21. change in temperature, change in color, or noticeable odor (after reaction has begun)
22. Picture... Wasn't so sure about this since no charge was given, so I suspected it has a charge of 2. If you have more information on this I would be glad to help, and I can't tell what is happening with the calculations given. It would be helpful if you can add spaces or put a line break.
23. An ion is an atom or molecule with an electric charge. It is formed when atoms lose or gain electrons. There are two types of ions: cations (net positive charge) and anion (net negative charge).
24. Picture
25. a) dinitrogen tetrahydride
b) phosphorus triiodide
c) lithium hydroxide
Answer: A chemical equilibrium exists when the rate at which reactants form products is the same as the rate at which products form reactants.
Explanation:
When the concentration of both the reactants and products do not change with time then it means the chemical reaction has reached to a state of chemical equilibrium.
For example, 
Therefore, we can conclude that a chemical equilibrium exists when the rate at which reactants form products is the same as the rate at which products form reactants.
The answer is (3) 7.64 days. The remaining mass of radon-222 is one fourth of the original mass, so two half lives have elapsed (because one half time one half equals one fourth). Each half life of radon-222 is about 3.82 days, so the total time elapsed is 3.82 * 2 = 7.64 days
