Effects of Acid Rain on Plant Growth Lab

To stimulate the effect of acid rain on plant life, students to set up a lab experiment to compare the effect of varying levels of acidity on plant seedlings and seed germination.

Industries and motor vehicles produce gaseous oxides of nitrogen and sulfur. For example nitrogen and oxygen in the air can combine under high-temperature engine condi­tions to produce nitrogen dioxide (NO₂). The equation for the reaction is:

N₂ + 20₂ --> 2NO₂.

Sulfides in fuels can combine with oxygen to make sulfur dioxide (SO₂) and sulfur trioxide (SO₃). Such oxides combine with water in the atmosphere to make acids. For example, nitrogen dioxide and sulfur trioxide combine with water. They form nitric acid (HNO₃) and sulfuric acid (H₂SO₄), respectively. The equations for these reactions are:

• 3NO₂ + H₂0 ---> 2HNO₃ + NO
• SO₃ + H₂0 ---> H₂SO₄.

The presence of these acids causes rain to be acidic. Acid rain damages trees, crops, and buildings. It can make lakes so acidic that fish cannot survive. In this investigation, students will moisten seeds and seedlings with acidic solutions and then observe how many seeds germinate and how well the seedlings grow.

Background

The acidity of solutions is measured using the pH scale, which extends from 1 to 14. The pH of a solution is defined as the negative of the logarithm of the hydrogen-ion concentra­tion (-log [H+]). For example, [H+] in pure water is 1.0 x 10^-7. The pH of pure water is therefore -log (1.0 x 10^-7), or 7. The pH of acidic solutions is less than 7, and that of basic solutions is greater than 7. The lower the pH below 7, the more acidic is the solution.

MATERIALS (per group)

* Seeds and Seedlings
* Filter Paper and Petri Dishes
* pH Solutions (6.0, 4.0, 2.0) and Droppers

Procedure

Effect of pH on Seed Germination

1.   Label 3 petri dishes for each pH solution.

2. Place filter paper in each the petri dish with 10 seeds. Cover the seeds with another piece of filter paper. Use a dropper to moisten the filter paper through all layers with the appropriate pH solution. Put the dishes in a sealed plastic bag and place them in a dark place.

3.   Four days later, count the number of seeds that have germinated.

4.   Measure the root length of each of the seedlings. Record observations about seedling condition (color and overall growth and appearance of seedling leaves).

5.   Tally class averages for each pH for germination and root length. Graph your data and the class averages. Photograph seedlings.

Effect of pH on Seedling Growth

1.   Label seedlings pots with each of the pH solutions.

2.   Measure stem and leaf lengths of seedlings. Take photos of plants.

3.   Water the seedlings and moisten the soil each day with the appropriate pH solution each day for two weeks.

4.   Measure stem and leaf lengths at the end of the week. Compile class data. Calculate averages and graph data. Take photos of plants.

5.   Observe leaf color, lesions and spotting on leafs and stems, presence of dead leafs, and any other signs of damage.

Data Analysis: Construct tables and graphs for data. Write a discussion of the data answering the following questions.

1.   What relationship did you see between seed germination and pH? Between seedling growth and damage and pH?

2.   Some rain has been found to be as acidic as pH 2.8. Given what you have observed, what is the likely effect on seed ger­mination, growth rates and plant reproduction?
 

This lab can be used as a part of an AP Environmental Science Class.

Contributed by: Paula Wang, Sidwell Friends School

For additional information, click here.