Chlorine production in the kitchen

Chlorine is a very common element on earth, but fortunately it does not occur anywhere in nature as a pure gas. It is in fact extremely reactive, i.e. "aggressive": Dyes decompose when they come into contact with chlorine bleach and pathogens die when treated with chlorinated detergent. In industry, chlorine is the starting material for numerous products such as the well-known plastic PVC (polyvinyl chloride). The chlorine is obtained using electricity from a solution of table salt in water - you can even try it out at home!

That's what you need:

• A glass of water
• Table salt
• A piece of cardboard
• Two pencils sharpened at both ends
• A 4.5-volt battery
• Two pieces of thin electric wire or stranded wire, stripped at the ends (i.e. the plastic sheath must be removed over a length of approx. 2 cm)

This is how it's done:

1. Add about two teaspoons of table salt to the glass of water and stir well.
2. Punch two holes in the cardboard. Then place it on the glass with the salt water and put the pencils through it so that the lower pencil tips are immersed in the salt solution.
3. Connect the upper pencil tips to one pole of the battery each. To do this, wind one end of the electric wire around the tip of the pencil and the other around one of the metal pads of the battery. The graphite lead in the pencil conducts the current into the solution and serves as an electrode.

Important: Conduct the experiment in a well-ventilated room, as chlorine irritates the respiratory tract and is toxic in higher concentrations. Do not use anything other than a 4.5-volt battery as a power source. Sodium hydroxide solution forms in the glass, so you must never drink the water after the experiment.

Well observed

After a few seconds, small bubbles begin to form on the pencil tips in the water: Hydrogen rises at the negative electrode (cathode) and chlorine at the positive electrode (anode).  A slight "chlorine smell" is perceptible.

And what’s the reason?

In this experiment, you use electricity to trigger a chemical reaction between the table salt and water. In the process, electrically charged particles are exchanged and recombine themselves. This is called electrolysis.

Scheme of the saline electrolysis.

Table salt dissolves in the water and the chlorine and sodium particles then float around as negatively charged chlorine ions (Cl^-) and positively charged sodium ions (Na⁺). As soon as the pencil electrodes are connected to the battery and current flows, the chlorine ions participate in this current circuit: They collect at the anode, give off an electron there (e^-) and form a chlorine molecule (Cl₂), which rises as a gas. At the same time, electrons enter the solution at the cathode and are carried away by the water (H₂O); hydrogen (H₂) escapes as a gas and hydroxide ions (OH^-) remain in the solution together with the sodium ions. They form a diluted sodium hydroxide solution.

The entire electrolysis reaction is: 2 NaCl + 2 H₂O → 2 NaOH + H₂ + Cl_2. Such a reaction in which electrons are transferred is called a redox reaction.

Download Experiment (german)

Source: SimplyScience. Idea: https://www.eurochlor.org/news/produce-your-own-chlorine. Text & photos: SimplyScience.ch editorial team. The Science Olympiads and SimplyScience share content with each other in a loose order.