Question

A precipitate forms in a double-displacement reaction when

a. hydrogen gas reacts with a metal.
b. positive ions combine with negative ions.
c. water boils out of the solution.
d. a gas escapes.

Answer 1

In a double-replacement reaction, a precipitate forms when the cations from one reactant combine with the anions from the other reactant to form an insoluble ionic compound.

Step-by-step explanation:

In a double-replacement reaction, a precipitate forms when the cations from one reactant combine with the anions from the other reactant to form an insoluble ionic compound. This reaction occurs in aqueous solutions, and one of the products is usually a solid precipitate, a gas, or a molecular compound like water. For example, when aqueous solutions of potassium iodide and lead (II) nitrate are mixed, a yellow precipitate of lead iodide forms:

2KI (aq) + Pb(NO3)2 (aq) → PbI2(s) + 2KNO3(aq)

So, the correct answer to the question is b. positive ions combine with negative ions.