. Carrying out the tests outdoors is best. You may spray the flowers or immerse them into your ammonia solution or vinegar. Important: the concentration of NH3 in household ammonia varies in a broad range of 0.5% to 10% depending on the brand. The more concentrated household ammonia smells stronger. Do not use concentrated NH3 (~30%) that has a pungent odor and is a strong eye irritant! 2.5.8. Exercises.
1. An electrolyte is a chemical compound that (a) readily gives off electrons; (b) dissolves in water to produce
a solution that conducts electricity; (c) always dissociates to give H+
; (d) always dissociates to give OH-
2. Define degree of dissociation (α) and strong, medium strength, and weak electrolytes.
3. Almost all salts and acids are strong electrolytes. True or false? Answer
4. Barium sulfate (BaSO4
) is very poorly soluble in water and is therefore a weak electrolyte. True or false? Answer
5. Identify strong, medium strength, and weak electrolytes among the following compounds: Ca(OH)2
, HCl, NaNO3
, NaOH, Ba(OH)2
. [Hint: Refer to Table 4 and text above]
6. Describe the process of dissolution and dissociation of NaCl. Draw sketches to illustrate the process.
7. Explain why a molecule of water is a dipole. Do you think a molecule of HBr is a dipole? [Tip: Consider the shape of HBr and the type of chemical bond between the Br and H atoms]
8. On dissociation in water, dibasic acids (a) lose both of their protons simultaneously; (b) lose the first proton much more readily than the second one; (c) lose only one proton and never both; (d) lose the protons step-wise, with the second proton coming off almost as easily as the first one. Answer
9. On dissociation in water dibasic bases such as Ca(OH)2
(a) lose both of their OH-
ions simultaneously; (b) lose the first OH-
much more readily than the second one; (c) lose only one OH-
and never both; (d) lose the OH-
ions step-wise, but the second OH-
comes off almost as easily as the first one. Answer
10. Provide an explanation for the difference in the correct answers to questions 8 and 9 above.
11. The pH value of an aqueous solution went up by two units after some chemical treatment. Consequently, the concentration of (a) H+
decreased by a factor of 2; (b) OH-
decreased by a factor of 2; (c) H+
increased by a factor of 2; (d) H+
decreased by a factor of 20; (e) H+
decreased by a factor of 100; (f) H+
increased by a factor of 100; (g) OH-
increased by a factor of 10; (h) OH-
increased by a factor of 100. Answer
12. Write chemical equations for electrolytic dissociation of Na2
, HBr, KOH, Mg(NO3
, and H2
. Indicate in the equations that the dissociation is reversible and specify what side of the equation the equilibrium is shifted to.
13. How is OH-
produced in aqueous solutions of ammonia? Is NH3
a strong base?
14. A laboratory technician distilled about 100 mL of water, placed it immediately in a clean 500 mL glass beaker, and measured its pH. The pH was 7, as expected for pure water. The technician then covered the beaker with a sheet of clean paper towel to protect the water from dust and went for lunch. When he returned to the lab an hour later, he decided to measure the pH again. To his surprise, the new pH measurement was 6, indicating that the water became slightly acidic. He thought that something went wrong and repeated the distillation the following day. The freshly distilled water had a pH of 7, as it was supposed to. As before, he poured the water into a clean beaker, covered it with a paper towel, and left for lunch. This time, he locked up the lab to make sure no one could enter it to play a joke on him by somehow slightly acidifying the distilled water. When he came back from lunch and tested the pH of the water, it was 6 again. The technician was totally puzzled. Can you explain the reproducible slight increase in the acidity of the distilled water on standing? Answer
15. Would you expect the following bonds to be prone to homolytic or heterolytic cleavage: (a) F-H; (b) Si-H; (c) F-F; (d) O-H; (e) C-H; (f) Te-H. Answer