THE PRODUCTION OF ACID RAIN
Pure water is neutral with a pH of 7. Many gases are soluble in water. Some of these gases, most notably CO2 nitrogen oxides (NOx) and sulfur oxides (SOx), form acids when they dissolve in water. Rain water falling through a atmosphere containing these gases will absorb the gases and become more acidic. This is what we call acid rain. The nitrogen and sulfur oxides from car exhaust and industry are the most serious causes of acid rain, but it is dangerous to generate those gases in the lab. Carbon dioxide forms carbonic acid in water, which also makes water slightly acidic. It is easy, safe and fun to generate CO2 in the lab and demonstrate its effect when bubbled though water, as a simulation of acid rain formation. In this reaction:
vinegar + baking soda ² ¨ CO2 + sodium acetate + H2O
HOAc + NaHCO3² ¨ CO2 + NaOAc + H2O
In this particular experiment, we would like to show how certain gases dissolved in water can make the water more acidic.
- To have the students lean about chemical reactions with water
- To show the students how this relates to the environmental world
- To help the students see the reaction which will further their understanding of the topic
- 500 ml bottle, or any clean soda bottle 16 oz or smaller
- Baking soda
- Vinegar (White vinegar is a 5% solution of acetic acid.)
- 25 ml graduated cylinder
- Two l5O ml beakers
- Bromothymol blue indicator solution*
*This is easily made from a small amount ( spatula tip or an amount the size of a small peppercorn) of the bromothymol blue sodium salt in 10 ml of water). This should be more than enough for a class of 35 students working in units of two or three.
- Pour 50 ml of vinegar into the bottle
- Put about 40 ml of water into each beaker
- Add a few drops ( it may need as much as half a medicine dropper ) of indicator to the water in the beakers. It should turn the water blue or blue-green.
- Using the funnel put two heaping teaspoons of baking soda into the balloon
- Carefully place the end of the balloon over the top of the bottle, taking care to prevent the baking soda from falling in
- Once the balloon is in place, lift it up to allow the baking soda to fall into the bottle
- The carbon dioxide formed in the reaction between vinegar and baking soda should blow up the balloon
- Pinch or twist the balloon to save the gas, and while holding the gas in the balloon, take the balloon off the bottle
- Twist the end of balloon around one end of a straw. Then place the other end of the straw in the water in one of the beakers
- SLOWLY release the pinch or twist that is holding the gas in the balloon, allowing it to bubble into the water
The indicator should turn yellow, indicating that the solution has become more acidic. You can compare this to the color of the water of the control beaker.
The carbonic acid formed in water by the dissolution of carbon dioxide is not a strong acid and only makes the pH of the water around pH = 5. In fact, most water from the faucet is initially pH = 5-6 because of the CO2 which is dissolved in it. The nitrogen and sulfur oxides from car and industrial emissions form much stronger acids when they dissolve in water, making rain with a pH as low 3, an acid level very close to vinegar.
The students could go to the library or look on the internet and look for information on acid rain. Each student could then create a booklet on the new information that they have collected, including what they could do to help the problem.
There are several other labs in this booklet that have other experiments dealing with the effects of acid rain on the environment. These include "Effects of Acid Rain on Marble Statues", "Must it Rust? The Reaction Between Iron and Oxygen", and "A Green Penny?"
There are also labs in the booklet dealing with the water cycle and this experiment on acid rain could be used in conjunction with 'The Water Cycle" to show how the acid rain is effecting the environment.