As the solution is diluted with water, its color changes strikingly from reddish to pale blue. These changes are related to changes in the absorption bands in the visible electronic spectra. Students will be able to conclude that the dissociated fraction of a weak acid increases with dilution. View Author Information. Cite this: J. Article Views Altmetric -. 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? Use a flat toothpick to pick up as much sodium carbonate as you can on the end of a toothpick. Explain what students will do in the next activity and discuss the purpose of having a control. Ask students: Why is it important to have a control? The control is left alone and not changed so that any color changes in the other wells can be compared to the original color in the control.
Have students test increasing concentrations of citric acid solution. Question to Investigate How does the concentration of citric acid affect the color of universal indicator solution?
Materials for Each Group Universal indicator solution pH color chart Citric acid solution At least 6 toothpicks Spot plate 2 droppers Procedure Test your citric acid solution Use one of your droppers to nearly fill 6 wells in your first spot plate with the universal indicator solution. Record the color of the indicator, the number of toothpicks of citric acid, and the pH number in the chart on the activity sheet for well 2. Expected Results The color of the indicator should turn yellow-green or yellow.
Record Observations Help students fill out the chart on their activity sheet. Ask students to make a prediction: 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? Procedure Test a more concentrated citric acid solution Add another toothpick scoop of citric acid to the citric acid cup.
Record the color of the indicator, the number of toothpick scoops of citric acid added, and the pH number in the chart for well 3. Continue adding toothpicks of citric acid and testing the solution in the last three wells to see how many different colors you can get. Expected Results 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.
Table 1. The color and pH of different concentrations of citric acid Well Number Number of toothpicks of citric acid used in 5 mL of water Color pH 1 0 Green 7 2 1 Yellow-green 6. As the citric acid solution becomes more concentrated, the color moves from green toward red on the pH color chart. How does the number on the pH scale change as the concentration of citric acid solution increases?
As the citric acid solution becomes more concentrated more acidic , the number on the pH scale decreases. Have students test increasing concentrations of sodium carbonate solution. Question to Investigate How does the concentration of sodium carbonate affect the color of universal indicator solution? Materials for Each Group Universal indicator solution pH color chart Sodium carbonate solution At least 6 toothpicks Spot plate 2 droppers Procedure Test your sodium carbonate solution Use a dropper to nearly fill the 6 wells in your other spot plate with universal indicator solution.
You will not add anything else to the first well. Record the color of the indicator, the number of toothpicks of sodium carbonate used to make the solution, and the pH number in the chart for well 2. Expected Results The color of the indicator should turn green-blue or blue.
Ask students to make a prediction: 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? Test a more concentrated sodium carbonate solution Add another toothpick of sodium carbonate to the sodium carbonate cup. Add 1 drop of sodium carbonate solution to the next well. Record the color of the indicator, the number of toothpicks of sodium carbonate used, and the pH number in the chart for well 3.
Continue adding toothpicks of sodium carbonate and testing the solution in the last three wells to see how many different colors or shades you can make. Expected Results The more concentrated sodium carbonate solution should cause the color to change to a darker blue moving toward purple.
Table 2. As the sodium carbonate solution becomes more concentrated, the color moves from green toward purple on the pH color chart. How does the number on the pH scale change as the concentration of base increases? As the sodium carbonate solution becomes more concentrated more basic , the number on the pH scale increases. Explain Explain how water molecules interact with each other to form ions. Project the animation Proton Transfer in Water. Play the first part of the animation. Project the illustration Water Molecules Trade Protons.
Explain the reformation of two H 2 O molecules. Explain how acids and bases cause the indicator to change color. Project the animation Acids Donate Protons. Project the animation Bases Accept Protons. Extend Have students slowly pour their remaining acidic and basic solutions into the indicator solution to introduce the idea that acids and bases can neutralize each other.
Ask students to make a prediction: How do you think the color will change if you pour a small amount of each leftover solution into your universal indicator solution?
Materials for Each Group Universal indicator solution pH color chart Citric acid solution Sodium carbonate solution Procedure Pour a small amount of either your citric acid solution or sodium carbonate solution into your indicator solution.
Pour a small amount of the other solution into your indicator solution. Swirl and compare the color to your color chart. Expected Results 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. Downloads Lesson 6. Student Reading Use this related reading to extend student comprehension after completing the lesson.
Student Reading for chapter 6. A parade of the color intensities is shown below:. The equivalence point is when the color changes most rapidly, not when the solution has changed color. Improper use of indicators will introduce inaccuracy to titration results. Indicators change color gradually at various pH. Let us assume that the acid form has a blue color and the basic form has red color.
The variation of colors at different pH is shown below. The background color affects their appearance and our perception of them.
If a solution has a color matching this, the pH would be the same as the p K ai of the indicator, provided that the conjugate forms of the indicator have the BLUE and RED colors.
Learning Objectives Explain color changes of indicators. Determine the acidic dissociation constants K a or K ai of indicators. Solution First, you should estimate the pH at the equivalence point, at which the solution is 0. Colors of an Indicator Solution Indicators change color gradually at various pH.
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