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When determining the color of a precipitate, the first thing we need to know is what the precipitate is. We need to look at the ions present in the reactants and see if there is a combination of ions that will result in the formation of an insoluble product.
Let's first look at CaCl2 + H2SO4. The possible products are CaSO4 and HCl because we have to have one cation and one anion and this is the only other possible combination other than the reactants.
HCl is soluble in water, but CaSO4 is only slightly soluble which means that there will be enough precipitate to see with the nake eye. (see link).
For the second reaction, we have CaCl2 and K2CrO4 which are both soluble. The possible products are CaCrO4 and KCl, of which only the former is soluble. Many compounds containing transition metals are colored and CaCrO4 is no exception as a bright yellow solid.
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