Amphoteric oxides are both acidic oxides (oxides that can react with an alkali solution to form salt and water are called acid oxides) and basic oxides (oxides that can react with an acid solution to form salt and water). Silica can react with strong alkali solutions such as sodium hydroxide to form sodium silicate and water, making it an acidic oxide. Still, it cannot react with an acid solution to form salt and water. So it's not basic, so it's not amphoteric. In addition: a reaction with hydrofluoric acid to form silicon tetrafluoride is not a salt. Instead of amphoteric oxides, silica is an acidic oxide, and the corresponding acid is silicic acid or orthosilicic acid. Amphoteric oxide refers to the oxides that are basic in the presence of strong acids and acidic in the presence of strong bases. They can interact with both acids and bases. They all produce salt and water. The amphoteric elements' oxides and the variable metals' intermediate valence are usually amphoteric. Silicon dioxide is colourless, solid at room temperature, chemical formula SiO₂, and insoluble in water. Insoluble in acid but soluble in hydrofluoric acid, hot concentrated phosphoric acid, and molten base action. There are two kinds of crystalline silica and amorphous silica in nature. If you are looking for high quality, high purity and cost-effective silica, or if you require the latest price of silica, please feel free to email contact mis-asia.