Step 3: Build a model of the reactant. a) Use gumdrops and toothpicks to build a model of CaCO3. b) Select one color of gumdrop for calcium, a second color for carbon, and a third color for oxygen. Be sure to include a key of your chosen color scheme with your model. c) Use the model below as your guide. The lines represent the bonds. Step 4: Build models of the products. a) Use gumdrops and toothpicks to build a model of CaO and a model of CO2. b) Be sure to use the same colors of gumdrops for calcium, carbon, and oxygen as you did in Step 3. Include a key of your chosen color scheme with your models. d) Use the models below as your guide. The lines represent the bonds. Step 5: Type one to two paragraphs that describe your models and explain the conservation of mass in the chemical reaction. will give brainliest i have 2 minutes left