16 Mar 2007

Does "light" really mean light? - part II

A follow-up on the last entry, comments and Brian's Diet Coke Floating blog entry

The comments on the last entry on Coke density spurred my curiosity, and I decided to follow up with a few experiments:
  1. What's the density of different kinds of Coke? I did a simple experiments, weighing measured amounts of Coke (ca. 100 ml) and got the following result. I did the same measurement for water and corrected this according to standard water density values from a web based water density calculator:

  2. What about water density as function of temperature? The density changes according to the reference values are small, far too small to make a difference (my cans/bottles held between 13 and 21 deg. C), as seen from the density curve as function of temperature (the same density calculator used):

    And, of course, the density-temperature difference should not be very different from water to Coke, so at temperatures of high water density, the same should apply to Coke.

  3. Experiments should always be tested for repeatability, so I used two cans of coke. Also, I tested whether the same would happen for Coke bottles (500 ml plastic bottles), and if the same would happen for Coke Zero. The pictures below tell the whole story. The difference between Coke Light and Coke Zero, from the ingredients list, seems to be the sweeteners. Coke Light contains Sucralose and Acesulfame K, while Coke Zero contains Aspartame and Acesulfame K.

What seems really strange to me is the measurement of Coke light and Coke Zero having densities lower than water, especially the light variety which is well below any temperature dependent variations. How can it be that a water solution with dissolved matter has lower density than water? Carbon dioxide? I don't think so. Carbon dioxide is still matter dissolved in water and should contribute to a higher density rather than lower (regardless of its density in pure, gaseous form).

Anyway, the safe explanation to the floating Coke light is of course the air pocket (both in cans and bottles), and I think I'll stick to this as the main explanation rather than densities of Coke. Ordinary Coke is a clean cut case, anyway.


12 Mar 2007

Does "light" really mean light?

In a simple demonstration playing around with light products floating/sinking in water a fascinating contrast emerges.

Try submerging the following two pairs of products: ordinary and light mayonnaise, and diet coke (or other cola vs. cola light).

Light mayonnaise floats lower than ordinary mayonnaise. In the case of the soft drink, cola light floats while ordinary cola sinks! What's going on?

The term in question is density:

The main ingredients in mayonnaise are water and fat/oil:
Ordinary mayo: 80% fat, 16% water
Light mayo: 40% fat, 50% water

Fat floats in water. A larger proportion of fat makes the mixture closer to pure fat and vice versa.

Cola can be considered as water with some dissolved material. Ordinary cola is, as such, a sugar solution with a few other additives (taste, aroma, colour etc.).

Cola light contains the artificial sweeteners Sucralose and Acesulfame K. Both are far sweeter than table sugar, sucrose (650 times and 180-200 times, respectively, ref. Belitz). Thus, far less sweetener is needed. If we assume that all other ingredients are the same, then far less material is dissolved in the light version. The same total volume with less material --> lower density. Thus: pure cola light would float up in ordinary cola taken that they didn't mix.

In both the mayo and cola cases, there is some air (or trapped gas) inside the tube/can. This makes the tube/can float higher than in the case of the pure mayo/cola. However, as long as the volumes are the same, this doesn't make any difference. If it wasn't for the air, both cans would in fact sink, and the experiment wouldn't be.

A question of using and understanding scientific concepts
The concepts relevant to this is not only "light", but also (amongst others) "(chemical/dietary) energy" and "density". If something floats, we usually say that it's "lighter than water". However, two kilograms of wood is heavier than one kilogram of water, but it still floats. To me as an adult, it's probably easy to grasp, but placing this in an educational context makes it important to use the correct terms. So, "cola has higher density than cola light" would be more correct.

It's quite easy to put this to the test: measure both the volume and weight of a can/bottle of cola and compare. The one that sinks (highest density) weighs the most taken the same volume. Another version of this experiment is concealing the labels, letting the students know the content without telling which is which. The task is then to use knowledge and reasoning to deduce which is which.

Using the term "diet" rather than "light" would of course make the whole case less diffuse, but then a fascinating aspect in the experiment and following discussion is lost. This is the reason for using "light" instead of "diet" in the first hand. However, this may be a nice way of introducing the energy concept of (chemical) energy, kcal and kJ, and contrast this against "light" used in different contexts.

Natural sciences are evidently not only concerned with nature itself, but just as much the language describing nature.


- Belitz et al., Food Chemistry 3rd ed., Springer 2004
- A Swedish version of the cola experiment at SkolKemi pages of University of Umeå