Limestone is mainly made up of calcium carbonate, CaCO3. When it is heated, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide. But if u wanna know how it is formed? It's formed by erosion and weathering which is a type of 'Sedimentary Rock'. Erosion and weathering, carried rocks and end up at the sea or ocean. Which is the end way to go! At there, rocks combined by adding more and more just under the sea. The combination of rocks made it harden. First, it was just small little eroded particles end up at the sea. But at last, it became a very hard rock which isn't easy to break!
