Acids and Bases

Vanderbilt Student Volunteers for Science

Training Presentation

Spring 2014

Important!!!

• Please use this resource to reinforce your understanding of the lesson! Make sure you have read and understand the entire lesson prior to picking up the kit!

• We recommend that you work through the kit with your team prior to going into the classroom.

• This presentation does not contain the entire lesson—only selected experiments that may be difficult to visualize and/or understand.

I. Intro Demo—Defining acids and bases

• Have 2 VSVS volunteers hold the 3 clear plastic glasses up so students can see them.

• Tell the students that you have three liquids.

• Pour the contents of the bottle labeled V (acid – vinegar) into the first glass.

• Pour the contents of the bottle labeled W (water) into the second glass.

• Pour the contents of the bottle labeled B (base) into the third glass.

• Have a student describe the three liquids they see. Describe liquids based on visual cues only.

I. Intro Demo—Defining acids and bases (cont.)

• Ask students if they have ever heard of litmus paper. If so, what is it used for?

– Litmus paper is used to test whether something is acidic or basic.

• Test each glass of clear liquid by dipping first the red and then the blue litmus paper into the liquid and noting what changes, if any, occur. (You may want to record the changes on the board for later reference.)

• Do not identify the liquids at this point. The vinegar should turn the blue litmus paper pink; the water should not change the color of either paper, and the baking soda water should turn pink litmus paper blue.

II. Discussion of Acids and Bases

• Note: Select the information that is appropriate for the class you are teaching. Try to keep the information on a level that the students can understand. Feel free to add other appropriate information. Keep this discussion brief.

• ACIDS– Weak acids like the natural acids in food give foods a

sour, sharp flavor. – Strong acids can burn your skin.– Many acids are corrosive. They eat away metals and

other substances. – Some acids can be helpful. The acid in your stomach

aids in digestion.– Two acids (sulfuric acid and nitric acid) cause damage

in acid rain.

• BASES– Bases taste bitter and feel slippery.– Weak bases are used to settle upset stomachs.– Detergents and many cleaning solutions are basic.– Strong bases can burn the skin.

III. Testing Acids and Bases

• Ask: How can scientists tell which solutions are acidic or basic?– (Based on the first experiment,

some students may be able to reason that scientists could use the different color of litmus to tell how acidic or basic a solution is.)

• Ask students, “When blue pH paper is used as the indicator, what color do the acid solutions turn it?” (pink)

• Ask students, “When pink pH paper is used as the indicator, what color do the base solutions turn it? (blue)

Some pH Indicators

Adapted from http://www.ausetute.com.au/indicata.html

Acid Neutral Base

pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Universal indicator

cyanidin (red cabbage)

blue litmus

red litmus

phenol- phthalein

thymol blue

phenol red

bromo-thymol blue

methyl red

bromo-cresol green

methyl orange

bromo-phenol blue

cresol red

pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

There are many indicators that can be used to show different pH’s. • Tell students to look at the

handout. Point out a few examples of how different indicators can be useful at different pH’s.

For example:

• Litmus can tell us if something is either a base or an acid

• Methyl orange indicator changes color at ph3/4 . It is red below pH 4 and yellow above pH 4.

• Universal indicator has a large number of color changes at different pH’s and can more precisely tell us what the pH of a solution is.

III. Testing Acids and Bases (cont.)• Put the Testing Household Items

Results chart (see worksheet) on the board.

• Organize the class into 8 groups. Students will conduct the experiments in pairs but will share dropper bottles.

• Distribute the following materials to each group:– 2 well trays (1 per pair) , 4

observation sheets and 4 Results sheets (1 per student).

– 1 bag containing dropper bottles of household substances (1 per group of 4)

– 2 containers of each: red litmus paper, blue litmus paper, and pH paper (at least 15 of each)

III. Testing Acids and Bases (cont.)

• Tell the students to:– Arrange the bottles of

household chemicals in numerical order (1-12: there will not be #s 2, 9, or 10).

– Place their well-plate on the instruction sheet on top of the diagram, so that they can see the numbers through the bottom of each well.

III. Testing Acids and Bases(cont.)

• Squirt a small amount of liquid from the #1 dropper bottle (vinegar) into the first well.

• Test its acidity and basicity by dipping a red and blue piece of litmus paper into the liquid in the well.

• Test its pH by dipping a piece of universal indicator paper into the liquid in the well. The color of the pH paper should immediately be compared to the label on the vial to figure out the correct pH number.

III. Testing Acids and Bases(cont.)

• Place the 2 litmus papers and the pH paper in the rectangles marked #1 (vinegar).

• Report findings– Circle the color change and

record the pH number on the Results sheet.

– VSVS will complete the chart on the board as each chemical is tested.

– VSVS team members need to monitor students for correct results.

• Repeat with all the liquids

IV. Neutralization of an Acid with a Base

• Ask students if they know what happens when an antacid is taken for an upset stomach– Stomachs contain hydrochloric acid, with a resting

pH of 4-5. – This pH drops after eating because HCl and

proteases are released to aid in digestion. – pH can get down to 1-2 after a high-protein meal. – Too much acid in stomach indigestion. Antacids

react with stomach acid to neutralize it.

IV. Neutralization of an Acid with a Base (cont.)

• Pass out dropper bottles of HCl, NaOH and Universal Indicator liquid.• Tell students to:

– 1. Put a squirt of 0.1 M HCl in the first well on the bottom row (D1), add a few drops of universal indicator, and note the color (red).

– 2. Do the same for 0.1 M NaOH in D2 (color should be blue).

– 3. In D3, add just 5 drops of HCl and a few drops of indicator. Add 0.1 M NaOH one drop at a time, counting drops added, and observing the color change until it is at neutral color (green). Stir with a toothpick after each drop.

D1

D2

D3

Discussion

• Ask students:• Are the foods that we have tested acidic or basic?

– Mostly acidic.

– Are the cleaners that we have tested acidic or basic?– Mostly Basic (cleaners used on your body are sometimes

slightly acidic).

• Is there any difference in pH of distilled water or rain water?– Rain water usually tests slightly acidic because of dissolved

carbon dioxide. However, pollutants from burning fossil fuels such as sulfur oxides and nitrogen oxides can cause the rain water to be more acidic (hence acid rain).

– Distilled water should be neutral.