Acid and Base pH Indicators Lab Activity 1
Indicators give scientists a quick way to determine if a solution is an acid or a base without having to taste them or performing time-consuming chemical tests. The most common of these indicators are litmus and universal indicator.
Litmus paper comes in both red and blue colors. To use this paper, simply dip the strip into the liquid and observe any changes. If the substance is a solid/powder, then you will need to dissolve it in a small amount of water first. The table below shows you a summary of what could expect to see.
| Type of litmus | Acidic Solution | Basic Solution |
| Red litmus | Stays red | Turns blue |
| Blue litmus | Turns red | Stays blue |
Universal Indicator has a much larger color range than litmus. This is due to its ability to determine the strength of the acid or base as well as if the substance is neutral. The table below gives you a summary of what you can expect to see for an acidic, basic, or neutral solution.
| Type of solution | Color of Universal Indicator |
| Acid | Red, Orange, yellow or light green |
| Base | Dark green, blue, purple, violet |
| Neutral | Green |
Acid and Base Indicators Lab Equipment
- Spot tile
- Dropper
- Safety glasses
- Universal indicator solution or paper
- Red and blue litmus paper
- A variety of household chemicals (clearly labelled)
Method
- Add 2-3 drops of the chemical being tested to a spot on your tile.
Note: If a substance is solid or powder you will need to add a few drops of water and mix it to form a solution before testing.
- Test the chemical with both colors of litmus paper and record your results in the table.
- Test the chemical with universal indicator and record results in the table below.
Results/Observations
| Household chemical being tested | Color in red litmus | Color in blue litmus | Color in Universal Indicator |
Indicators for Acid and Base Lab Activity 2
Indicators can be obtained from many plant tissues.
Red cabbage juice is a dull purple color and contains a natural pH indicator. This indicator changes colors according to the acidity of the solution due to the presence of the pigment molecule called flavin (an anthocyanin). Flavin is water-soluble and is also found in apple skins, beets, plums, poppies, cornflowers, and grapes.
In very acidic solutions, red cabbage juice indicator will turn a red color. Neutral solutions result in a purplish color. Basic solutions appear in greenish-yellow. Therefore, it is possible to determine the pH of a solution based on the color it turns the anthocyanin pigments in red cabbage juice.
The spice turmeric contains the yellow pigment curcumin which changes from yellow in an acidic and neutral solution to red in basic solutions.
Indicators for Acid and Base Lab Equipment
- beakers
- pipettes/plastic droppers
- safety glasses
- spot tiles
- universal indicator
- cabbage juice
- milk
- soapy water
- lemon juice
- vinegar
- soda
- tap water
- baking soda
- hand sanitizer
Safety
- Cabbage juice and universal indicator are highly pigmented and stain both clothing and skin.
- Treat all food stuffs being tested in this lab as contaminated, avoid tasting/eating any samples.
- Wear safety glasses for the duration of the lab.
Method
- Use a pipette to place 2-3 drops of the first sample to be tested into two wells on the spot tile.
- In the first well add a drop of universal indicator solution and record any color change.
- Using a clean pipette, put 3 drops of cabbage juice into the second well. Record any color changes.
- Repeat steps 1-3 for each of the sample substances and record all observations in the table below. The first row has been completed as an example.
Results:
| Name of substance | Observations and Prediction | Test with Universal Indicator | Color in cabbage juice indicator | Acid, base, or neutral? |
| Example: Grapefruit Juice | Sour tasting – Acid | Orange | Pink | ACID |
| Milk | ||||
| Soapy water | ||||
| Lemon juice | ||||
| Vinegar | ||||
| Soda | ||||
| Tap water | ||||
| Baking soda | ||||
| Hand sanitizer |
The Indicators of Acid and Base Activity:
Students record their predictions and results of the lab activity. Once the activity is finished, discuss the lab activity and answer post-lab questions.
We also provided some of the Resources below for you to use in class. iTeachly Members can access all the content for the activity and the following lesson.
Students must come up with a unanimous answer for each question. Discussing the answers usually, results in some excellent dialogue on this topic.
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