weigh the powder CaCO_3.
There may be a discrepancy between the scales. Weight may not be required. Then pour the powder down CaCO_3 vitro by using cones to help pour the mouth tube is small. This may cause the powder CaCO_3 attached to the cone or in a beaker. It was a mixture does not meet the requirements and may be inaccurate cup water and bring the mixture of alum solution volume is concentrated at fault. Powder particles are sinking slow down or speed up to reality. And the mixture has precipitated the crash. The experimental subjects were not carefully pour the powder in the powder CaCO_3 CaCO_3not swept out of the beaker and powder CaCO_3not knock down cones attached edge down until the test tube. Make solid particles that are less than scheduled. And may measure the volume of water and make an alum solution mixture is concentrated false start. The concentration of sediments and less precipitation than it should be.