Color Change Chemical Reaction (Alex Kim)

Title: Mystery Pitcher - Color Change Chemical Reaction

https://docs.google.com/spreadsheets/d/1XYFfvrUKNAZ481eEf9gFByJb9J4q6fDnQxIEydtONLU/edit#gid=0

https://docs.google.com/presentation/d/1aw6zA7BRay-t7Xfw9BK9mOGEsIp80gjuLq9cSCMyJHA/edit#slide=id.p

Principle(s) Investigated:

-pH, acids, bases, color changes

Standards:

HS-PS1-2

PS1.B: Chemical Reactions

Materials:

6 Drops of Phenolphthalein

1/8 Teaspoon of Sodium Carbonate (washing soda works too)

3 Full droppers of Vinegar

4 Glass beakers or clear, see-through glasses

1 Clear, see-through pitcher of water (~1200mL)

1 Stirring rod

Procedure:

1. In the first beaker, pour about 300 mL of water.
2. In the second beaker, put about an 1/8th of a teaspoon of sodium carbonate,
3. In the third beaker, put 6 drops of phenolphthalein solution.
4. In the fourth beaker, put three droppers-full of vinegar.
5. Add a few drops of water to the second beaker and stir to dissolve the sodium carbonate.
6. Fill the second, third, and fourth beakers with 300 mL of water each.
7. Then pour all of the contents from the beakers into the pitcher EXCEPT for the one with vinegar.
8. Refill the three empty beakers – the water will be red!
9. Now pour all four beakers back in the pitcher (the one with vinegar last).
10. Refill the beakers one last time — the liquid will be colorless again.

Student prior knowledge:

-pH

-acid-base indictors (what causes a color change in solution)

-chemical reactions (how they work)

-identifying lab equipment/knowledge of lab safety

Explanation:

Before any visible reaction takes place, the first thing worth noting is the reaction between water and sodium carbonate (washing soda). Out of the three beakers that are initially poured into the pitcher, one of them contains this combination of water and sodium carbonate. It is important to note the following reaction.

Na2CO3 + H2O → 2 NaOH + CO2

Once the solid sodium carbonate has been mixed with the water, sodium hydroxide and carbon dioxide result. What is left is essentially a basic solution containing of hydroxide (OH^- ) ions. Next, once both this beaker and the one containing water and phenolphthalein combine, the following reaction takes place.

2 OH- + H2In → In2- + 2 H2O

The reason why the solution turns pink at this point is due to the fact that the solution has become more basic and phenolphthalein turns pink in the presence of bases with a pH higher than 8.2. At this point, you should be left with a pink solution in your pitcher.

After equally distributing the pink solution back into the two beakers to in fact show that it is pink, pour the pink solution back into the pitcher and combine the beaker containing the vinegar/water solution. Once this happens, the vinegar (weak acid) combines with the pink solution and lowers the pH of the overall solution. The solution turns from pink back to colorless because the pH is reduced due to the addition of the vinegar.

Questions & Answers:

Why did the solution turn pink?

To understand why the solution turned pink, you first have to understand the reaction that took place between sodium carbonate and water (shown above in the "Explanation" section). Once you know that you end up with sodium hydroxide, a base, and some CO2, you'll know that the solution turns pink when the phenolphthalein reacts with the sodium hydroxide since phenolphthalein shows a pink color in the presence of a base (as indicated by the second equation above).

Why did the solution turn colorless?

Once the solution was a pink color, it turned back to being colorless because the addition of vinegar, an acid, lowered the pH back down to a level to show a colorless state. Since phenolphthalein is colorless once the pH is reduced to lower than about 8, you know that the pH indeed decreased upon seeing a change from pink to colorless.

How would you get the solution to turn back from colorless to pink?

In order to achieve this, you would need to first think about how you initially got from colorless to pink. Knowing that sodium carbonate and water react to form sodium hydroxide and carbon dioxide, adding more sodium carbonate would turn the solution back into a pink state. Adding sodium carbonate would once again raise the pH and the phenolphthalein would indicate a pink color again, indicating that the pH has risen above 8.2.

Applications to Everyday Life:

Concrete

Phenolphthalein, which is the indictor used in this experiment to indicate either an acidic or basic solution (pH) based off of the color the solution turns, is also used to determine the carbonation of concrete. Concrete has a high pH due to the calcium hydroxide that forms when cement reacts with water. Over time, the concrete reacts with carbon dioxide in the atmosphere which decreases the pH. When a 1% phenolphthalein solution is applied to normal concrete, it will turn bright pink because of the high, basic pH. However, if the concrete remains colorless, the concrete's pH has decreased and the concrete itself has undergone carbonation.

Disappearing Ink

Phenolphthalein is also used in disappearing inks, which are most commonly found in children's toys. One example involves Barbie dolls whose hair is coated with phenolphthalein. When sprayed with a "magic mist" that contains ethyl alcohol, water, and a base such as sodium hydroxide, the doll hair turns pink. Once the hair dries, the pink fades back and "disappears".

Food

Indicators such as phenolphthalein are required to perform titrations. This extends far past the acid-base titrations most chemistry students will do at some point during their educational careers. Titrations are also widely used in the food industry. They allow manufacturers to determine the amount of a reactant in a given sample. For example, through the process of a titration, the amount of salt in a product can be determined as well as the concentration of a given vitamin. For vitamins, this also ties in with color change as the concentration can have an effect on the overall color.

Photographs: Include photos and diagrams that illustrate the how the investigation is performed.

IMPORTANT NOTE:

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DO upload photos into the album linked in the left sidebar. Once you have uploaded a photo, click on the same album link, find your photo, right click on it, and "copy image address" (copy image URL) and then use this link when inserting the photo. The photo must be visible when viewing your page. A link to the photo does not suffice.

Videos:

https://www.youtube.com/watch?v=eh7sd0bfKKY

Sources:

http://www.chymist.com/Toystore%20part3.pdf

https://worldwide.espacenet.com/publicationDetails/biblio?CC=US&NR=6531528&KC=&FT=E&locale=en_EP

https://www.newfoodmagazine.com/article/1485/titration-techniques-in-the-food-industry/