What is the mass of one milliliter of water?

• A. 0.9 gram
• B. 1 gram
• C. 1.1 gram
• D. 10 gram

Answer: (B)

Mass relates to how much stuff is packed into a given volume, through density. Water has a density of about 1 gram per milliliter, meaning each milliliter of water has roughly one gram of mass. Because density is mass per volume, multiplying by a volume of 1 milliliter gives a mass of about 1 gram. In fact, under the traditional definition tied to water at 4°C, 1 milliliter is defined as exactly 1 gram. So the correct takeaway is that a milliliter of water masses about one gram. The other numbers would correspond to densities quite different from that of water, which isn’t how dense water is.