If a solid forms when two ionic solutions are mixed together in a test tube, what does this indicate?

If a solid forms when two ionic solutions are mixed together in a test tube, it is an indication of a chemical change

The precipitaion os the solid indicates that a new product has been formed which is not soluble.

When a solid ionic compound is put in water, the compound is dissolved by ionizing (the ions separate and remain in aqueous phase). If you combine two ionic solutions and see a solid precipiate is becasue the ions have combined and form a new compound which is not soluble.