Key Concepts

pH is a measure of the concentration the H3O+ ions in a solution.Adding an acid boosts the concentration the H3O+ ions in the solution. Adding a base decreases the concentration that H3O+ ion in the solution.An acid and also a base are like chemical opposites. If a base is included to one acidic solution, the solution becomes much less acidic and moves towards the middle of the pH scale. This is dubbed neutralizing the acid.If an mountain is added to a basic solution, the equipment becomes less straightforward and moves towards the center of the pH scale. This is referred to as neutralizing the base.

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Students will usage citric acid and sodium carbonate services to see that including a basic to an acidic solution makes the solution much less acidic. Students will certainly then use a base to aid them determine which of 2 acidic remedies is much more concentrated.


Students will have the ability to explain, top top the molecular level, that pH is influenced by the concentration that the H3O+ ions in water. They will also be maybe to explain why adding a base to an acid or an acid to a base deserve to make the pH the the solution closer come 7.


Download the student activity sheet, and also distribute one per student as soon as specified in the activity. The task sheet will certainly serve together the “Evaluate” component of every 5-E great plan.


Be sure you and also the student wear correctly fitting goggles throughout the task and wash hands afterwards. Sodium carbonate may irritate skin. Citric acid is one eye irritant. Universal indicator is alcohol-based and also flammable. Read and follow all security warnings on the label. At the finish of the lesson, have actually students to water their provided solutions in a rubbish container. Dispose of this waste down the drainpipe or follow to neighborhood regulations. The leftover citric acid and sodium lead carbonate powders can be disposed of through the great trash.

Materials for the Demonstration

4 clean plastic cupsGraduated cylinderUniversal indicatorWaterSodium carbonateCitric acidFlat toothpicks2 droppersMasking tape and pen or long-term marker

Materials for the every Group

Universal indicator equipment in cupCitric mountain in cupSodium lead carbonate in cupWater equipment A, sodium carbonate solutionSolution B, much more concentrated salt carbonate solutionAt the very least 8 flat toothpicksGraduated cylinderSpot plate4 droppers3 clean plastic cupsMasking tape and pen or irreversible marker

About the Materials

Each group will require Universal Indicator Solution, Flinn Product #U0002, citric acid (anhydrous), product #C0136 (500 grams) and sodium carbonate (anhydrous–Laboratory grade), Product #S0052. Each group will likewise need a spot plate. A porcelain 6-well clues plate is easily accessible from NASCO, Product #SB40727M. A polystyrene 12-well point out plate is obtainable from Flinn Scientific, Product #AP6399.


Do a demonstrate to show students that an acidic solution becomes much less acidic once drops of a base room added.

Materials for the Demonstration

4 clean plastic cupsGraduated cylinderUniversal indicatorWaterSodium carbonateCitric acidFlat toothpicks2 droppersMasking tape and also pen or irreversible marker

Teacher Preparation

Make indicator equipment for college student groupsMake a dilute global indicator equipment for this demonstration and for every student team by combining 125 mL water through 5 mL global indicator solution.Pour around 15 mL of this dilute universal indicator solution right into a clean cup for each student group.

Note: Your local tap water is most likely fine because that the demonstrate and tasks in this lesson. If the indicator systems you make is not green, this means that her water is either acidic or basic. If this happens, use distilled water, which is easily accessible in supermarkets and also pharmacies.

Note: In the engage and extend activities, students will fill 6 wells with universal indicator solution. Check to make sure that 15 mL of systems is enough. You will need about 25 mL the indicator solution for her demonstration. If 125 mL of solution is no enough, make much more using the very same proportions.

Prepare because that the Demonstration

Divide the staying indicator solution right into two clean plastic cup for you to usage in the demonstration. Usage masking tape and a pen to label two empty cups citric acid and sodium carbonate.Use your graduated cylinder to add 5 mL the water to every labeled cup.Use a level toothpick to pick up as lot citric mountain as you have the right to on the end of the toothpick as shown. Add this citric acid to the water in the citric mountain cup. Gently swirl till the citric acid dissolves. Usage a flat toothpick to pick up as much sodium lead carbonate as you deserve to on the finish of a toothpick. Add this sodium carbonate come the water in the sodium carbonate cup. Gently swirl until the sodium carbonate dissolves.


Hold up the two cups of global indicator solution, which room both green.Also display students that you have a citric acid solution and also a sodium carbonate solution.

Ask students:

What color will the green indicator solution rotate if I include a couple of drops that citric acid solution?The indicator systems will readjust color towards red.


Add 3–5 drops of citric acid systems to one of the cups.

Expected Results

The shade of the solution should readjust from green to reddish.

Ask students:

What execute you think girlfriend could include to the reddish indicator to do it less acidic and go ago toward green? Students should suggest including sodium carbonate (a base) come the acidic (red) solution.


While holding increase the cup of red indicator solution, add 1 drop of salt carbonate solution, swirl, and compare the shade of the systems to the shade of the control.

Have students prepare the options for the activity.

Teacher Preparation

Students will certainly need small amounts of sodium carbonate and also citric mountain for the activity.

Label two tiny plastic cups citric mountain solution and sodium carbonate solution for each group. Place around ¼ teaspoon of citric acid and also sodium lead carbonate in the labeled cups. Distribution the cups v universal indicator equipment to each college student group.

Materials for Each Group

Sodium carbonate in cupCitric acid in cupUniversal indicator in cupWater3 clear plastic cupsGraduated cylinderFlat toothpicks2 droppersSpot plateMasking tape and pen or irreversible marker


Label your equipment

Use masking tape and also a pen to brand one cup citric acid solution and also another cup sodium carbonate solution.

Use a little piece the masking tape and also a pen to brand one dropper citric mountain solution and also the various other dropper sodium carbonate solution.Make a citric mountain solutionUse your i graduated cylinder to add 5 mL the water come the cup labeling citric acid.

Use a level toothpick to choose up as lot citric mountain as you can on the end of the toothpick together shown.

Add this citric acid to the water in the citric acid cup. Gently swirl until the citric acid dissolves. Make a salt carbonate solution

Use your i graduated cylinder to add 5 mL the water to the cup labeled salt carbonate.

Use a flat toothpick to choose up as lot sodium lead carbonate as you deserve to on the finish of a toothpick.

Add this sodium carbonate to the water in the salt carbonate cup. Gently swirl till the sodium carbonate dissolves.


Have student neutralize one acidic solution.

Question to Investigate

How plenty of drops of sodium carbonate solution will it require to neutralize her citric acid solution?

Materials for Each Group

Universal indicator solutionCitric acid solutionSodium carbonate solutionAt least 6 level toothpicksSpot plate3 droppers


Use a dropper to practically fill two little wells in her spot plate through universal indicator solution. Perform not add anything else to the very first well. This will certainly be your control. Include 3 fall of citric acid systems to the indicator in one of the wells. Use a clean toothpick come mix the solution. If that is no reddish, add more drops, but be sure to counting the total number of drops added.

Ask students:

What could you then add in bespeak to do the indicator solution much less acidic?Adding a base, like the salt carbonate solution, will certainly make the solution less acidic.Should you add one drop of sodium carbonate solution at a time or a couple of drops in ~ once?You should include one drop in ~ a time to better monitor how many more drops the the salt carbonate solution need to be added. How will you know when the equipment is neutralized?The shade of the equipment will be similar to the shade of the control. Call students that if the solution turns blue, it has gone native an acid, previous neutral, and also is currently a base. If this happens, try adding one or an ext drops the citric acid until the color is close come neutral. Be certain to store track that the total number of drops that acid and also base girlfriend add.

Add solitary drops of sodium carbonate to the same well in i m sorry you added the acid. Be sure to counting the drops girlfriend use and also stir through a toothpick after including each drop.


Expected Results

With every drop of salt carbonate, the citric acid equipment will relocate toward neutral, eventually coming to be green.

Note: The systems may acquire close to the green shade of the control, however will more than likely not it is in exact. This is due to the fact that the citric acid and sodium carbonate solutions are not specifically equal in the means they act as acid and base. Also, to be very exact, student would require to be able to use half-drops or even quarter-drops, i m sorry is not possible with the droppers the students room using. As lengthy as students see a trend toward the green control color, the is an excellent enough.

Table 1. How many drops of sodium carbonate walk it require to neutralize her citric acid solution?Acidic solutionNumber of autumn of citric mountain solution included to the indicatorNumber of autumn of sodium carbonate solution essential to neutralize the citric acid solutionFirst citric mountain solutionSecond citric mountain solution
3 drops

Discuss college student observations.

How numerous drops of salt carbonate walk it take it to lug the color earlier to the color of the control?Results will certainly vary yet it should take under drops of salt carbonate 보다 drops of citric acid to neutralize the solution. Does the systems become an ext acidic or less acidic as each drop of sodium carbonate is added to the indicator?The systems becomes less acidic.How do you use the color of the regulate to help you neutralize an acid?When the color of the universal indicator systems becomes close to green, the acidic solution has been neutralized. Explore

Have student compare how many much more drops of a base it takes to neutralize a much more concentrated acidic solution.

Question to Investigate

How many more drops of sodium carbonate equipment will it require to neutralize a an ext concentrated citric acid solution?

Materials for Each Group

Citric acidCitric acid solutionSodium carbonate solutionUniversal indicator solution2 flat toothpicks3 droppersSpot plateNeutralize a citric acid solution

Use a flat toothpick to include two scoops that citric acid to her citric acid solution to do it even an ext acidic. Tenderness swirl till the citric acid dissolves.

Add global indicator equipment to a clean fine in the clues plate. Include 3 fall of the an ext concentrated citric acid systems to the indicator and stir with a clean toothpick.

Ask students:

Do you think it will take more, less, or the very same amount of sodium carbonate solution to neutralize this an ext concentrated citric mountain solution?It will certainly take much more drops that the basic to neutralize the an ext concentrated citric acid solution.Thinking about the animation, why will certainly you need an ext drops of salt carbonate solution?Since the solution is more acidic, over there are much more H3O+ ions. So it takes an ext molecules of the base to expropriate the extra protons and also move an ext toward neutral.Neutralize a more concentrated citric acid solution

Add solitary drops of salt carbonate systems to the exact same well in which you added the acid. Be certain to count the drops you use and stir through a toothpick after adding each drop. Document this number in the chart.


Have college student neutralize two straightforward solutions to determine which is most concentrated.

Materials for each group

Universal indicator solutionCitric acid solutionSolution ASolution BAt the very least 6 toothpicksSpot plate3 droppers

Teacher Preparation

Make two an enig solutions using different amounts of salt carbonate.

Label 2 cups systems A and Solution B because that each group.Make a class collection of remedies A and B.Solution A: 50 mL the water and 5 toothpicks of sodium carbonateSolution B: 50 mL of water and 10 toothpicks of sodium carbonatePlace about 5 mL that each equipment in your labeled cups.

Ask students:

Solutions A and B are both an easy solutions made with sodium carbonate and also water. One of these services has an ext sodium carbonate 보다 the other. How can you number out which solution is much more concentrated? students should explain a procedure very comparable to the one they used to neutralize the two citric mountain solutions. Castle should indicate that lock neutralize each salt carbonate systems with autumn of citric acid and count how numerous drops that takes come neutralize every solution. As soon as the color of the equipment is close come the color of the control, the systems is neutralized. Just how will you recognize which equipment is the many concentrated? The equipment that needs the greatest variety of drops that citric mountain to end up being neutral is the many basic.

Discuss what students will certainly do:

For finest results, have students ar 2 fall of solution A in one well filled with indicator solution and 2 drops of systems B in another well filled with indicator solution. Then they need to add solitary drops that citric acid solution, stir, and also compare the shade to the shade of the control. Students must keep monitor of the number of drops that citric acid it takes come neutralize each solution.


Add global indicator solution to three wells in a clean spot plate.

Leave the very first well alone so the it can be provided as a control. Add 2 fall of equipment A to the second well.

Add 2 fall of solution B come the 3rd well. Neutralize systems A. Record the number of drops supplied in the chart. Neutralize solution B. Document the number of drops used in the chart. Table 2. Which systems is the many concetrated?SolutionNumber of fall of solution included to the indicatorNumber of fall of citric acid solution needed to neutralize the salt carbonate solutionSolution ASolution B
2 drops
2 drops

Ask students:

Which equipment is the most concentrated? how do friend know?Students should uncover that the takes more drops of citric acid to neutralize systems B. Therefore, equipment B need to be an ext concentrated than equipment A.Antacids space medicines people take when the acid in your stomach is causing them discomfort. One advertisement claims that the medicine offers relief for mountain indigestion and also sour stomach. What kind of chemical do you think is in the medicine?Bases neutralize acids, therefore the chemical is most likely a base.Extra Extend

Place an Alka-Seltzer® tablet computer in indicator solution and have students interpret what the color transforms say around the pH that the solution.

Explain that Alka-Seltzer® contains powdered acids and also a base. The acids room citric acid, i beg your pardon tastes a small sour, and acetylsalicylic acid, i beg your pardon is aspirin. The base is baking soda, i m sorry is likewise known by its chemistry name sodium bicarbonate.

Tell students the they will certainly observe an Alka-Seltzer tablet in a global indicator solution. Then they will use what castle know about universal indicator and also its color alters to describe whether the systems is acidic or an easy as the substances in the tablet react.

Question come Investigate

How does the pH that the solution adjust during a chemistry reaction between the ingredients in an Alka-Seltzer tablet in water?

Materials because that Each Group

Universal indicator equipment in cupWaterAlka-Seltzer tabletGraduated cylinderSnack-sized zip-closing plastic bag


Add 20 mL of global indicator solution to a snack-sized zip-closing plastic bag.Seal the bag.

Note: So the students perform not handle the Alka-Seltzer, i m sorry is a medicine, you should place an Alka-Seltzer tablet in each group’s bag.

Procedure for the Teacher

Add one Alka-Seltzer tablet computer to every group’s bag by opening the edge of the bag just sufficient so the the tablet can fit through.

Remove as much air as possible and fall the Alka-Seltzer tablet through the little opening.Seal the bag and also hand it to one of the students. Instruct this student to shake the bag and pass it approximately so that each group member has actually an possibility to hold the bag.

Expected Results

As quickly as the Alka-Seltzer tablet computer is put in the bag, the shade of the indicator solution transforms to red. Bubbles show up in the solution and the bag inflates. The solution additionally becomes cold. With time the solution becomes orange, yellow, and also finally returns to green.

Discuss college student observations.

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As the colors are changing and the bags are inflating, asking students:

What execute the color alters tell you about the pH that the systems at the beginning, middle, and end of the chemical reaction?Beginning: The systems is acidic.Middle: The systems is coming to be less acidic.End: The systems is neutralized.

Students should conclude the the acid and also base ingredients in the tablet neutralized one another.