Silica $(S i O_{2})$ is dissolved in a concentrated solution of NaOH under pressure.
Is silica precipitated by NaOH? Write the appropriate reaction.

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Solution

Extraction of silica from ash as sodium silicate using caustic soda. This reaction is carried out at a temperature in the range $180^{0} - 200^{0} C$ and pressure ranging from 6-8 atmosphere.

The reaction is:

$S i O_{2} + 2 N a O H + h e a t (180 - 200^{0} C) \to N a_{2} S i O_{3} + H_{2} O$

But high reaction temperature and pressure can be avoided if ash obtained by burning rice husk at $650^{0} C$ is used. This ash is mostly amorphous silica which is reactive around $100^{0} C$ with NaOH solution to yield sodium silicate.

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