Milk & Cola

Obtain a colorless liquid that smells like cola!

Difficulty:
Danger:
Duration:
15 minutes
Experiment's video preview

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

Is the liquid that filtered through drinkable?

Not at all. Safety rule #1 is never drink or eat from chemical laboratory glassware. However, you may repeat this experiment in the kitchen: pass the mixture through a coffee filter and collect the resulting liquid into a regular cup. Now, you may drink the obtained colorless cola – though, we can’t promise it would be tasty!

Step-by-step instructions

The excess gas in cola will hinder the experiment procedure, so release this excess by stirring the cola with the stirring rod.

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To “purify” our cola, we will need milk.

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The food coloring in cola is called “caramel color”. It readily adheres to the protein molecules in milk.

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As cola has an acidic medium, the milk coagulates. We will need a paper filter to filter it out.

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Coagulated milk absorbs the food coloring and together they are left behind on the filter paper.

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We have obtained colorless cola!

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Disposal

Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

What happens if cola is added to milk?

Apart from sugar and water, cola contains some other ingredients, including phosphoric acid (H3PO4).

According to the manufacturers, orthophosphoric acid gives tartness to such drinks as Dr. Pepper and Coca-Cola (classic, diet and zero). In all, one can agree with the manufacturers, because this component makes the drink acidic -- in other words, it increases the acidity of the drink.

What is the acidity of the solution?

We characterize the number of protons (hydrogen ions H+) a solution contains (also known as its “acidity”) using the potential of hydrogen (pH).

Water molecules are not the only particles present in water-based solutions; these solutions also contain other molecules and ions. The pH of a solution is determined by the concentrations of hydrogen ions H+ and hydroxide-ions OH- it contains.

If a solution contains an excess of hydrogen ions H+, its pH will be less than 7. Such solutions are called acidic.

If a solution contains an excess of hydroxide ions OH-, its pH will be greater than 7. Such solutions are called basic.

The H+ and OH- ions together form water H2O. So if neither of them are present in excess, the pH of the solution will be the same as that of water, i. e. 7. Such solutions are called neutral.

Milk consists of proteins, fats, microelements, and water. Despite its diverse contents, milk is almost a neutral medium (pH≈7), nearly the same as water. When milk is added to cola, the acid in the cola changes the structure of the milk proteins. These proteins stick to each other and to the drops of fat. As a result, the milk curdles.

What is left in the filter?

The brownish curd is the result of the interaction between the milk proteins and the protons of orthophosphoric acid. In fact, we have obtained curd by a process called “curdling”. This process is often used in manufacturing. Most of the dairy products you can find in grocery stores have been made using this method.

As opposed to our experiment with calcium chloride, we do not need to heat this mixture. Since cola has a rather low pH, the milk curdles much faster than in the experiment with calcium chloride, without needing heat.

Why did cola become colorless, while the precipitate turned brownish?

Cola contains a caramel colorant internationally classified as E150. This colorant adheres to and precipitates together with the curdled proteins. This happens because of the hydrogen bonds that form between some of the atoms of the colorant and the proteins.

More about hydrogen bonds:

What are hydrogen bonds? The answer to this question is rather complex. We will try not to go deep into scientific terms and difficult calculations. To this end, let’s study a less complicated molecule than a protein, for example, ammonia NH3.

A hydrogen bond (indicated by the dotted line) is a relatively weak bond between a hydrogen atom and any other atom of any neighboring or even the same molecule (in the latter instance, the hydrogen bond is intramolecular). Pay attention to the fact that the hydrogen atom in question is already bonded with another atom (the “O-H” bond in the molecule of water or “N-H” in the molecule of ammonia):

ammonia_fountain_hydrogen_bonds

Hydrogen bonds and regular bonds between hydrogen atoms and other molecules differ regarding the distance between the atoms forming this bond, and the binding energy. The binding energy is the quantity of energy needed to break the bond.

In a water molecule, a hydrogen atom is located about 1 Å away from the oxygen atom to which it is bound by a regular bond. An angstrom (Å) is a very small unit of length − there are as many as 10 000 000 000 angstroms in one meter. However, it is a really notable value for atoms and molecules, comparable with their own sizes. The distance between a hydrogen atom and an oxygen atom from a neighboring molecule bound by a hydrogen bond is twice as great (about 2 Å).

Interestingly, it is thanks to these hydrogen bonds that water is a liquid at room temperature.

The molecules of the flavoring agent do not interact with the milk proteins in any way, so they pass through the filter into the flask. This is why the filtered liquid distinctly smells like cola.