Key concepts
Chemistry
Physics
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 coloring disappear with the power of bleach!

Background
The first question to ask is, what actually makes the stains? Of course there are many answers to that, which include 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 (400-700 nanometers). 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 those specific wavelengths (if they fall in the visible spectrum). Grass, for example, appears green to us because its molecules absorb all wavelengths except in the green color range.  

Chemical molecules that appear a certain color usually contain 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 of such compounds. However, the question still remains: How can bleach get rid of these color-giving chromophores in the stains?

Most household bleaches are based on chlorine and contain sodium hypochlorite (NaClO). This is an oxidizing agent, which causes a reaction to form 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 oxidation reaction changes the chemical structure of the chromophore. The resulting molecule 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 show a color, and the stain magically disappears. Want to see for yourself? Then do this activity and watch food coloring turn colorless!

Materials

  • Apron
  • Safety glasses
  • Measuring cup
  • Tap water (cold)
  • Masking tape
  • Pen
  • Five glasses or bowls that can hold 500 milliliters
  • Liquid food coloring (green, yellow, blue and red)
  • Six glasses or cups that can hold 100 milliliters
  • Bleach detergent (about 8-9 percent sodium hypochlorite)
  • Medicine dropper
  • Teaspoon
  • Paper towels
  • Adult helper
  • Microwave, timer, additional bowl and glass (optional)

Preparation

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

Procedure

  • Place the five glasses containing the different colored solutions next to each other on the table. How intense are the different colors?
  • Take the medicine dropper and carefully suck up a little bit of bleach solution. Then slowly add three drops of bleach to the cup with the yellow solution. Carefully swirl the cup slightly. What happens with the yellow solution after you swirl the cup? Does the color change? How fast does the reaction happen?
  • Next, add three drops of bleach to the orange solution and swirl the cup. What color is the solution after adding the bleach? Does the solution turn colorless?
  • Repeat the bleach addition (three drops) with the green food coloring solution. Swirl the cup afterwards. How does the color change this time?
  • Continue with adding three drops of bleach solution to the blue and red solutions. Carefully swirl the cups and observe both. What happens with the color in these cups? Do they stay the same or change? Does the reaction occur quickly or slowly?
  • 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?
  • To each cup that still contains a colored solution at this point of the activity, add one full teaspoon of bleach (approximately five milliliters). Swirl each cup afterwards. Do you see any changes happening? Are the colors disappearing? Which color changes fastest?
  • 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 milliliters of cold tap water. Add one drop of blue and one drop of red food coloring to make a purple solution. Then add three drops of bleach to this solution with the medicine dropper. Based on your previous observations, what color change do you expect? How quickly 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-milliliter glasses from your tests by pouring the solutions down the sink and rinsing the cups with water. Then fill two cups with 100 milliliters of the leftover blue food coloring solution from your preparations. Heat one of the cups in the microwave for about 30 seconds to warm it up a little bit. Put the cold and hot food coloring 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 whether the solution is hot or cold? What is different?
  • Extra: Repeat this activity with all colors but this time add one teaspoon of 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 that 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 that make them appear blue, red or yellow. When adding three 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 dye 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 a visible hue for quite some time afterwards. The reason for this is that each chemical reaction happens at a different rate. Yellow reacts very quickly 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 did not happen immediately, you should have seen that after a while these 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 more quickly 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—and because 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, since 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 was 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
You can dispose of all your color solutions (including the ones you added bleach to) in the sink. Let the water run for an additional 30 seconds after you pour all the solutions into the sink. Wipe down your work area with wet paper towels.

More to explore
Eating with Your Eyes: The Chemistry of Food Colorings, from ACS Chemistry for Life
How Chlorine Bleach Works, from The Seattle Times
Rate of Reaction, from Chem4Kids
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies