Page 57 - Curriculum Visions Dynamic Book

(Right) Rainwater is weakly acidic. As it percolates through limestone rock, it reacts to form soluble calcium bicarbonate. When this solution reaches a cave, some of the carbon dioxide escapes from the water, and the calcium bicarbonate changes to insoluble calcium carbonate, which is precipitated. In this picture a cave straw about 5 mm across is forming. It is the starting point for a stalactite.
Some materials also give out great amounts of heat when they absorb water. When water is added to lime (calcium oxide), it heats up dramatically. At the same time, the calcium oxide blocks swell and crack apart. Within a few minutes the hard lime block has been changed into a soft white powder. This is how lime is produced for use as a fertilizer on fields.
Acid water
Many gases dissolve in water. This is especially important in the case of water droplets in the air because the billions of droplets produce such a large surface area through which gases can dissolve.
The most common gas in the air, which will dissolve in water and create an acid, is carbon dioxide. It reacts with the water to produce carbonic acid (H2CO3). This is the acid that is mainly responsible for sculpturing limestone landscapes throughout the world.
Stronger acids can be produced by the reaction of industrial pollutant gases and water. Nitrogen dioxide reacts to produce nitrous acid, and sulfur dioxide reacts to produce sulfurous acid, which in turn change to nitric and sulfuric acid. These are the gases that produce acid rain. Acid rain
is primarily responsible for the deterioration of buildings containing limestone, for example, churches and cathedrals.
The reaction of these gases with water can be shown in a simple demonstration. If an indicator solution is added
to water, it first shows a blue colour, indicating lack of acidity. However, as gas is bubbled through the water,
the colour of the indicator changes until the colour is orange, showing acid conditions.
(Right) Where air pollutants increase the acidity of rainwater, the corrosion of certain stones such as limestone speeds up. Damage due to acid rain can weaken buildings structurally, such as is seen here.
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