Which statement best describes a key assumption of serial dilutions?

• A. Each step reduces concentration by the same factor
• B. Temperature must be exactly 20°C
• C. The solvent must be water
• D. Dilutions only work for solids

Answer: (A)

The main idea being tested is that in a serial dilution, each step reduces the concentration by the same factor. This happens because you transfer a fixed volume from one tube into a fixed volume of diluent in the next tube, creating a constant dilution ratio every time. For example, if you perform a 1:10 dilution, you take 1 part sample and add it to 9 parts diluent; the next step uses the same 1:10 ratio, so the concentration becomes a constant fraction of the previous step (and the overall concentration drops by that same factor with each step). This consistent factor makes calculations predictable: after k steps, the concentration is the original concentration multiplied by the dilution factor raised to the k-th power.

Temperature is not the defining assumption; while temperature can influence volumes slightly, the standard concept relies on consistent transfer volumes and diluent, not a specific temperature. The solvent being water is also not required; any suitable diluent can be used, as long as the dilution factor is applied consistently. Dilutions are not limited to solids; they apply to solutions and suspensions of solutes in a liquid diluent, including microbial suspensions or chemical solutions.