Obtain chlorine and study its chemical properties!

30 minutes


  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray and in a well-ventilated area.
  • Keep a bowl of water nearby while working with fire.
  • Avoid inhaling chlorine gas from the beaker (steps 5–7).
  • Remove protective gloves before lighting the candles (step 5).
  • Do not extinguish candles with water.
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

The NaHSO4 aggregated into lumps. How can I measure it?

Use a stirring rod from the kit to break these lumps apart. Keep stirring the salt in the bottle until it becomes fine enough to measure.

I smell gas. Is everything okay?

This experiment produces a negligible amount of chlorine. The amount of gas released is enough to create a noticeable smell but too small to cause any harm to health.

As a side note, never sniff chemical products directly! The proper technique for smelling any chemical substance is called “wafting”: gently fan the air above a chemical container toward your face. This will suffice to allow you to detect any scent.

The iodine stain doesn’t want to disappear completely. What should I do?

Adjust the filter paper to position the stain away from the edge of the beaker, right above the chlorine vapors. Make sure all three candles are lit. Give it some more time.

I dripped the thymol blue on the same filter as the potassium iodide drop. Will I still get a result at the end of the experiment?

If the drops don't overlap, you may continue the experiment. However, if they blended together, take a new filter paper and deposit the drops again.

Step-by-step instructions

Perform the experiment in a well-ventilated area. Keep a pitcher of water nearby.


Sodium chloride NaCl and sodium hydrogen sulfate NaHSO4 can react to produce hydrochloric acid HСl, but not when dry.


Hydrochloric acid HCl, in turn, can react with manganese dioxide MnO2 to produce chlorine Cl2. To produce HCl and make it react with MnO2, you need to dissolve the reagents in water.


You’ll need some heat in order to start the reactions to produce chlorine. The candles can provide just enough heat. But before heating the beaker, apply some potassium iodide KI to help you detect the emerging chlorine.


The reaction begins when you light the candles. You can determine the presence of chlorine by observing the color changes in the KI spot on the filter paper.


To stop the production of chlorine, simply stop heating the beaker and add cold water. But even after you do this, there will be enough chlorine gas in the beaker to observe one more reaction—with thymol blue as an acid-base indicator.


Thymol blue turns yellow-red in the presence of acids.


Expected result

Chlorine changes the color of the indicator thymol blue and oxidizes iodide into molecular iodine.


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

Scientific description

If you arrange all the chemical elements by how much they like to take electrons from others, chlorine Cl will come up in the top three. In KI, iodine I had taken hold of potassium K's electron and was existing happily as an iodide ion I-… until the chlorine gas Cl2 arrived (with your help, of course). It took that extra electron from iodine, making it form I2 molecules, which have a characteristic iodine-brown color. And chlorine didn't stop there. It even took electrons from the newly-formed I2, forcing it to form a compound with hydrogen H and oxygen O — HIO3, which is colorless again.

Not only can Cl2 ruin many compounds by taking their electrons, Cl2 forms acids upon dissolving in water (remember the acid-base indicator color change?). These properties make chlorine quite dangerous, but also very useful: most microbes can't survive in water treated even with moderate and safe (for humans) amounts of chlorine.

How is chlorine produced?

Chlorine Cl2 was first produced as early as the XVIII century by a Swedish chemist, Carl Wilhelm Scheele. Even though the chemist did not realize that he had discovered a new element, the method he used is very similar to the method used in our experiment. The chemical reaction in Scheele’s method is shown in the following chemical equation:

4HCl + MnO2 → MnCl2 + 2H2O + Cl2

When manganese dioxide is added to a solution of hydrochloric acid, half of the chlorine ions are oxidized to form molecular chlorine, while the other half bond with manganese.

How is the hydrochloric acid for this experiment produced?

First, NaCl (table salt) is mixed with MnO2 (manganese dioxide). Next, we add NaHSO4 (sodium hydrogen sulfate) and water. This mixture is then heated. When NaHSO4 (sodium hydrogen sulfate) is dissolved in water, it releases a hydrogen ion, or proton (H+):

NaHSO4 → Na+ + H+ + SO42–

At the same time, sodium chloride splits into two ions, Na+ and Cl-:

NaCl → Na+ + Cl

When the chloride and hydrogen ions meet, they bond to form hydrochloric (HCl) acid:

Cl + H+ → HCl

In summary, hydrochloric acid forms according to the following reaction:

NaHSO4 + NaCl → Na2SO4 + HCl

Why does the drop of thymol blue solution change from blue to red?

Thymol blue is a pH indicator, which means it changes color depending on the concentration of protons in a solution. The initial thymol blue solution does not have many protons, so the color of the thymol blue indicator remains blue. However, after a while, a notable amount of chlorine forms. Some of the chlorine dissolves in the drop of thymol blue solution. When chlorine dissolves in water, it yields two acids:

Cl2 + H2O → HCl + HClO

This causes the number of protons to increase. When thymol blue is exposed to a high concentration of protons, its color changes from blue to red.

What happens to the solution of potassium iodide?

If you look at a modern periodic table, you will notice there are six halogens: fluorine, chlorine, bromine, iodine, astatine, and tennessine. As astatine is extremely rare, and tennessine is very radioactive with an extremely short life span, the best-studied halogens are fluorine, chlorine, bromine, and iodine.

In general, element reactivity decreases as we proceed down a column of the periodic table. This also applies to halogens: their reactivity decreases as we go down from fluorine to chlorine and then to iodine.

Halogens have an interesting chemical property: a more reactive halogen pushes a less reactive one out of its salts. This is exactly what happens in our experiment: chlorine is more reactive than iodine, so it pushes the iodide ions out of potassium iodide to form molecular iodine. This gives the spot on the filter paper its characteristic brown color:

Cl2 + 2KI → I2 + 2KCl

Why do iodine and thymol blue lose their color over time?

Chlorine is a very reactive element. It can react with iodine to produce a colorless compound HIO3.

Chlorine can also oxidize thymol blue. The oxidized form of thymol blue is colorless, and therefore the drop of thymol blue solution gradually loses its color.