Nerdy Science Blog

Back in secondary school, “acid rain” was one of my presentation topics. The fact that acid rain erode and ruined the statues and buildings amazed me. Why is it happening?

What is acid rain?

Distilled water which is neutral has pH 7. Anything that is less than pH 7 is considered as acidic. Normal rain which is unpolluted is slightly acidic, somewhere around pH 5.6. This is due to the reaction of carbon dioxide and water forming carbonic acid.

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Acid rain is the result of human activity. It is normally used to refer to the acidic deposition in rain, snow, dew, fog, or dry particles. There are different types of acidic precipitation that can be found in acid rain, which are:

1. Sulfurous acid [H₂O₂ (aq) + H₂SO₃ (aq) → H₂SO₄ (aq) + H₂O (l)]
2. Sulfuric acid [H₂SO₄]
3. Nitric acid [3NO₂ (g) + H₂O → 2HNO₃ (aq) + NO (g)]

Normal rainwater is however weakly acidic because rainwater contains dissolved CO₂ from the air.

What is rock?

Rock here is referred to marble and limestone, which are both consisted of calcium carbonate, CaCO3 (just like eggshell). They are commonly used in architecture, especially in constructing buildings, monuments and statues. However calcium carbonate is reactive to acid solutions. When acid rains have contact with the limestone or marble, they will erode.

For example, limestone building contacts with sulfuric acid rain.

CaCO₃ (n) [building] + H₂SO₄ (aq) [acid rain] → Ca²⁺ (aq) + SO₄^2- (aq) + H₂O + CO₂

Limestone or marble (calcium carbonate) buildings or monuments surface will be damaged and destroyed due to erosion.  Image (right) showed the corrosive effects of acid rain and wind that damage the carving of St. Peter (source).  The cleaning of limestone buildings cannot be done with acid-based cleaning chemicals.  Only neutral or mild alkaline-based cleaner can be used to clean the buildings or monuments.

Experiment

Simple experiment can be done to observe the effect of acid rain on limestone.  All you need are:

• 3 drinking glasses
• lemon juice
• vinegar
• water
• 3 chalks

Procedures:

1. Fill glass 1 with 1/2 glass of lemon juice, glass 2 with 1/2 glass of vinegar, and 1/2 glass of water in glass 3.
2. Then chuck one piece of chalk into each glasses (make sure part of the chalk is in the liquid).
3. Check the glasses few days later.

After couple of days, you will see that the chalks in the lemon and vinegar glasses are dissolved.  This is due to the calcium carbonate (chalk is consisted on calcium carbonate) reacts with the acids in lemon juice and vinegar.

Chinese Idiom.

There is a Chinese idiom “the drops of water penetrate the rock”.

Rock bastard, “Hey, what are you doing here?”

Water, “I want to pass through you.”

Rock bastard, “Haha, what makes you think you can pass through me?  You are just a tiny little rain drop.”

Water, “I will pass through you!  You just need to wait and see!”

After thousands of years, the water finally made a hole through the rock and passed through it.

The meaning of this idiom is to show that if you have power of perseverance, you can do anything.

England’s White Cliffs of Dover

England’s White Cliffs of Dover is composed of chalk (calcium carbonate) and the cliff face erodes at the rate of one centimetre per year.  If you lean against the cliffs, your body will be covered with white powder.