Acid oxide is its corresponding hydrate, which is acid.
The definition of acid anhydride is: an acid removes one molecule of water, and the remaining part is called the acid anhydride. Usually, inorganic acid is a molecule of acid. When a water molecule is directly lost, an anhydride is formed, and the valence of the elements that determine the acidity in the anhydride remains unchanged. Organic acids are formed by intermolecular dehydration of two or more molecules of acids. Only oxyacids have anhydrides. Anaerobic acids have no anhydride.
Silicon dioxide is an inorganic substance, and its chemical formula is SiO2. The long-range ordered arrangement of silicon atoms and oxygen atoms forms crystalline silicon dioxide, and the short-range ordered or long-range disordered arrangement forms amorphous silicon dioxide. In the silica crystal, the silicon atom is located at the center of the regular tetrahedron, and four oxygen atoms are located at the four corners of the regular tetrahedron. Many of these tetrahedrons are connected by oxygen atoms in the corners, and each oxygen atom belongs to two tetrahedrons, that is, each oxygen atom is combined with two silicon atoms. The simplest form of silicon dioxide is SiO2, but SiO2 does not represent a simple molecule (it only represents the ratio of the number of atoms of silicon and oxygen in the silicon dioxide crystal). Pure natural silica crystal is a hard, fragile, insoluble, colorless and transparent solid, which is often used to manufacture optical instruments.
The chemical properties are relatively stable. Does not react with water. It is an acidic oxide and does not react with ordinary acids. Hydrofluoric acid reacts with silicon dioxide to produce gaseous silicon tetrafluoride. React with hot concentrated alkali solution or molten alkali to form silicate and water. React with various metal oxides at high temperature to form silicates. Silicon dioxide is inactive, and does not react with halogen, hydrogen halide, sulfuric acid, nitric acid and perchloric acid except fluorine and hydrogen fluoride (except hot concentrated phosphoric acid).