Which of the following substances will most likely ionize completely when dissolved in water?

• A. HBr
• B. KCl
• C. CaBr2
• D. MgCl2

Answer: (A)

Verified step by step guidance

1. Understand that substances that ionize completely in water are strong electrolytes, typically strong acids, strong bases, or soluble ionic salts.
2. Identify the nature of each substance: HBr is a strong acid, while KCl, CaBr2, and MgCl2 are ionic salts.
3. Recall that strong acids like HBr dissociate completely into ions in aqueous solution, for example: \(\text{HBr} \rightarrow \text{H}^+ + \text{Br}^-\).
4. Recognize that ionic salts such as KCl, CaBr2, and MgCl2 also dissociate into their respective ions, but their degree of ionization depends on their solubility and strength as electrolytes.
5. Compare the substances and conclude that HBr, being a strong acid, will ionize completely in water, whereas the salts may dissociate but are not classified as ionizing completely in the same sense as strong acids.