The rapid oxidation of the ribbon as it burns is shown by the small pieces of ribbon that are still bright and shiny where the tongs were holding it in the flame. When magnesium ribbon is dropped into dilute hydrochloric acid, a reaction takes place in which hydrogen gas is given off along with a lot of heat. 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. In some cases the joints are widened enough to produce holes big enough to swallow entire rivers. The faster effects of pollution Weathering is an example of a general chemical process called corrosion.