When two or more chemicals are mixed, changes like heat production, gas formation, or precipitate formation provide evidence that a chemical reaction has occurred, forming new materials. The document describes experiments mixing calcium chloride, baking soda, and citric acid in water. It was observed that calcium chloride and baking soda mixtures produced carbon dioxide gas and calcium carbonate precipitate. Citric acid reacted similarly with baking soda. Evaporation of filtrates showed the presence of salt crystals, indicating the solutions contained ionic compounds.