1. The problem statement, all variables and given/known data In a tube filled with a dissolved substance, there is a probability for each molecule to disappear in time dt equal to kdt. Write an equation for dc/dt for the case in which c is not spatially uniform. 2. Relevant equations When concentration is spatially uniform dc/dt=-kc. k is a constant. 3. The attempt at a solution Only hint the text gives is that the answer is said to be of reaction-diffusion form. This is the first part of a multi-part problem and as the latter parts don't use the non spatially uniform equation it makes me think it should be something I could just write down. Just not sure how to think about this question.