Key Concepts
Reaction rate, color

Introduction

Have you ever wondered why your dirty clothes come out of the washing machine white and clean? What makes all the stains disappear? The answer to that question is bleach - an ingredient that is present in most laundry detergents. Bleach is responsible for the whitening effect that occurs during washing and removes most of the stains. But how does it work? In this science activity you will find out by making food color disappear with the power of bleach!

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

The first question to ask is what actually makes the stains? Of course there are many answers to that like ketchup, syrup or grass – but what makes the ketchup stain red, the syrup stain brown and the grass stain green? The simple answer is that the color comes from what ketchup, syrup or grass is made of, their molecules. Some molecules can function as dyes which have the ability to absorb light in the visible wavelength range (400-700nm). When light interacts with such a molecule, part of the light spectrum is absorbed by its chemical structure. The wavelengths that are not absorbed are reflected instead, which results in us seeing this specific wavelength (if it falls in the visible spectrum). Grass for example appears green to us because its molecules absorb all wavelengths, except in the green color range.   

The unique feature of chemical molecules that appear colored is that they usually contain so-called chromophores in their chemical structure, which are able to absorb light in the visible wavelength range (instead of the invisible light spectrum). Often times, chromophores are aromatic compounds or molecules containing a series of alternating single and double bonds. Food dyes such as Blue 1 (Brilliant Blue), Red 3 (Erythrosine) or Yellow 5 (Tartrazine) are good examples for such compounds. However, the question still remains, how can bleach get rid of these colored chromophores in the stains?

Most household bleaches are based on chlorine and contain sodium hypochlorite (NaClO). This is an oxidizing agent, meaning that it is able to oxidize another chemical compound. During oxidation, the oxidizing agent removes one or more electrons from its reaction partner. This means that when bleach reacts with a “stain molecule”, the oxidization reaction changes the chemical structure of the chromophore. The resulting molecule now either does not contain a chromophore anymore or the chromophore is no longer able to absorb light in the visible range. In both cases the reaction product will not be colored and the stain magically disappeared. Want to see for yourself? Then do this activity and watch food color turn colorless!

Materials

  • Apron
  • Safety glasses
  • Measuring cup
  • Tap water (cold)
  • Masking tape
  • Pen
  • Glasses or bowls that can hold 500 mL (5)
  • Liquid food color (green, yellow, blue and yellow)
  • Glasses or cups that can hold 100 mL (6)
  • Bleach detergent (about 8-9% sodium hypochlorite)
  • Medicinal dropper
  • Teaspoon
  • Paper towels
  • Optional: Microwave, timer, additional bowl and glass

Preparation

  1. Choose a working surface that can tolerate small spills of food color and bleach.
  2. Put your safety glasses and the apron on to protect you from any color and bleach spills.
  3. With the pen and masking tape label all five 500 mL bowls or glasses from 1-5.
  4. Use your measuring cup to fill all five bowls/glasses with 500 mL (ca. 2 cups) of cold tap water.
  5. In glass 1, add 2 drops of blue food coloring. Mix the solution with a spoon until all the color is equally distributed.
  6. In glass 2, add 2 drops of red food coloring. Again, mix with a spoon.
  7. In glass 3, add 2 drops of yellow food coloring and mix with a spoon.
  8. In glass 4, add 2 drops of green food coloring. Again, mix with a spoon.
  9. In glass 5, add 1 drop red and one drop yellow food coloring. The resulting color after mixing should be orange.
  10. Using the measuring cup again, fill 100 mL (about ½ cup) of the blue, red, yellow, green and orange solution into a separate glass each. Quickly rinse the measuring cup in between colors.
  11. In a separate glass pour a little bit of the bleach. Be careful not to spill anything. If spills occur on the surface, remove them with a wet paper towel immediately. In case you get some bleach on your skin, immediately wash the affected area with plenty of water.

Procedure

  1. Place the 5 glasses containing the different color solutions next to each other on the table. How intense are the different colors?
  2. Take the medicine dropper and carefully suck up a little bit of bleach solution. Then slowly add 3 drops of bleach to the cup with the yellow solution. Swirl the cup slightly. What happens with the yellow solution after you swirled the cup? Did the color change? How fast did the reaction happen?
  3. Next, add 3 drops of bleach to the orange solution and swirl the cup. What color does the solution have after adding the bleach? Does the solution turn colorless?
  4. Repeat the bleach addition (3 drops) with the green food color solution. Swirl the cup afterwards. How does the color change this time?
  5. Continue with adding 3 drops of bleach solution to the blue and red solutions. Swirl the cups and observe both solutions. What happens with the color in these cups? Do they stay the same or change? Does the reaction occur fast or slow?
  6. Compare the colors of each of the cups after adding the bleach. What do you notice? Are any of the colors the same or are they all different? Can you explain your results?
  7. To each cup that still contains a colored solution at this point of the experiment, add 1 full teaspoon of bleach (approximately 5 mL). Swirl each cup afterwards. Do you see any changes happening? Are the colors disappearing? Which color changes fastest?
  8. Repeat adding bleach to the cups until all the solutions become colorless. Do you have to add more bleach to one colored solution compared to another?

Extra: Prepare an extra bowl with 500 mL of cold tap water. Add 1 drop of blue and 1 drop of red food coloring to make a purple solution. Then add 3 drops of bleach to this solution with the medical dropper. Based on your previous observations, what color change do you expect? How fast does the reaction occur? How much bleach do you have to add to remove all the color from the solution?

Extra: Clean out all the 100 mL glasses from your experiment by pouring the solutions down the sink and rinsing the cups with water. Then fill 2 cups with 100 mL of the left-over blue food color solution from your preparations. Put one of the cups in the microwave for about 30 seconds to heat it up a little bit. Put both, the cold and hot food color solutions next to each other and add about 20 drops of bleach to each of them. How does the color change in both cups? Do they react differently depending on if the solution is hot or cold? What is different?

Extra: Repeat this experiment with all colors but this time add 1 teaspoon bleach to every color at the same time. Then start a timer and measure how long it takes for each color to disappear. Which color is removed the fastest and which solution takes longest to become colorless? Can you explain your observations?

Observations and Results

Were you successful in making all the different colors disappear? Most likely, yes. All the food dyes are molecules with lots of chromophores in their chemical structure which make them appear blue, red or yellow. When adding 3 drops of bleach to each color you should have noticed that the yellow solution almost immediately turned colorless after swirling the cup. This is due to the above mentioned oxidative power of bleach that breaks up chemical bonds and alters the chemical structure of the dyes so it is not able to absorb light in the visible spectrum anymore. The blue and red solutions however, did not turn colorless right after adding the bleach and stayed colored for quite some time afterwards. The reason for this is that each chemical reaction happens at a certain rate, meaning that some can happen faster or slower. Yellow reacts very fast with bleach which is why you see it turning colorless right away. Blue 1 and Red 3, the dyes in the blue and red solutions, react at a much slower rate. Although the color change does not happen immediately, you should have seen that after a while the colors faded as well.

Orange and green as well as purple are mixtures of two different colors. Once bleach is added to these solutions, one of the colors in the mixture will react faster with the hypochlorite than the other one, which is why one color component disappears and the other one remains visible. Orange for example is a mixture of yellow and red – as yellow is oxidized much faster, the orange solution turns red. Similarly, the green solution, which is a mixture of blue and yellow, will turn blue after the yellow dye is oxidized. And if you did the extra step with the purple solution, you should have observed that the solution turned blue as the red dye reacts faster with bleach than the blue dye. If you measured how long it takes to make each color disappear, you should have found that yellow is oxidized the fastest, followed by red and then blue.

One way of speeding up a chemical reaction is adding more of the reactants, which is why the blue and red colors disappear faster if you add more bleach to the solution. Another way of increasing the reaction rate is an increase in temperature. Once you heat up the blue food coloring solution, the color disappears much faster compared to a cold solution even if you add the same amount of bleach.

Cleanup

  1. You can dispose of all your color solutions (including the once you added bleach to) in the sink. Let the water run for an additional 30 seconds after you poured all the solutions into the sink. Wipe down your work area with wet paper towels.

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Credits

Svenja Lohner, PhD, Science Buddies

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Key Concepts
Reaction rate, color
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