If tap water containing calcium ions are used to dissolve reactants it will have effects on the chemical reaction that occur.
Tap water is typically consist of many ions which originate from the soil, water pipes, etc. If tap water is used for mixture of reactants, the ions present in the water will interfere with the chemical reaction. Deionized water on the other hand has have all its ions removed, so it is a suitable choice for mixing reactants in the laboratory.
