Magic liquid

Make water change color!

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

Reagents

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

I didn’t get five different colors.

Proportions are crucial in this experiment. If we add too much or not enough of any given substance, this will affect the color of the resulting solution. You can repeat any specific reaction using a new cup. Make sure to measure the quantities of solutions and dry reagents as described in the instructions.

Now, pour 50 mL of water into a beaker, add 3 drops of thymol blue solution, and mix thoroughly. Pour the resulting solution from the beaker into a disposable plastic cup.

Also, try using clear, bottled, non-carbonated water. Tap water, carbonated water, or water with any sort of additives often has a non-neutral pH (pH != 7).

The liquid in the third cup is hissing! What is it?

Mixing citric acid with sodium carbonate Na2CO3 produces carbon dioxide CO2, and this gas forms bubbles in the solution.

Step-by-step instructions

First, prepare the pH indicator thymol blue solution.

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Now, prepare samples of substances that might affect the solution pH in different ways.

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Add the pH indicator solution to each of the samples.

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pH indicators are substances which, when added to a solution, change color corresponding to the solution’s pH value. One can determine a solution’s pH according to the indicator color.

Here are the various possible colors of the pH indicator thymol blue:

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Expected result

Thymol blue dye changes the color of solution depending on its acidity level.

Disposal

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

Scientific description

Why does the liquid change color?

To recap: we prepared five disposable plastic cups, each with small amounts of different substances, and then added the solution of thymol blue to each of them. Since each cup contained a different substance, the molecules of thymol blue found themselves in quite diverse neighborhoods and therefore behaved differently in each.

Let us analyze in detail what happened in each cup. We will refer to thymol blue as “Ind” (Indicator).

Cup #1 (red)

In the first cup, we put 5 drops of sodium hydrogen sulfate NaHSO4 solution. When dissolved in water, it dissociates cheerfully into three charged particles (called ions):

NaHSO4 → Na+ + H+ + SO42-

As a result, many H+ particles (hydrogen ions or protons) are released into the solution in the cup. Due to the large quantity of protons H+, the acidity of the medium increases and the thymol blue (Ind2- – blue) becomes red H2Ind.

Cup #2 (orange)

The citric acid C6H8O7 from the second cup also dissociates in water, producing protons H+. Moreover, each of its molecules can form three whole H+! But, unlike NaHSO4, citric acid does so unwillingly, and as a result, much less H+ is released into the solution in the second cup than in the first. Therefore, some of the thymol blue becomes red H2Ind, and some becomes yellow HInd-. A mixture of yellow and red, just as when we mix paint colors, yields orange.

Cup #3 (yellow)

In the third cup, the mixture of citric acid C6H8O7 and sodium carbonate Na2CO3 produces even fewer protons H+. Therefore, all the thymol blue in such an environment turns into yellow HInd-. In fact, the medium in the third cup is almost neutral. You might think that we would obtain the same result if we just added the thymol blue to a cup of plain water. However, even clear drinking water contains some dissolved carbon dioxide, which makes its medium slightly acidic. That is why we use the mixture of citric acid and sodium carbonate. Incidentally, when these two substances are mixed, carbon dioxide forms - causing the mixture to bubble.

Cup #4 (green)

Sodium hydrogen carbonate NaHCO3 dissolves in water, dissociating into Na+, H+ and CO32-. Its solution is slightly basic. Therefore, we can observe a slightly green-colored solution resulting from a mixture of yellow HInd- and blue Ind2-.

Cup #5 (blue)

In the last cup, thymol blue is present in its natural form Ind2- and is, as its name suggests, blue — indicating that the medium in the cup is basic. This is the result of water’s interaction with sodium carbonate Na2CO3, which yields the OH- ions responsible for the basic medium:

Na2CO3 + H2O → 2Na+ + HCO3- + OH-

Follow up

Did you know that you can change the color of the liquids in some of the cups by just blowing air through them?

With air, we breathe out a significant amount of carbon dioxide. When carbon dioxide dissolves in water weak carbonic acid is formed. Despite its low concentration, this acid can neutralize the basic medium in the solution. So if you blow some air through the green solution in the fourth cup for about two minutes, it will change its color to yellow. Such color change indicates that the solution has become more acidic. The same method may be used to change the color in the last cup with blue solution, but in this case, more carbon dioxide will be needed and as a result, more time to blow it through the solution.

You can a soda straw to perform this part of the experiment. But be careful: you must only breathe out through the straw and not breathe in so as to avoid swallowing the reagents.