Fox tail gas

Synthesize nitrogen dioxide and learn how it adsorbs onto activated carbon

25 minutes
Experiment's video preview
Chemical formula



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray and in a well-ventilated area.
  • Use only the clean and dry flask.
  • Observe safety precautions when working with boiling 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

It’s been 10 minutes but the flask isn’t yet filled with gas. What’s wrong?

First of all, make sure you are using a dry flask. Nitrogen dioxide NO2 is very well soluble in water, which means it will be absorbed by water to yield nitric and nitrous acid. In other words, if you use a wet flask you might have not noticed the gas.

If it isn’t moisture, then double check that you are using really hot water. To be sure, carefully set a thermochrome sticker onto a beaker. If the sticker turns yellow, then water is indeed hot. However, if it hasn’t changed its color, then water has cooled down. Carefully pour it out and fill the beaker up to 100 mL mark with hot water. Wait 5–10 min.

Still isn’t working? Carefully open the vial and mix the contents again with a stirring rod. Now, close the vial and immerse it in hot water.

Step-by-step instructions

Prepare the flask to collect gas.


To create nitrogen dioxide NO2, first mix calcium nitrate Ca(NO3)2 and sodium hydrogen sulfate NaHSO4. This yields nitric acid HNO3, which reacts with copper Cu to form NO2.


Heat the mixture to speed up the reaction.


To stop the reaction, neutralize the acid using sodium carbonate.


The red gas in the flask is nitrogen dioxide NO2. This gas condenses easily, adsorbing onto activated carbon. This carbon has a very large surface area, which allows it to easily adsorb almost all of the gas.



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

Scientific description

How does nitrogen dioxide form?

In this experiment, it all begins when the solutions of sodium hydrogen sulfate NaHSO4 and calcium nitrate Ca(NO3)2 are mixed with copper powder Cu. At first, sodium hydrogen sulfate reacts with calcium nitrate to form nitric acid HNO3:

2NaHSO4 + Ca(NO3)2 → CaSO4 + Na2SO4 + 2HNO3

Learn more

This reaction is not as straightforward as it may seem. When sodium hydrogen sulfate NaHSO4 mixes with water, it splits (or dissociates). The particles, which form as a result of dissociation, are called ions:

NaHSO4 → Na+ + HSO4

These ions may be positively charged, for example, like sodium ions Na+, or negatively charged like hydrosulfate ions HSO4. Positively charged ions are called cations and negatively charged ions are called anions.

After the initial dissociation of sodium hydrogen sulfate NaHSO4, the hydrosulfate anion also dissociates:

NaHSO4 → H+ + SO42−

In this process, the hydrogen cation (or proton) is released. Hydrogen cations are the key components of an acid. When there are many protons (hydrogen cations) in a solution, chemists say that it is acidic. The solution of sodium hydrogen sulfate contains a large number of protons, and is thus acidic. When calcium nitrate Ca(NO3)2 is added, nitric acid forms.

When calcium nitrate dissolves, ions are produced:

Ca(NO3)2 → Ca2+ + 2NO3

When nitrate anions NO3 come in contact with protons H+, nitric acid is formed:

H+ + NO3 → HNO3.

Next, nitric acid reacts with copper, producing nitrogen dioxide NO2 – the brownish gas which collects in the flask.

Why do we heat the reaction mixture?

In most cases, when a chemical reaction is heated, the reaction notably speeds up. If the reaction in this experiment wasn’t heated, it would take very long for the flask to fill with nitrogen dioxide. Thus, it is the heat that increases the rate of reaction.

How does heat increase the reaction rate?

If we could look closely at the microscopic action inside a reaction mixture, we would notice an enormous number of small particles constantly moving around. These particles are called molecules. A chemical reaction is a process in which a molecule of one substance meets the molecule of another substance to form either one new molecule or several new molecules. These are called product or products depending on how many different kinds of molecules are formed. When the reaction mixture is heated, the molecules move around more quickly, so there is a much greater chance that the molecules of the reacting substances meet each other to form the product(s) molecules. A chemical rule called “The Van’t Hoff Rule”, states that an increase of 10o C in temperature of a reaction mixture, increases the reaction rate between 2-4 times. Just imagine, how we have increased the rate of reaction in our experiment by heating it!

Why should we neutralize the reaction mixture?

Nitrogen dioxide will keep evolving even when the flask is full of the brownish gas. Once we no longer need any more gas, we should stop the reaction. The best way to do this is to neutralize the nitric acid. This is exactly what sodium carbonate Na2CO3 does:

Na2CO3 + 2HNO3 → 2NaNO3 + H2O + CO2

When there is no more nitric acid left in the reaction mixture, the reaction ceases.

Why does the nitrogen dioxide disappear from the flask?

Activated carbon is very porous, so it has great adsorption properties. As we shake the flask, more and more nitrogen dioxide molecules become trapped in the pores of the activated carbon. As a result, the contents of the flask become colorless and the brownish nitrogen dioxide seems to disappear.