How do you know what products will result from a chemical reaction such as a double replacement reaction involving sodium carbonate plus sulfuric acid?

Sodium carbonate ionizes to yield two ions. What are they? Hint: write the formula for sodium carbonate.

Sulfuric acid also ionizes to give two ions. What are they? Hint: write the formula for sulfuric acid.

Now, you have anions (negatively charge ions) and cations (positively charged ions). You could put them back together the way they are, or you could mix them up. Try mixing them up.

Normally, the possible species would be in equilibrium. However, sometimes something happens that will "drive" the chemical reaction to one side or the other. In this case, one of the possible products can decompose. If you drive off one product, the reaction will be forced to completion.