1. Physical and chemical properties
Silica, also known as silica, has the chemical formula SiO₂. There are two kinds of crystalline silica and amorphous silica in nature.

Crystalline silica is divided into three types: quartz, tridymite and cristobalite due to different crystal structures. Pure quartz is a colorless crystal, and the large and transparent prismatic quartz is called crystal. If the crystal contains trace impurities with different colors, there are amethyst, citrine, ink crystal and so on. Ordinary sand is fine quartz crystal, including yellow sand (more iron impurities) and white sand (less impurities and purer). In the silicon dioxide crystal, the 4 valence electrons of the silicon atom form 4 covalent bonds with 4 oxygen atoms, the silicon atom is located in the center of the regular tetrahedron, the 4 oxygen atoms are located at the 4 corners of the regular tetrahedron, and many Each such tetrahedron is connected by oxygen atoms at the top corners, and each oxygen atom is shared by two tetrahedra, that is, each oxygen atom is combined with two silicon atoms. SiO₂ is the simplest formula to express the composition, which only expresses the ratio of the number of atoms of silicon and oxygen in the silicon dioxide crystal. Silica is an atomic crystal.

The bond energy of the Si-O bond in SiO₂ is very high, and the melting point and boiling point are high (melting point 1723 °C, boiling point 2230 °C). The refractive index is about 1.6.

The refractive index of various silica products is: quartz sand is 1.547; powder quartz is 1.544; vein quartz is 1.542; diatomite is 1.42~1.48; fumed silica is 1.46; precipitated silica is 1.46.

Diatomaceous earth existing in nature is amorphous silica, which is the remains of diatoms of lower aquatic plants. It is a white solid or powdery, porous, light and soft solid with strong adsorption.

2. Chemical Properties
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. Reacts with hot concentrated alkali solution or molten alkali to form silicate and water. Reacts with various metal oxides at high temperature to form silicates. Used in the manufacture of quartz glass, optical instruments, chemical utensils, ordinary glass, refractory materials, optical fibers, ceramics, etc. The nature of silica is inactive, and it does not interact with halogens other than fluorine and hydrogen fluoride, hydrogen halides, and sulfuric acid, nitric acid, and perchloric acid (except hot concentrated phosphoric acid). Common concentrated phosphoric acid (or pyrophosphoric acid) can corrode silicon dioxide at high temperature to generate heteropolyacid [2], and molten borate or boric anhydride can also corrode silicon dioxide at high temperature. Salt can be used as a flux in ceramic firing. In addition, hydrogen fluoride can also be used as an acid that dissolves silicon dioxide, resulting in fluorosilicic acid that is easily soluble in water: SiO₂ + 4HF = SiF4↑ + 2H₂O

3. Acid and oxygen permeability
Silica and Alkaline Oxides
SiO₂ + CaO = (high temperature) CaSiO₃ silicon dioxide can be dissolved in strong hot alkali solution:
SiO₂ + 2NaOH = Na₂SiO₃+ H₂O
(The reason why the reagent bottle containing alkali can't use a glass stopper but a rubber stopper)

At high temperatures, silica can be reduced by carbon, magnesium, and aluminum:
SiO₂+2C= (high temperature) Si+2CO↑
SiO₂+2Mg=(high temperature) Si+2MgO
3SiO₂+4Al= (high temperature) 3Si+2Al₂O₃↑
If c is in excess, the reaction occurs:
Si+C=high temperature=SiC (corundum)

Silicic anhydride
Silicon dioxide (SiO₂).
Silica does not react with water, that is, it does not generate silicic acid in contact with water, but it is artificially specified that silica is an anhydride of silicic acid.