Students may say that the color of the solution in well 2 is yellow or yellow-green. Then have students assign a number for pH. Tell students that if the color in the well seems to be between two colors on the chart, they should assign a pH value between the two. Tell students that in the next part of the activity they will add a little more citric acid to the citric acid solution. This will make the citric acid solution more concentrated. Just as they did before, they will add one drop of citric acid solution, but this time the citric acid solution will be more concentrated.
How do you think the color will change if you add one drop of a more concentrated citric acid solution to the universal indicator in the next well? Add another toothpick scoop of citric acid to the citric acid cup. Add 1 drop of this more concentrated citric acid solution to the third well. Gently mix the solution with a clean toothpick. As the citric acid solution becomes more concentrated, the color should change to variations of yellow-green, yellow, yellow-orange, orange, orange-red, and red.
The colors obtained will vary from group to group because of the different amounts of citric acid students can pick up on the end of a toothpick. Students may be able to get 4 or 5 different colors. The answers and colors included in the chart below will vary. Note : The differences in color on the base side of the pH scale for universal indicator are not as obvious as those on the acid side. Students will have to look harder to see the difference between green-blue, blue, blue-purple, and purple.
Add 1 drop of sodium carbonate solution to the second well. How do you think the color will change if you add one drop of a more concentrated sodium carbonate solution to the universal indicator in the next well? Tell students that if you add more base to the same amount of water, the concentration of the base increases.
Add another toothpick of sodium carbonate to the sodium carbonate cup. The more concentrated sodium carbonate solution should cause the color to change to a darker blue moving toward purple. Answers and colors in the chart below will vary. Tell students that pH has to do with the way acids and bases interact with water. Explain that first you will show students how water molecules interact with each other before an acid or a base is added.
Remind students that each hydrogen atom in a water molecule has both a proton and an electron. The hydrogen atoms share their electrons with the oxygen atom. Water molecules continuously move and bump into one another. Sometimes when two water molecules come together, a proton from one hydrogen atom leaves its water molecule and becomes part of another water molecule.
Only the positively charged proton moves; the negatively charged electron stays behind. This illustration shows the chemical equations that explain how water molecules can become ions and how ions can become water molecules again. Explain to students that the first chemical equation shows two water molecules coming together.
Point out the chemical formula for each water molecule, H 2 O. The reason why the number of hydrogen atoms changed from two the subscript in H 2 to three the subscript in H 3 is because having an extra proton is like having an extra hydrogen atom, even though the electron did not come over with it.
Because one proton was added, there is one more proton than electrons, making this a positive ion. The reason why the number of hydrogen atoms changed from two the subscript in H 2 to one no subscript after the H means 1 hydrogen is because losing a proton is like losing a hydrogen atom. Because only the proton was transferred, there is one more electron than proton, making this a negative ion. Explain to students that water molecules and ions are always colliding. At any given time in an ordinary sample of water, a small percentage of water molecules are transferring protons and becoming ions.
Tell students that when an acid is added to an indicator solution, the acid donates protons to the water molecules. How do you think the color will change if you pour a small amount of each leftover solution into your universal indicator solution? Continue pouring small amounts of the acid and base solutions into your indicator until the solutions are used up. The colors of the indicator solution will vary, but students should see that acids and bases mixed together cause the color of the indicator to change toward neutral.
Have students describe what they did and their observations. Then explain that in Chapter 6, Lesson 9, they will combine acids and bases in an indicator solution with the goal of making the pH of the final solution neutral. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 6. Materials for the Demonstration 3 clear plastic cups Citric acid Sodium carbonate Universal indicator solution Water Note : Your local tap water is likely fine for the demonstration and activities in this lesson.
Teacher Preparation Make indicator solution for student groups Make a dilute universal indicator solution for this demonstration and for each student group by combining mL water with 10 mL universal indicator solution. Pour about 25 mL of this dilute universal indicator solution into a clean cup for each student group. Prepare for the Demonstration Pour about 50 mL indicator solution into a clear plastic cup for you to use in the demonstration.
Do not tell students that you have added anything to the cups. Expected Results The citric acid turns the indicator from green to reddish. Reveal to students that you put something in the cups beforehand. Ask students: Do you think this was a chemical reaction? Why or why not? A color change is often a clue that a chemical reaction has taken place. So the color change in each cup is likely the result of a chemical reaction.
This point is made in Chapter 6, Lesson 6. Would you say that the substances that were in the cups before the liquid was added were the same or different? The liquid in each cup turned a different color during the reaction. Because substances react chemically in characteristic ways and the substances reacted differently, the substances in each cup must be different. Have students compare the color of the solutions made in the demonstration to the colors on the Universal Indicator pH Color Chart.
Hold up the cups from the demonstration and ask the following questions: What does the color of the liquid in each cup tell you about the substance that was already in the cup when the indicator was added? The cup that turned reddish initially contained an acid, and the cup that turned purple initially contained a base. What does the green color of the indicator tell you about the water in that cup? Is it acidic, basic, or neither?
The green indicator left in the cup is neither acidic nor basic, so it must be neutral. Introduce the acid and base used in the demonstration and discuss how the color of universal indicator may change with other common acids and bases.
Acids and Universal Indicator Solution Explain that citric acid is in citrus fruits such as lemons, limes, and oranges. Ask students: What are some other common examples of acids? Universal indicator has many different colour changes, from red for strongly acidic solutions to dark purple for strongly alkaline solutions. In the middle, neutral pH 7 is indicated by green. When universal indicator is added to a solution, the color change can indicate the approximate pH of the solution.
Acids cause universal indicator solution to change from green toward red. Bases cause universal indicator to change from green toward purple. In general, litmus paper is red below a pH of 4. If the paper turns purple, this indicates the pH is near neutral. Label four small beakers 1 to 4.
In 1 and 3, put in 5 drops of phenolphthalein. In 2 and 4, put in 5 drops of ammonia. In one large beaker, put in 20 drops of vinegar. Fill the other large beaker with water for pouring into small beakers. Instructions: Pour water into each beaker so each is half full. Ask students to observe carefully and describe the colour as you pour 1 into 2, and 4 into 3.
Watch as they both turn purple-pink! Dump both solutions together into the large beaker laced with vinegar and watch as they become colourless again! Extensions Can you predict the colour phenolphthalein will turn when added to milk? How about orange juice? Try it out with some other common solutions. Try making your own cabbage juice acid-base indicator.
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