Generation of nitrogen dioxide gas and then nitric and nitrous acids. In the conical flask at left, nitric acid is dripped onto copper metal, producing copper nitrate and brown nitrogen dioxide gas. This gas passes through a glass tube into a jar of water, which also contains universal indicator. In neutral water, the indicator has a green colour. When the gas contacts the water it dissolves and reacts to form nitrous and nitric acids, which rapidly change the colour of the indicator to red. This reaction with water also occurs in the atmosphere, where nitrogen dioxide is an important industrial pollutant. The resulting acids fall as acid rain, acidifying soils and damaging building materials.