Acids and Bases
Materials:
- A minimum of one prepared leader
- Litmus paper
- Cup of water - in beaker
- Cup of milk - in beaker
- Cup of baking soda w/ water in beaker
- Cup of lemon juice - in beaker
- Cup of bleach (mild) - in a beaker
- Cup of coffee - in a beaker
- Cup of vinegar in a beaker
- Another cup of vinegar in a separate beaker
- Paper towel to clean up
Concepts Used:
- pH
- Forming Hypothesis
- Chemical Reactions
Skills Emphasized:
- Chemistry
Background:
In the world, there are millions of chemical substances. List some. Some of them have acidic properties, others have basic properties. These are acids or bases. The acid or basic level of substances is measured in pH. The pH scale spans from 0 to 14. The substances with pH lower than 7 are considered acids, those with pH equal to 7 are considered neutral and those with pH higher than 7 are considered bases (Draw this on a board). Substances with low pH are more acidic; those with high pH are more basic. Concentrated acidic and basic substances are corrosive and dangerous. When you mix acids and bases you get a chemical reaction, this can also be very dangerous (talk about chemical reactions). Luckily there is a paper called litmus paper that will change different colors if the liquid is an acid or a base.
Description of activity:
* Estimated prep time (before student arrival): 40min
* Estimated activity time: 15-35 min depending on complexity and number of students.
Teacher Preparation:
Fill beakers with liquids and LABEL THEM. Test a few for varying pH levels.
CAUTIONS:
Never use dangerous substances. Do not use strong acids or bases. Do not put dangerous chemicals in containers for alimentary use (i.e. cups, glasses, bottles) because they could be mistaken for beverages or foods. At the end of the experiment, empty the beakers and wash them. Place the containers of the remaining substances which you want to preserve in suitable places.
Experiment #1:
This first experiment serves to introduce the concept of chemical reaction.
1) Ask the class what they think will happen when you mix an acid and a base. Let them know that in this experiment you are mixing relatively mild acid with a mild base.
2) In a glass of water (half full), put some teaspoons of baking soda and mix in order to obtain a concentrated solution.
3) In the same glass, pour a spoon of vinegar - there will be an abundant production of foam. What happened? A chemical reaction between baking soda (a basic substance) and vinegar (an acid substance) occurred. These two substances reacted with each other, producing a salt, water, and carbon dioxide. It was this gas which produced the little bubbles. In general, acid and basic substances react with each other, producing a salt and often other substances like water and carbon dioxide. If you want a livelier the reaction, use warm (not hot) water.
Experiment #2
1) Assign groups of students to different beakers of liquid.
2) Have them make a hypothesis (explain what a hypothesis is) about if their liquid is an acid or a base.
3) Have them use a piece of litmus paper to test the solution (follow litmus paper instructions).
4) Have them assign the liquid a pH using the litmus key.
5) Have each group rotate to the different beakers so they can see the litmus paper from each solution.
Review and Questions:
Talk about what substances turned out to be acids and which were bases. Talk about why acid and bases are often used in cleaning products and drain cleaners (corrosive properties).
What color did the litmus paper turn with different liquids?
Why was there a chemical reaction? (stronger acid and base mixed together will usually create a reaction)
Additional Resources:
For more fun experiments with acids and bases check out:
http://www.funsci.com/fun3_en/acids/acids.htm
Alternatives for older/younger ages:
* For older students: Talk about acids and bases on a chemical level (Acids are substances which free hydrogen ions when they are poured in water. Bases are substances which free hydroxide ions when they are poured in water - ext.) Go into depth on what a chemical reaction is.
* For younger students: Leave out the stuff about ph and just stress the distinction between acids and bases and what happens when you mix them.
Idea for the activity came from:
Chemistry classes and