This is an example of a double replacement or precipitation reaction. The solutions of potassium carbonate and calcium nitrate both contain dissolved ions so they will appear as clear, colorless solutions. When the two solutions are combined the calcium ions will combine with the carbonate ions to form an insoluble precipitate, which will appear as a white cloudiness. Potassium ions and nitrate ions will remain dissolved in the solution. They're called "spectator ions" because they're present as both reactants and products - they don't participate in the chemical reaction.