What Does A Color Change Indicate The Formation Of?

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Lesson half dozen.8

pH and Color Change

Key Concepts

• Whether a solution is acidic or basic can be measured on the pH scale.
• When universal indicator is added to a solution, the colour change can bespeak the approximate pH of the solution.
• Acids cause universal indicator solution to alter from green toward ruby-red.
• Bases cause universal indicator to change from greenish toward regal.
• Water molecules (H_iiO) tin interact with one some other to class H₃O⁺ ions and OH⁻ ions.
• At a pH of 7, there are equal numbers of H_threeO⁺ ions and OH⁻ ions in water, and this is chosen a neutral solution.
• Acidic solutions take a pH below 7 on the pH scale.
• Basic solutions take a pH above seven on the pH scale.

Summary

Students volition see a demonstration of a color modify using universal pH indicator. Students will change the concentrations of an acid and a base and use universal indicator to examination the pH of the resulting solutions. Students will come across an animation showing that water molecules collaborate and separate into the H_threeO⁺ ion and the OH⁻ ion. Students will run across that the pH of a solution is related to the concentration of these ions in water.

Objective

Students will be able to explain, on the molecular level, that pH is a measure of the concentration of the H₃O⁺ ions in water and that adding an acid or a base to water affects the concentration of these ions.

Evaluation

Download the student activeness sheet, and distribute 1 per educatee when specified in the activity. The activeness sheet will serve every bit the "Evaluate" component of each 5-E lesson programme.

Condom

Exist sure you and the students wear properly fitting goggles during the activeness and launder hands afterwards. Sodium carbonate may irritate pare. Citric acid is an centre irritant. Universal indicator is alcohol-based and flammable. Read and follow all safety warnings on the label. At the end of the lesson, accept students pour their used solutions in a waste container. Dispose of this waste product downwardly the drain or according to local regulations. The leftover citric acid and sodium carbonate powders tin be disposed of with the classroom trash.

Materials for the Demonstration

• 3 articulate plastic cups
• Citric acrid
• Sodium carbonate
• Universal indicator solution
• Water

Materials for the Each Group

• 3 clear plastic cups
• Masking tape and pen or permanent marker
• Universal indicator solution
• pH colour chart
• H2o
• Citric acid
• Sodium carbonate
• Graduated cylinder
• At least 12 flat toothpicks
• 2 6-well spot plates or 1 12-well spot plate
• 3 droppers

About the Materials

For this lesson, each group will need a Universal Indicator pH Color Chart. Print plenty pages of these charts on a color printer so that each group can have its ain nautical chart, or buy them from Flinn Scientific, Product #AP8765.

Each grouping will likewise need Universal Indicator Solution, Flinn Product #U0002, citric acid (anhydrous), Product #C0136 (500 grams) and sodium carbonate (anhydrous, laboratory class), Product #S0052. Each group volition need either two half-dozen-well spot plates or one 12-well spot plate. A porcelain vi-well spot plate is available from NASCO, Product #SB40727M. A polystyrene 12-well spot plate is available from Flinn Scientific, Production #AP6399.

Almost this Lesson

Because of their chemical backdrop, reactions involving acids and bases are different from the chemical reactions students have seen so far in Affiliate half dozen. In the previous lessons, information technology was always the electrons that were being shared or transferred when atoms interacted. In the next iii lessons about acids and bases, things are a little different. With acids and bases, it is a proton from a hydrogen cantlet that is transferred from 1 substance to another.

The main aspect of acids and bases that students volition explore in the next three lessons deals with the influence of acids and bases on h2o. The reactions of acids and bases with water are measured using the pH scale. Understanding pH on the molecular level will requite students a better appreciation for some of the environmental problems involving acids and bases. The significant of pH and the style it is affected by acids and bases can be a little tricky, just by using animations, drawings, and some simplifications, students should be able to sympathize the main ideas.

1. Add universal indicator solution to an acid and a base subconscious in "empty" cups to demonstrate how an acrid and a base can change the color of a pH indicator.

   Materials for the Demonstration

   • three articulate plastic cups
   • Citric acid
   • Sodium carbonate
   • Universal indicator solution
   • Water

   Annotation: Your local tap water is likely fine for the demonstration and activities in this lesson. If the indicator solution y'all brand is not green, this means that your h2o is either acidic or basic. If this happens, use distilled water, which is available in supermarkets and pharmacies.

   Instructor Grooming

   1. Make indicator solution for student groups
   2. Make a dilute universal indicator solution for this demonstration and for each student group by combining 250 mL water with 10 mL universal indicator solution.
   3. Pour about 25 mL of this dilute universal indicator solution into a clean cup for each student group.

   Notation: In the action, students will make full 12 wells with universal indicator solution. Cheque to make certain that 25 mL of solution is enough. You will demand about l mL of indicator solution for your demonstration. If 250 mL of solution is non enough, make more than using the same proportions.

   Prepare for the Demonstration

   1. Pour about 50 mL indicator solution into a articulate plastic cup for you to use in the demonstration.

      
   2. Using two empty articulate plastic cups, add about ⅛ teaspoon of citric acid to 1 loving cup and ⅛ teaspoon of sodium carbonate to the other. Do non tell students that you have added anything to the cups.

   Procedure

   1. Pour about ⅓ of the indicator solution into the citric acid cup and ⅓ into the sodium carbonate cup. Go out ⅓ in the indicator cup.

      

   Expected Results

   The citric acrid turns the indicator from green to reddish. The sodium carbonate turns the indicator from green to purple.

   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 modify is often a clue that a chemical reaction has taken place. So the color alter in each cup is probable the result of a chemical reaction. (This point is made in Chapter 6, Lesson half-dozen.)

   Would you say that the substances that were in the cups before the liquid was added were the same or different? Why?

   The liquid in each cup turned a dissimilar color during the reaction. Considering substances react chemically in characteristic means and the substances reacted differently, the substances in each cup must be different.

   Tell students that the green solution was made by adding a substance called universal indicator to water. Explain that you put a minor corporeality of a substance, one an acid and one a base, in each loving cup. Don't tell students which loving cup contained the acid or base.

   Tell students that when you poured universal indicator solution into the cups, the acid and base of operations each reacted with the indicator and changed its color. Ordinarily, when two substances are mixed and a colour change results, that is a clue that a chemical reaction has taken place. The cause of this color change volition be discussed later in this lesson when students practise their own action.

   Tell students that they will use an acid, a base, and universal indicator solution to learn about how acids and bases affect h2o. They volition as well larn how to measure the upshot with colors and numbers on the pH scale.

2. Have students compare the color of the solutions made in the demonstration to the colors on the Universal Indicator pH Colour Chart.

   Distribute one Universal Indicator pH Color Nautical chart to each group. Explicate that the chart shows the range of colour changes for universal indicator when acidic or basic solutions are added to the indicator. Signal out that each color has a number associated with information technology and that students volition larn more than about these numbers later in the lesson. As the solution becomes more than acidic, the color changes from green toward red. As the solution becomes more basic, the colour changes from light-green toward purple.

   

   Hold up the cups from the demonstration and ask the following questions:

   What does the color of the liquid in each loving cup tell you about the substance that was already in the cup when the indicator was added?

   The cup that turned carmine initially contained an acid, and the cup that turned royal initially contained a base.

   What does the green colour of the indicator tell you lot about the h2o in that cup? Is information technology acidic, basic, or neither?

   The green indicator left in the cup is neither acidic nor bones, and so it must be neutral.

3. Introduce the acrid and base used in the sit-in and discuss how the colour of universal indicator may modify with other common acids and bases.

   Explicate that before class, you placed a modest amount of citric acid in the cup that turned scarlet and a small amount of sodium carbonate in the loving cup that turned purple. Then citric acid is an acrid and sodium carbonate is a base of operations.

   Acids and Universal Indicator Solution

   Explain that citric acrid is in citrus fruits such as lemons, limes, and oranges.

   Ask students:

   What are some other common examples of acids?

   Students might say that vinegar is an acid. You could point out that in that location are also stronger acids, like sulfuric acrid used in machine batteries.

   What colors would you expect to meet if you placed any of these substances in universal indicator?

   The color may change to yellow, orange, or red for these acids.

   Bases and Universal Indicator Solution

   Explain that sodium carbonate is one of the chemicals ordinarily used in detergents made for dishwashing machines.

   Inquire students:

   What are some other common examples of bases?

   Students may not know any examples of bases but yous tin tell them that soaps, ammonia, and other cleaners are often bases.

   What colors would you look to see if yous placed whatsoever of these substances in universal indicator?

   The color may change to nighttime greenish, blue, and purple for any of these bases. (For universal indicator, the changes in color for bases are not every bit unlike equally they are for acids.)

   Tell students that next they will explore the colour changes of universal indicator with minor amounts of citric acid and sodium carbonate.

   Requite each student an Activity Sheet.

   Students volition record their observations and answer questions about the activity on the activity canvass. The Explain It with Atoms & Molecules and Take Information technology Farther sections of the activity sheet will either be completed every bit a form, in groups, or individually, depending on your instructions. To find the answers to the action sheet, become to the downloads area within the online version of this lesson.

4. Have students prepare the solutions for the action.

   Explain to students that they will offset make their solutions for the activity. Either go through each stride with them or have them follow the process described on their activity sheet.

   Teacher Preparation

   Students will demand small amounts of sodium carbonate and citric acid for the activity.

   • Characterization two small plastic cups citric acid and sodium carbonate for each group.
   • Identify about ¼ teaspoon of citric acid and sodium carbonate in the labeled cups.
   • Distribute the cups with universal indicator solution to each educatee group.

   Materials for Each Group

   • 2 clear plastic cups
   • 3 droppers
   • Masking record and pen or permanent marker
   • Universal indicator in cup
   • H2o
   • Graduated cylinder
   • Sodium carbonate
   • Citric acid
   • 2 flat toothpicks

   Procedure

   1. Label your equipment

      1. Utilise masking tape and a pen to characterization one cup citric acrid solution and another cup sodium carbonate solution.

         

      2. Use a pocket-sized piece of masking record and a pen to label 1 dropper citric acid solution and the other dropper sodium carbonate solution.

   2. Make a citric acid solution

      1. Employ your graduated cylinder to add v mL of h2o to the cup labeled citric acid.

      2. Use a flat toothpick to pick upwardly as much citric acrid as you can on the end of the toothpick as shown.

         

      3. Add together this citric acid to the water in the citric acrid loving cup. Gently swirl until the citric acid dissolves.

   3. Make a sodium carbonate solution

      1. Use your graduated cylinder to add 5 mL of water to the loving cup labeled sodium carbonate.

      2. Apply a flat toothpick to pick upward as much sodium carbonate every bit you tin on the finish of a toothpick.

      3. Add together this sodium carbonate to the h2o in the sodium carbonate cup. Gently swirl until the sodium carbonate dissolves.

         

5. Explain what students will do in the adjacent activity and talk over the purpose of having a control.

   Explain to students that in this activeness they will fill up the wells in each spot plate with universal indicator solution. Then in the first spot plate, they will test how different concentrations of citric acid affect the color of universal indicator solution. In the other spot plate, they volition test how different concentrations of sodium carbonate affect the colour of universal indicator solution.

   Tell students that in each spot plate, they will add nothing to the indicator solution in the first well. This is because the first well volition serve as the control.

   Ask students:

   Why is it of import to have a control?

   The control is left solitary and non changed and so that any color changes in the other wells tin can exist compared to the original colour in the control.

6. Have students test increasing concentrations of citric acrid solution.

   Question to Investigate

   How does the concentration of citric acrid bear upon the color of universal indicator solution?

   Materials for Each Grouping

   • Universal indicator solution
   • pH color chart
   • Citric acid solution
   • At to the lowest degree 6 toothpicks
   • Spot plate
   • 2 droppers

   Procedure

   1. Examination your citric acid solution

      1. Apply one of your droppers to nearly make full vi wells in your commencement spot plate with the universal indicator solution. Place the Universal Indicator pH Color Chart in front of the spot plate.

      2. Use your dropper to add 1 drop of citric acid solution to the 2d well. Gently mix the liquid with a clean toothpick.

         
      3. Compare the colour of the liquid to the control and to the Universal Indicator pH Color Nautical chart. Tape the color of the indicator, the number of toothpicks of citric acrid, and the pH number in the chart on the activeness sheet for well 2.

      Expected Results

      The colour of the indicator should plow yellow-greenish or yellow. If there is no obvious color change afterward adding a toothpick of citric acid, have students add a little more citric acrid to the solution. Tell them to be sure to option upward as much citric acid as they can on the end of a toothpick.

      Record Observations

      Aid students fill up out the chart on their action sheet. Students may say that the color of the solution in well 2 is yellow or yellowish-green. So have students assign a number for pH. Tell students that if the color in the well seems to be between two colors on the nautical chart, they should assign a pH value between the two.

      Tell students that in the adjacent role of the activeness they volition add a little more than citric acid to the citric acrid solution. This will make the citric acid solution more concentrated. But equally they did before, they will add ane drop of citric acid solution, but this time the citric acid solution volition be more than concentrated.

      Ask students to make a prediction:

      How do you think the color volition change if yous add one drib of a more concentrated citric acrid solution to the universal indicator in the next well?

   2. Procedure

      1. Exam a more full-bodied citric acid solution

      2. Add together another toothpick scoop of citric acrid to the citric acid cup. Gently swirl until the citric acid dissolves.

         

      3. Add together i drop of this more concentrated citric acrid solution to the tertiary well. Gently mix the solution with a clean toothpick.

         
      4. Compare the color of the solution to the control and to the Universal Indicator pH Color Nautical chart. Record the color of the indicator, the number of toothpick scoops of citric acid added, and the pH number in the chart for well three.
      5. Continue adding toothpicks of citric acid and testing the solution in the concluding three wells to see how many different colors y'all can become.

      Expected Results

      As the citric acid solution becomes more than concentrated, the colour should change to variations of yellow-green, yellowish, yellow-orange, orangish, orange-red, and ruby. The colors obtained will vary from group to group because of the different amounts of citric acrid students can pick upwards on the terminate of a toothpick. Students may be able to become 4 or v different colors. The answers and colors included in the chart below will vary.

      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
      one	0	Light-green	seven
      two	ane	Yellow-green	six.five
      iii	2	Yellow	vi
      four	3	Light orange	5.5
      5	4	Peach	five
      6	5	Pink	4

      Enquire students:

      How does the color of the indicator solution change every bit the citric acrid solution becomes more concentrated?

      As the citric acid solution becomes more than full-bodied, the color moves from greenish toward red on the pH color chart.

      How does the number on the pH scale change every bit the concentration of citric acid solution increases?

      As the citric acid solution becomes more than concentrated (more acidic), the number on the pH scale decreases.

7. Accept students examination increasing concentrations of sodium carbonate solution.

   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 take to look harder to see the divergence betwixt green-blue, blue, bluish-purple, and purple.

   Question to Investigate

   How does the concentration of sodium carbonate impact the color of universal indicator solution?

   Materials for Each Group

   • Universal indicator solution
   • pH color chart
   • Sodium carbonate solution
   • At to the lowest degree 6 toothpicks
   • Spot plate
   • 2 droppers

   Process

   1. Examination your sodium carbonate solution

      1. Use a dropper to nearly fill up the vi wells in your other spot plate with universal indicator solution. You will not add anything else to the first well.

      2. Add 1 driblet of sodium carbonate solution to the 2nd well. Gently mix the solution with a clean toothpick.

         
      3. Compare the color of the solution to the control and to the Universal Indicator pH Colour Chart. Record the colour of the indicator, the number of toothpicks of sodium carbonate used to make the solution, and the pH number in the nautical chart for well two.

      Expected Results

      The color of the indicator should turn green-blueish or blue.

      Ask students to brand a prediction:

      How exercise you think the colour will alter if you add one drib of a more than concentrated sodium carbonate solution to the universal indicator in the next well?

      

      Tell students that if you add together more base of operations to the same amount of water, the concentration of the base increases.

   2. Test a more concentrated sodium carbonate solution

      1. Add another toothpick of sodium carbonate to the sodium carbonate cup. Gently swirl until the sodium carbonate dissolves.

         
      2. Add one drop of sodium carbonate solution to the adjacent well. Gently mix the liquid with a clean toothpick.
      3. Compare the color of the liquid to the control and to the Universal Indicator pH Color Chart. Record the color of the indicator, the number of toothpicks of sodium carbonate used, and the pH number in the chart for well 3.
      4. Go on adding toothpicks of sodium carbonate and testing the solution in the terminal three wells to see how many dissimilar colors or shades y'all can brand.

      Expected Results

      The more than concentrated sodium carbonate solution should cause the color to change to a darker blueish moving toward majestic. Answers and colors in the chart below will vary.

      Tabular array 2. The color and pH of different concentrations of sodium carbonate

      Well Number	Number of toothpicks of citric acrid used in 5 mL of h2o	Colour	pH
      1	0	Green	7
      2	1	Dark-green-blue	eight
      3	2	Blue	viii.5
      4	3	Blue-majestic	9
      five	four	Regal	nine.five
      6	5	Regal	10

      Ask students:

      How does the color of the indicator solution change as the sodium carbonate solution becomes more concentrated?

      As the sodium carbonate solution becomes more concentrated, the colour moves from dark-green toward imperial on the pH color chart.

      How does the number on the pH scale alter equally the concentration of base increases?

      Every bit the sodium carbonate solution becomes more than full-bodied (more basic), the number on the pH scale increases.

8. Explain how water molecules collaborate with each other to form ions.

   Tell students that pH has to do with the way acids and bases collaborate with h2o. Explain that first y'all will bear witness students how h2o molecules interact with each other earlier an acid or a base is added.

   Project the blitheness Proton Transfer in H2o.

   Play the first role of the animation.

   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.

   Click "next" to evidence how the h2o molecules become ions.

   H2o molecules continuously move and crash-land into 1 another. Sometimes when 2 h2o molecules come up together, a proton from one hydrogen atom leaves its water molecule and becomes part of some other h2o molecule. Simply the positively charged proton moves; the negatively charged electron stays backside. And so, these two H₂O molecules become the ions H₃O⁺ and OH⁻.

   Click "next" again to bear witness how the ions get water molecules once again.

   Explain that when these ions bump into each other, the proton from the H₃O⁺ can move over to the OH⁻ ion, forming ii regular water molecules again. Because protons go back and forth between the water molecules or betwixt ions continuously, there is e'er the same corporeality of H_threeO⁺ and OH^⅛ ions in water.

   Projection the analogy H2o Molecules Trade Protons.

   This illustration shows the chemical equations that explicate how water molecules tin can become ions and how ions tin can become water molecules again.

   Explicate to students that the start chemical equation shows two water molecules coming together. Betoken out the chemical formula for each h2o molecule, H₂O.

   1. Explicate the germination of the H₃O⁺ ion.

      After the proton is transferred, the h2o molecule that at present has the extra proton is chosen an H₃O⁺ ion. The reason why the number of hydrogen atoms changed from two (the subscript in H_ii) to three (the subscript in H_three) is because having an actress proton is like having an extra hydrogen atom, even though the electron did non come over with information technology. Because one proton was added, there is i more proton than electrons, making this a positive ion.

   2. Explain the formation of the OH⁻ ion.

      The water molecule that lost a proton now has an actress electron, and so it is called the OH⁻ ion. The reason why the number of hydrogen atoms changed from two (the subscript in H₂) to one (no subscript afterward the H means 1 hydrogen) is considering losing a proton is similar losing a hydrogen atom. Because only the proton was transferred, at that place is 1 more electron than proton, making this a negative ion.

      

   Tell students that the second chemical equation shows an H₃O⁺ ion and OH⁻ ion meeting to become water molecules once again.

   1. Explain the reformation of two H₂O molecules.

      Explicate to students that water molecules and ions are e'er colliding. When an H₃O⁺ ion and an OH⁻ ion bump into each other, the proton can be transferred from the H₃O⁺ ion over to the OH⁻ ion and so that each ion becomes an H₂O molecule again.

      

   At any given time in an ordinary sample of water, a small percentage of water molecules are transferring protons and becoming ions. Also, the H₃O⁺ and OH⁻ ions are transferring protons and becoming h2o molecules over again.

9. Explain how acids and bases cause the indicator to alter color.

   Project the animation Acids Donate Protons.

   Tell students that when an acid is added to an indicator solution, the acid donates protons to the h2o molecules. This increases the concentration of H₃O⁺ ions in the solution. The H_iiiO⁺ ions donate protons to the indicator molecules causing the indicator to alter color toward crimson.

   Project the animation Bases Accept Protons.

   When a base is added to an indicator solution, it accepts protons from the water molecules, creating OH⁻ ions. The H₃O⁺ ions and indicator molecules donate protons to the OH⁻ ions, causing the indicator to alter color toward imperial.

10. Have students slowly cascade their remaining acidic and bones solutions into the indicator solution to introduce the idea that acids and bases can neutralize each other.

    Enquire students to brand a prediction:

    How do you lot recollect the colour will alter if you lot cascade a minor amount of each leftover solution into your universal indicator solution?

    Materials for Each Group

    • Universal indicator solution
    • pH color chart
    • Citric acrid solution
    • Sodium carbonate solution

    Procedure

    1. Pour a pocket-size corporeality of either your citric acid solution or sodium carbonate solution into your indicator solution. Swirl and compare the color to your Universal Indicator pH Colour Chart.
    2. Pour a modest amount of the other solution into your indicator solution. Swirl and compare the color to your color chart.

    3. Continue pouring modest amounts of the acid and base of operations solutions into your indicator until the solutions are used up.

       

    Expected Results

    The colors of the indicator solution will vary, merely students should meet that acids and bases mixed together crusade the color of the indicator to change toward neutral.

    Take students describe what they did and their observations. Then explain that in Chapter half-dozen, Lesson 9, they volition combine acids and bases in an indicator solution with the goal of making the pH of the final solution neutral.

Source: https://www.middleschoolchemistry.com/lessonplans/chapter6/lesson8

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