According to the given information, the gradient of the graph is steadily decreasing because the calcium carbonate is being consumed during the reaction, it means that every time there is less calcium carbonate that before.
Also, as long as the amount of calcium carbonate is consumed, the rate of reaction decreases, so the gradient steadily decreases too.
If powdered calcium carbonated was used, the graph would probably have steeper slopes, this is because powdered calcium carbonate has a smallest particle size than the lumps, making the reaction be faster.
