Structure and properties of silicates
Silicates are salts of silicic acids. Their crystal structure is based on polymer chains of $(SiO_3)^{2n-}$ anions. The space between these extended anionic chains is filled with cations of the corresponding metals, such as sodium ions.
Special attention should be paid to the solubility of these compounds. Only alkali metal silicates are soluble in water. A solution of sodium silicate is often referred to in everyday life and industry as water glass.
Hydrolysis occurs in aqueous solutions of silicates, during which long polymer chains break down into shorter fragments. Nevertheless, the presence of such polymer structures causes the high viscosity of the solutions. A classic example of such a viscous solution is the well-known silicate glue, which is a solution of sodium silicate.
Silicic acids and silica gel formation
If an acid is added to a sodium silicate solution, weak silicic acids begin to precipitate. It is important to note that insoluble forms of these acids do not form a typical loose or flocculent precipitate. Instead, they bind water and turn into a gel (jelly).
Among all silicic acids, orthosilicic acid ($H_4SiO_4$) stands out as the only one soluble in water. However, its existence is extremely short-lived: it is unstable and very rapidly undergoes polymerization according to the following equation:
$$nH_4SiO_4 = (H_2SiO_3)_n + nH_2O$$
If the resulting silicic acid gel is heated to remove water, silica gel—an active form of silicon dioxide—is formed. Due to its structure, silica gel is a powerful adsorbent. It actively absorbs water vapor from the surrounding air, which is why it is widely used as a desiccant in containers for storing moisture-sensitive devices and products.
Glass production
Silicates are characterized by high thermal stability. This property forms the basis of glassmaking. The process of obtaining glass involves melting sand (silicon dioxide) with oxides or carbonates of various metals.
Upon cooling, molten silicates behave in a special way: they do not crystallize, but instead transition into an amorphous state, forming glass. By varying the set of silicates in the initial mixture, different types of glass can be obtained: from ordinary tableware and window glass to colored mosaics, optical lenses, and even bulletproof glass.
The chemical composition of ordinary window glass is conventionally described in two ways:
- As salts (silicates): $Na_2SiO_3 \cdot CaSiO_3 \cdot 4SiO_2$
- As a set of oxides (this notation form is much more convenient for chemical calculations): $Na_2O \cdot CaO \cdot 6SiO_2$
Briefly: answers to topical questions
What is the application of silica gel in medicine and pharmacy?
Silica gel is used as an active adsorbent that absorbs water vapor from the air. When storing moisture-sensitive devices and products, it is used to dry the air in containers. In addition, silica gel is part of the final structure of hardened glass ionomer cement and surrounds the aluminosilicate glass particles.
What types of dental cements are made on the basis of silicates?
Several types of dental cements used for filling and fixation are made on the basis of silicates.
- Silicate cements are materials whose powder contains silicon dioxide, which ensures the transparency and gloss of the filling.
- Silicophosphate cements are a combination of zinc phosphate and silicate cements, occupying an intermediate position in their physicochemical properties.
- Glass ionomer cements are a class of modern materials created by combining the properties of silicate and polyacrylic systems, incorporating aluminosilicate glass into their structure.
To remember the composition of window glass in oxides ($Na_2O \cdot CaO \cdot 6SiO_2$), use the "1-1-6" rule: one sodium, one calcium, and six sands.
Class and exam questions
Why is sodium silicate solution viscous?
The viscosity of the solution (silicate glue) is due to the presence of polymer anion chains in it, which, even with partial hydrolysis, retain sufficient length to thicken the liquid.
What is the qualitative reaction for silicates?
A qualitative reaction is the addition of ammonium chloride ($NH_4Cl$) to a sodium silicate solution ($Na_2SiO_3$). This enhances hydrolysis and releases a gas with a pungent odor of ammonia ($NH_3$).
How is silica gel obtained from silicic acid?
First, a gel of insoluble silicic acids is obtained, and then it is heated. The water evaporates, leaving behind the active form of silicon dioxide—silica gel.
What else to study on this topic
- Mechanism of orthosilicic acid polymerization
- Process of alkali metal silicate hydrolysis
- Differences in the properties of crystalline and amorphous silicates
- Interaction of sodium silicate with ammonium chloride (ionic equation)
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