Explore the effects of acid rain on rocks and minerals.
What you need
Be careful not to rub your eyes when handling vinegar. Wash your hands immediately after finishing this experiment.
What to do
Break chalk into approximately 1/2 cm pieces.
Crush each 1/2 cm piece of chalk into smaller pieces, keeping each crushed 1/2 cm portion separate from each other.
Pour 100mL of vinegar into a container.
Add one crushed 1/2 cm piece of chalk to the container and observe the changes that take place.
Record what you see happening.
Add another piece of chalk to the container. Record what you see happening.
Continue to add chalk until you do not see any more changes taking place.
When the chalk (which is made of a base called calcium carbonate) is added to an acid like vinegar, a chemical reaction occurs. This reaction is called a neutralization reaction and occurs when an acid (pH less than 7) and a base (pH more than 7) are combined. During the reaction a gas and a neutral solution (pH equal to 7) are formed.
The gas produced in this activity is carbon dioxide and can be seen as bubbles in the vinegar. When the chalk is crushed before being added to the vinegar, the reaction will take place faster. If you keep adding more caulk, eventually you will notice that the bubbles stop forming. This shows that all the acid in the vinegar has been used up and the remaining liquid is now neutralized, meaning no further reaction is taking place. The solid chalk seems to disappear because it has been changed into another substance that has dissolved in the remaining liquid.
This neutralization reaction occurs naturally in the environment when weak acids in rain react with limestone and other rocks, resulting erosion (the wearing away of rock). This reaction occurs very slowly and the effects are not normally seen for hundreds or even thousands of years.
Acid rain is formed when certain pollutants dissolve in rain creating stronger acids. Acid rain is one factor that can increase the rate of erosion, with effects that can be seen in just a few decades. Limestone and marble are composed calcium carbonate, the same material as chalk, and are commonly used for constructing buildings and statues. Rain that is too acidic will “eat away” at these structures very quickly, the same way the vinegar ate away at the chalk.
Why does it matter?
Acid rain erosion of limestone and marble can result in a loss of artwork (outdoor statues, monuments, plaques, etc) that cannot be recovered. This increases the cost of maintaining buildings. Destruction of ecosystems like lakes and rivers can occur when they become too acidic, which can result in fish and other water-dwelling creatures not surviving, and no longer being a food source for birds and other animals.
On the positive side, regions where the bedrock or soil contain large amounts of limestone are less likely to have polluted water due to acid rain than areas with igneous bedrock. This is because the limestone (which is a base) is able to neutralize acid rain before it gets into the lakes and rivers, in much the same way the chalk in our experiment neutralized the vinegar. This means that damage due to acid rain depends on both the pH (amount of acid in a substance) of the rain and the type of soil/bedrock.