I started out by asking a couple of review questions about liquids: Do they have a definite shape? Do they have a definite volume? Do they take up space? I then told the class that almost all liquids they encounter will have either acidic or basic properties. One exception to this is distilled/pure water that is neutral.
We moved on to discuss some acids the students knew about - lemon juice and other citrus fruits and vinegar. They also knew there is acid in batteries. I told them about amino acids that make up proteins and can be found in proteins in our bodies and our foods.
Fun fact: The word acid comes from the Latin acidus meaning sour or sharp. Another word for basic is alkali.
Fun fact: The term pH was introduced in 1909 by the Danish chemist Soren Sorenson. pH stands for pondus hydrogenii meaning potential hydrogen.
Why "potential hydrogen?" When an acid is placed into an aqueous solution, it breaks up into positive hydrogen ions (H+) and another compound. A base, on the other hand, will break up into negative hydroxide (OH-) ions and another compound.
The pH Scale
This scale is used to determine how acidic or basic something is. The scale goes from 0 (most acidic) to 14 (most basic). 7 in the middle is neutral. Solutions that are very basic or very acidic are also very toxic. More H+ ions indicate a lower pH number or more acidity. A greater number of OH- ions indicate a higher pH and a more basic compound. For example, battery acid has a pH below 1. Drain cleaner is a very basic compound that has a pH near 14.
Although there may be many different ions in a substance, the pH scale only focuses on those two listed above: hydrogen and hydroxide.
Green Pennies - This had to sit for 24 hours so I prepped it at home then brought in the pennies to show the class.
For this, I doused a folded paper towel in vinegar then placed four pennies on the towel. I let them sit for almost 24 hours. The acetate in vinegar reacts with the copper of the pennies to form a green coating, copper acetate.
Cleaning Pennies labs - We used two acids to clean dirty pennies; carbonic acid in Coca-Cola and acetic acid in vinegar.
Before we started cleaning, though, I asked the students what they thought causes pennies to become dirty. I gave them a hint: oxidation. The students know oxidation (the reaction between iron and oxygen) causes metal objects to rust. Dirty pennies are caused by a similar reaction. The copper in the pennies reacts with oxygen in the air to form copper oxide. This is a black powder that coats the pennies.
Carbonic acid - For this we just poured some Coke into a cup and added a couple of pennies. It takes a while but the pennies will become nice and shiny.
Acetic acid - We poured 1/4 cup of vinegar into a cup and then stirred in 1 tsp. of salt until it dissolved. Each student then dipped a penny halfway into the vinegar-salt solution and held it for 20 seconds. This is all it took to clean the half of the coin that was in the vinegar.
The acid (either carbonic or acetic) eats away at the copper oxide leaving the pennies clean and shiny. You could also do this with lemon juice (vitamin C is ascorbic acid).
Copper-Plated Nails lab - This is fun to try at home if you have steel nails or screws. After cleaning the pennies with the vinegar and salt, keep the liquid mixture. Positively charged copper ions from the pennies are now floating around in the salt and vinegar. If you place a steel nail or screw in this mixture, the acids will dissolve some of the iron from the steel and will leave a negative charge on the nail. The positive copper ions are then attracted to the negative ions in the nail. The copper then attaches to the nail giving it a slight copper color.
This also creates a little hydrogen gas (the bubbles you will see) as the hydrogen ions in the acid react with the metals in the nail.
I was a little worried this wouldn't work since the website I got this from said to use steel nails/screws. I had boxes upon boxes of galvanised nails and zinc wood screws but no plain old steel ones. We tried it with all of the different nails and screws and actually got a nice result on the wood screws. The galvanised nails are coated with something that protects them from the acids in wood so I had a good feeling this would not work on them. Worth trying, though, since it's part of the learning process!
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| Before and after photo of the copper-plated wood screw. |
Properties of Acids
1) Acids taste sour. I prompted the students to think of a lemon. If you cut into a lemon and taste it, it's very sour. Taste, however, is not a good way to test for acids. :)
2) Acids are electrolytes. This means they can conduct electricity due to the presence of the positive hydrogen ions. Our bodies require four electrolyte minerals to stay healthy: sodium (Na+), potassium (K+), chloride (Cl-), and phosphorous (HPO-). I asked the students to identify the two electrolytes listed that are acids. We also discussed electrolytes in foods: salts, bananas, Gatorade (and similar products).
3) Acids are corrosive. Some acids react strongly with certain metals and can "eat away" at them. Acids can also be dangerous to human skin.
4) Acids react with certain compounds, called indicators, to produce predictable changes in color. More on this later.
Common Acids
We spoke about some of these at the beginning of class. Vinegar is acetic acid. The acid in car batteries is sulfuric acid. We also know about carbonic acid in sodas and ascorbic acid which is vitamin C. We also discussed stomach acid - hydrochloric acid (HCl). This is an extremely strong acid that helps break up and digest foods.
Properties of Bases
1) In a pure, undissolved state, most bases are crystalline solids. In solution, bases feel slippery and have a bitter taste.
2) Just like their acidic counterparts, bases are corrosive and strong bases such as drain cleaner can cause burns and tissue damage.
3) Remember Cl- and HPO- from above? Bases can be electrolytes, too. The ones with a negative charge (chloride and phosphorous) are the basic electrolytes that the body needs to stay healthy.
4) Bases also react with indicators to produce predictable color changes.
Common Bases
Some common bases are ammonia (NH3); sodium hydroxide (NaOH); and aluminum hydroxide [Al(OH)3]. Ammonia is a cleaner; sodium hydroxide is used in the manufacture of soaps, detergents, pulp for paper, and textiles; and aluminum hydroxide is an ingredient in anti-perspirant.
Acid and Base Testing lab - We used pH paper to test for the presence (and strength) of acids and bases in a variety of materials. This included some citrus fruits (orange, lemon, grapefruit); some coffee; ammonia; and vinegar. The pH paper allowed the students to see where on a pH scale the various items fell.
Testing for Bases lab - We used turmeric paper (a base indicator) to test for the presence of a base in ammonia. We dipped a piece of turmeric paper in some ammonia and watched it turn red.
How to make turmeric paper: Mix 1/3 cup of rubbing alcohol with 1/4 tsp. turmeric. Stir to dissolve then dip coffee filters (the unbleached ones work fine) into the mixture. Place the wet coffee filters on a cooling rack placed on a cookie sheet (to avoid yellow countertops) and let them dry. You can then cut the coffee filters up into strips and use them to test things around the house for bases.
Wet Only lab - This one showed that you must add water in order to test dry solids with the turmeric paper.
I passed around a small amount of baking soda in a cup and asked the students to test it with a dry strip of turmeric paper. The indicator did not react with the dry soda. However, when we dipped the turmeric paper in water then in the baking soda, it turned red.
Neutral lab - We actually did this last but it used the turmeric paper. In this we neutralized a base. I poured a tiny amount of ammonia into a plastic cup. We used a piece of turmeric paper to show that it really is a base. I then added a tiny amount of vinegar to the ammonia and stirred. The students then dipped a second piece of turmeric paper into the new mixture and saw that the color did not change.
The acidic vinegar and basic ammonia cancelled each other out and the products formed are neutral.
Baking with Acid? lab - This led into a discussion of leaveners and acids in baked goods.
Step 1: Mix 1 tsp. of baking powder with 2 tbsp. of water.
Step 2: Mix 1 tsp. of baking powder with 2 tbsp. of vinegar.
Step 3: Mix 1 tsp. of baking soda with 2 tbsp. of water.
Step 4: Mix 1 tsp. of baking soda with 2 tbsp. of vinegar.
We observed bubbling and foaming in steps 1, 2, and 4. Step 3 created a milky white solution but no bubbling.
Baking powder consists largely of sodium bicarbonate (baking soda) and an acid. If you're out of baking powder, you can actually make your own by mixing baking soda and acidic cream of tartar. Adding water to the baking powder activated the acid and created carbon dioxide gas.
When we added vinegar to the baking powder, it reacted with the sodium bicarbonate to produce CO2.
Adding vinegar to the baking soda (sodium bicarbonate) created, you guessed it, more CO2.
The carbon dioxide gas created from these reactions in baking pushes the batter of a cake, muffins, or bread up/causes it to rise. The heat of the oven then bakes the batter in this elevated state. When you're baking with baking soda alone, you need to add an acid to create the CO2. Recipes may call for buttermilk, cream of tartar, or vinegar to help the batter rise.
I finished up class with a really quick discussion on acid rain. Acid rain is caused by sulfur and nitrogen compounds (air pollution from factories, car exhausts, etc.) that mix with water vapor in earth's atmosphere to create nitric acid and sulfuric acid. These acids then return to earth as acid precipitation (this could include acid rain, acid snow, acid fog, etc.) Acid rain can cause all sorts of harm from damaging buildings, statues, and trees to leading to water pollution.
More on acid rain:
Next week:
We'll get back into physics next week by studying waves and light.
References:
The labs Green Pennies, Baking with Acid?, Wet Only, and Neutral are all from Chemistry for Every Kid. I also got the idea for making turmeric paper from this book.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.
The lab on cleaning pennies with vinegar and then copper-plating the nails came from this website:
More information on acids and bases: http://www.chem4kids.com/files/react_acidbase.html
How to make cabbage indicator paper (from Chemistry for Every Kid):
This will produce a testing paper that will indicate both acids and bases. It will turn green in the presence of a base and a pinkish-red in the presence of an acid.
You will need an uncooked red cabbage and distilled water for this.
Fill one jar (or glass bowl) with torn pieces of cabbage leaves. Boil the distilled water and carefully fill the jar or bowl containing the cabbage with the water. Let the jar sit until the water cools to room temperature.
Pour the liquid through a strainer into a second jar or bowl and discard the cabbage leaves.
You can now dip coffee filters into the cabbage juice and let them dry to create a pale blue acid-base testing paper.
How to make cabbage indicator paper (from Chemistry for Every Kid):
This will produce a testing paper that will indicate both acids and bases. It will turn green in the presence of a base and a pinkish-red in the presence of an acid.
You will need an uncooked red cabbage and distilled water for this.
Fill one jar (or glass bowl) with torn pieces of cabbage leaves. Boil the distilled water and carefully fill the jar or bowl containing the cabbage with the water. Let the jar sit until the water cools to room temperature.
Pour the liquid through a strainer into a second jar or bowl and discard the cabbage leaves.
You can now dip coffee filters into the cabbage juice and let them dry to create a pale blue acid-base testing paper.
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