Which weighs more milk or water?

Milk weighs more because it is more dense.

Answer:

Milk is a complex mixture containing sugars, salts, lipids and proteins. As a consequence its mass/volume or density and its specific gravity vary with the type of cow and the specific cow.

To minimize variables the milk or water must be at the same temperature as the density of water increases as the temperature lowers (until 4oC). The density of milk = 1.003073 - 0.000179t - 0.000368F + 0.00374N, where t = temperature in degrees C; F = percent fat; and N = percent nonfat solid. Comparing the resulting data for several types of cattle, the mean specific gravity of Holstein milk at 20 C/20 C is 1.0330 (range 1.0268 to 1.0385) and for Ayrshire milk is 1.0317 (range 1.0231 to 1.0357), while Jersey milk also has a mean of 1.0330 (range 1.0240 to 1.0369). [see Sherbon JW, Physical Properties of Milk, Chapter 8 in Wong et al, 1988]

Milk is more dense than water by 2% to 4%. This is about the weight of penny per litre.