What's compound streak mean in science

Streak is the color a solid substance leaves behind when it is used to scratch a white porcelain tile. This is often different than the apparent color of the substance upon first visual examination, and it is a more accurate way to identify a substance (usually rocks and minerals) because while external color often varies from sample to sample, streak color is consistent.

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Rate law for this reaction israte of the reaction = k[a]²[b].second order in a  means concentration of a in power of two, and  first order in b means concentration of b in power of one.