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Chemical properties of iron

The valence of pure iron is generally +2 and +3. Iron does not change in the air. Iron containing impurities gradually rusts in humid air, and rusts particularly quickly in humid air containing acid gas or halogen vapor. Oxygen will dissolve in iron at not too high a temperature, and iron and oxygen will form stable oxides (FeO and FezOs). For example, cast iron is easy to rust, and its rust composition is: 7.43% H:0, 11.49% Fe0, 81.08% FeO3.

Iron can be well dissolved in dilute sulfuric acid, dilute nitric acid and hydrochloric acid. Concentrated nitric acid or cold concentrated sulfuric acid can passivate iron. The so-called passivation is the process in which some metals are treated by anodizing or chemical methods (such as strong oxidant reaction) to change from an active state to an inactive state (passive state). The passivated metal (such as iron immersed in concentrated nitric acid) is not easy to corrode because a tight oxide protective film is formed on the surface.

When heated, iron can react with non-metals such as halogens, sulfur, phosphorus, silicon and carbon. Iron reacts with sulfur to form iron sulfide (FeS), which makes iron products brittle. Iron reacts with carbon to form carbides, which can increase the hardness, strength and resistivity of iron. The interaction between iron and carbon is the basis for the modern production of carbon steel with various properties. In industry, iron can be divided into pig iron (also known as "cast iron", with a carbon content of more than 2%) and industrial pure iron (with a carbon content generally below 0.04%) according to the different carbon content in iron. The carbon content between 0.04 and 2% is called "steel". When the carbon in the iron-carbon system is at 400°C, the FeO and F0O: contained in the iron can be reduced to generate gases 00 and 0O2. Since these gases are continuously released during the manufacture and use of electric vacuum devices, the vacuum properties of iron are poor, which is the main reason that hinders the widespread use of iron in the electric light source manufacturing industry.

Alkali acts very slowly on iron. Unlike many other metals, iron does not react with mercury, so it can be used to make mercury storage devices.

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