Sink or Swim

Density is a physical property of matter that quantifies the weight of the material in a given amount of space. The amount of space that the material takes up is described by a quantity called volume. Volume is measure the amount of space a substance takes up in three dimensions. The volume of solids is often measured in units of m3(meters cubed), in3 (inches cubed), or ft3 (cubic feet). The volume of liquids and gases are often measured in units of liters, quarts, or gallons. The most common units for density are grams (abbreviated with a g) per milliliter (abbreviate with mL).

Water has a density of the 1 g/mL, which means that 1 mL (~1/5 tsp) weighs one gram. mL is a measure of volume, where volume is to. In order for a material to float (swim) the density has to be less than the density of water, which means that it displaces its volume of water, but more mass of water than its mass. For a material to sink it displaces its volume in water, but less mass.

In the case of the soda cans, diet soda has a sugar substitute which can consist of aspartame, saccharin, sucralose, cyclamates (outside the US), acesulfame potassium ("Ace K"), and stevia. A mixture of aspartame and Ace K is used in Diet Coke and sucralose is used in Diet Pepsi. Since these compounds are 200-1,000 times sweeter than sugar, often only 200 milligrams (or 0.007 oz.) of sweetener is needed. This small amount of sweetener results in a soda that has similar density to water and results in a can that floats. In the case of a non-diet soda, the soda will often contain > 30 grams (1 oz.) of sugar. This amount of sugar is enough to add extra weight to the soda and make it denser than water, causing it to sink.