A colorless solution suddenly turns deep blue!
- Put on protective gloves and eyewear.
- Conduct the experiment on the plastic tray.
- Observe safety precautions when working with boiling water.
- 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.
- 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.
- 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
The temperature and the component concentrations are the main factors in the time needed for the solution to darken. Check out what happens if you add 10 drops of sodium thiosulfate Na2S2O5 instead of 5, or 1 spoonful of potassium iodide KI instead of 2.
This is actually a more complicated question than it would appear. To answer briefly, you won’t get the cool result this way. To learn more, please read the scientific description of this experiment.
If you don’t mix the final solution, the “night” will advance much more slowly. The solution will first darken to a brownish muddle, then eventually turn dark blue.
This happens if the reagents were added in the wrong proportions. It is not critical, but sometimes the reaction needs much more time to proceed this way. You can repeat the experiment, but pay close attention to the reagent amounts this time!
Prepare a starch solution. This will act as an iodine indicator. Starch dissolves slowly in water. Consequently, to ensure your solution has the necessary concentration of starch, first dissolve starch in boiling water and then dilute a portion of the solution with cold water.
Now, add two substances to the resulting solution: a substance containing iodide ions I-, potassium iodide KI, and a reductant, sodium thiosulfate Na2S2O3.
Prepare a second solution. This solution will contain the oxidant—hydrogen peroxide H2O2. Add some NaHSO4 to make the solution acidic.
Both of the solutions are colorless. Mix them. It seems like nothing is happening...
The solution in the beaker suddenly turns deep blue.
Leave the solution for 1 h, then mix the contents using a wooden stick. Add 10 drops of Na2S2O3 and mix again. Pour the solutions down the sink. Wash with an excess of water. Dispose of solid waste together with household garbage.
Why does the solution darken?
The dark blue coloring of the resulting solution appears due to the formation of a starch-iodine complex, which has an intense blue color. To make this complex form, three substances have to meet with one another: an iodide ion I-, molecular iodine I2, and starch. Iodine ions interact with molecular iodine, yielding complicated triiodide ions I3-. These ions are more soluble in water than iodine is, so they easily bind to starch and yield the final blue product.
What is starch?
Starch is a polymer — a substance with very large and branchy molecules. It has a very complicated spatial structure and is often of vegetable origin – starch can be found in rice, potatoes, and corn. Plants use this substance as an emergency nutrient source. Chemically, starch molecules consist of glucose fragments, making starch a polymeric carbohydrate.
Starch is practically insoluble in cold water, which is why we have to heat the water to produce the starch solution. The water molecules need this heat in order to penetrate between the starch molecules. This makes the polymer swell, then dissolve.
The interaction between starch molecules and triiodide ions is a qualitative reaction testing for the presence of starch. Thus, if you apply a solution containing I3- ions to a potato, for example, you will see the blue coloring, as potatoes are rich in starch.
What do we need the hydrogen peroxide for?
Hydrogen peroxide is a good oxidizer. This means that it is happy to take electrons from other substances around it. In this experiment, we also use it as an oxidizing agent.
What is the role of sodium hydrogen sulfate in the experiment?
Sodium hydrogen sulfate, being an acidic salt, has a hydrogen atom in its formula. This means that this salt releases protons when dissolved in water, forming an acidic medium. This acidic medium is necessary for the reaction to take place.
How does sodium hydrogen sulfate release protons? Firstly, when this salt dissolves, it splits into two ions: a potassium ion and a hydrogen sulfate ion:
NaHSO4 → Na+ + HSO4-
However, that is not the end: the hydrogen sulfate ion can also split into two ions: a sulfate ion and a proton.
HSO4- → H+ + SO42-
This is the proton that increases the acidity of the solution. The process of splitting into ions in water is called electrolytic dissociation.
How are triiodide ions formed?
Since we obtain our blue color through the formation of triiodide ions I3- the question becomes: how do we obtain the iodine and iodide ions needed to produce triiodide ions in the first place? Potassium iodide and hydrogen peroxide hold the answer. Iodide ions are already present in the solution of potassium iodide because this substance dissociates when dissolved:
KI → K+ + I-.
The next “ingredient” is iodine. It can be formed in the reaction between hydrogen peroxide and potassium iodide. Hydrogen peroxide acts as an oxidizer – it takes the electrons from iodide ions, transforming them into iodine molecules. However, this process wouldn’t occur without sodium hydrogen sulfate NaHSO4, which makes the reaction mixture acidic and thereby facilitates the process.
2KI + H2O2 + 2NaHSO4 → I2 + 2H2O + Na2SO4 + K2SO4
So, we have both iodide ions and iodine molecules and a triiodide complex forms:
I- + I2 → I3-
These processes proceed very fast; however, though one might expect the reaction mixture to darken immediately, it doesn’t. This is because of the presence of sodium thiosulfate Na2S2O3 – another key participant in the reaction.
What does sodium thiosulfate do?
In contrast to hydrogen peroxide, sodium thiosulfate is a reducer – it looks forward to sharing its electrons with any suitable substance. In our experiment, iodine is such a substance. Sodium thiosulfate reduces iodine to iodide ions (yielding sodium iodide):
2Na2S2O3 + I2 → Na2S4O6 + 2NaI.
Sodium thiosulfate immediately reduces all the iodine forming in the reaction with hydrogen peroxide; triiodide ions do not form and have no opportunity to interact with the starch, and the reaction mixture remains transparent. However, when the sodium thiosulfate is used up, nothing can hinder the iodine formation and the consequent triiodide ions’ formation. As a result, triiodide ions quickly accumulate in the reaction mixture, interacting with starch and yielding a blue-colored starch-iodine complex.
Why do we need boiling water?
Starch is poorly soluble in cold water, so we need hot water to make the starch dissolve.
Molecules in cold water are rather sluggish, just like people who prefer staying home when the weather outside is cold.
Water molecules need some heat to become more energetic and leave their comfortable houses. In our case, the hot water in the beaker provides this heat. Thus, the water molecules in the vial start to explore more and more new places, even venturing into territory that was previously occupied by starch molecules. There’s nothing for the starch molecules to do but make room for all their unexpected water molecule guests. As a result, the starch swells and dissolves. Indeed, starch cannot defend its territory anymore when there are so many water molecules around!