Learn the basics about the reactions of halogens, when learning about the periodic table as a part of properties of matter.

Group 7 of the periodic table is the halogens.

The reaction between diatomic chlorine gas and cold sodium hydroxide solution is very popular in industry because the resulting mixture of the three products works perfectly as a bleach.

Different halogens can react with each other. Chlorine and iodine can react together to form iodine monochloride, which is a brown liquid and vapour.

The halogen Fluorine can be reacted to produce hydrofluoric acid, which is a really dangerous acid. It cannot be stored in glass bottles, like other acids are. Hydrofluoric acid is so corrosive that it can attack and dissolve glass. It does this by forcing the silicon dioxide component of the glass, which is usually chemically inert, to react, forming silicon tetrafluoride and water. It’s the only acid capable of doing this.

Hydrofluoric acid can only be stored in certain types of plastic, which are resistant to its corrosive power. But we can exploit this power too: hydrofluoric acid is used industrially to create patterns on glass, a technique called ‘etching’.

Iodine is another halogen, and it is used to test for the presence of starch. An interesting use of testing for starch is to detect fake banknotes. The paper used to print official banknotes is starch-free, but most common types of paper contain starch and the ink will show up blue.

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