Page 12 - Curriculum Visions Dynamic Book

Dissolving limestone
Although calcium carbonate (limestone) will
not dissolve in pure water, it will react easily
with an acid. The reaction happens naturally as acidic rainwater seeps through soils and reaches limestone rocks.
Carbon dioxide, a gas found naturally in the atmosphere, dissolves in raindrops and produces carbonic acid. Its effect on limestone is slow but unceasing. More carbon dioxide is produced
in the tiny passageways of soil. This dissolves in the water, seeps down to rocks and causes underground limestone to react and dissolve faster than surface rock.
This slow natural solution of the rock is
called weathering. Sometimes the surface soil is stripped off a limestone rock and you can see the way the joints have been widened by chemical weathering (see next page). In some cases the joints are widened enough to produces holes
big enough to swallow entire rivers.
 Sink holes
Sink holes are depressions in the surface of limestone that have been produced by extreme solution of the limestone blocks.
As the blocks dissolve away, they are no longer able to support each other, and they collapse.
Sink holes may be the entrances to entire cave systems, and rivers may disappear into them. This example is in the Tarn Valley in southern France.
EQUATION: Dissolving limestone
Water + carbon dioxide ➪ carbonic acid H2O(l) + CO2(g) ➪ H2CO3(aq)
Hydrogen
Oxygen
Carbon
Water Carbon dioxide gas
12 12