Invisible Soda

Add milk to a dark cola and watch a chemical reaction clear up things in no time.

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A glass of cold milk goes great with a lot of things: cereal, cookies, cakes, pies, etc. However, it’s not such a good match-up with a dark-colored soda pop. Milk and soda pop are both mostly water but each has ingredients that make for an unexpected reaction when they combine. This experiment may help you understand why some people suggest that soda pop may be stealing a vital nutrient from your body. It’s time to “clarify” things.

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Here's What You'll Need

  • 20 oz (591 ml) bottle of any dark, non-diet cola
  • 2% milk
  • Scissors
  • Adult supervision

Let's Try It

  1. It’s easier to see all the chemical and physical action if you remove the label from the bottle. Snip an edge with the scissors and the label peels off – mostly. Slowly open the bottle and slowly – again – pour in enough milk to raise the level of the soda pop so it’s close to the top.

  2. Replace the bottle cap and screw it on tightly. You can gently rotate the bottle to mix things inside a little but it’s not essential. Let the bottle sit undisturbed in a quiet place. Keep track of what’s going on inside the bottle, however. A photo every 15 minutes or a video will track things for data collection. Plan on a couple of hours anyway. The longer you wait, the more distinct the differences you’ll see. The upper three-quarters of the bottle ends up with a clear liquid in it and the bottom quarter is filled with a solid material.

How Does It Work

You probably have seen what happens when you put drops of vinegar or orange juice into milk. The milk is curdled by the acid content of vinegar (acetic acid) and orange juice (citric acid) and a solid (a precipitate) forms where the drops are. When a cola is diluted with milk, the phosphoric acid in the soda pop curdles the milk into little globs. The surprising chemical change occurs throughout the bottle and that’s followed by a physical change as the curdled milk slowly falls to the bottom.

The chemical change is phosphoric acid in the soda pop reacting with calcium in the milk to make two new products: tricalcium phosphate and hydrogen.

The equation looks like this: 3Ca + 2H3PO4 —> Ca3(PO4)2 + 3H2

Tricalcium phosphate is the precipitate that falls out of the liquid and settles on the bottom taking almost all of the caramel coloring in the soda pop with it. The hydrogen gas bubbles to the top and fills the space under the cap along with the CO2 that used to be held in solution by the soda pop.

There are some studies that suggest that over time, the high levels of phosphoric acid in consuming repeated quantities of soft drinks might increase the chance of osteoporosis (chronically weak, porous, and brittle bones) in those who don’t get enough calcium in their diet. This simple demonstration shows how that might just be possible, too. An occasional soda pop may be OK but make sure you get the calcium you need first, everyday!

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