Silicon dioxide is widely distributed in nature, such as quartz, quartz sand, etc. White or colorless, and light yellow with high iron content. The density is 2.65 ~ 2.66. Melting point 1670 ℃ (lepidolite); 1710 ℃ (cristobalite). Boiling point 2230 ℃. It is insoluble in water and slightly soluble in acid. When it is fine, it can act with melting and alkali.


The chemical formula SiO2 of silicon dioxide, formula quantity 60.08, also known as silica, is a hard and insoluble solid. It often appears in three varieties of quartz, scaly quartz and cristobalite. Almost 65% of the ore is silicon dioxide 16 kilometers below the ground. Natural silicon dioxide can be divided into crystalline and amorphous. Crystalline silicon dioxide mainly exists in quartz ore. Pure quartz is colorless crystal, and large and transparent prismatic quartz is crystal. Silicon dioxide is an atomic crystal with tetrahedral structure formed by silicon atoms and four oxygen atoms. The whole crystal can be regarded as a huge molecule. SiO2 is a good simple formula, which does not mean a single molecule. Amorphous silica is a white solid or powder.


The chemical properties of silicon dioxide are very stable. It is insoluble in water and does not react with water. It is an acidic oxide and does not react with general acids. The nature of silicon dioxide is not active. It does not interact with halogen, hydrogen halide, hydrogen halide, sulfuric acid, nitric acid and perchloric acid except Fluorine, hydrogen fluoride and hydrofluoric acid.


Hydrogen fluoride (hydrofluoric acid) is an acid that can dissolve silicon dioxide to produce fluorosilicic acid that is easily soluble in water. The reaction formula is as follows:


SiO2 ＋ 4HF = SiF4↑ ＋ 2H2O

Silica and alkaline oxides:

SiO2 ＋ Cao = (high temperature) CaSiO3

Silicon dioxide can be dissolved in concentrated hot strong alkali solution:

SiO2 ＋ 2NaOH = Na2SiO3 ＋ H2O

(the reagent bottle containing alkali shall not use glass stopper but rubber stopper)

Under high temperature, silicon dioxide can be reduced by carbon, magnesium and aluminum:

SiO2＋2C=Si＋2CO↑

The chemical properties are relatively stable. It is insoluble in water and does not react with water. It is an acid oxide and does not react with ordinary acids. Gaseous hydrogen fluoride reacts with silicon dioxide to form gaseous silicon tetrafluoride. React with hot concentrated alkali solution or molten alkali to form silicate and water. It reacts with various metal oxides at high temperature to form silicate. Used for manufacturing quartz glass, optical instruments, chemical vessels, ordinary glass, refractory materials, optical fiber, ceramics, etc. The nature of silicon dioxide is inactive. It does not interact with halogen, hydrogen halide, sulfuric acid, nitric acid and perchloric acid other than fluorine and hydrogen fluoride (except hot concentrated phosphoric acid). Common concentrated phosphoric acid (or pyrophosphoric acid) can corrode silicon dioxide and generate heteropoly acid at high temperature. Molten borate or boric anhydride can also corrode silicon dioxide at high temperature. In view of this property, borate can be used as a flux in ceramic firing. In addition, hydrogen fluoride can also dissolve acid in silicon dioxide to generate water-soluble fluorosilicic acid.