In my science class, I have been taught that silica (silicon dioxide/sand) has three properties, namely

  1. It does not react with water.
  2. It does not decompose with heat.
  3. It does not react with acid.

However I think that if it does not react with acid, will it react with bases? I am just curious because if it does react, why does my textbook not say anything about that?

  • 1
    $\begingroup$ Silicon dioxide most certainly reacts with HF (hydrofluoric acid) - HF is the common liquid etchant for SiO2 in semiconductor processing. (HF is also nasty stuff, so be careful). $\endgroup$ – Jon Custer Nov 14 '14 at 14:37
  • $\begingroup$ But I thought silicon dioxide is acidic. How it going to react with acid? $\endgroup$ – Simon-Nail-It Nov 15 '14 at 1:21
  • $\begingroup$ Silicon dioxide is not attacked by acids in general but it does react with HF. The distinguishing feature here is F-. So, yes, silica is acidic, and it reacts with a very special (Lewis) base. $\endgroup$ – Abel Friedman Nov 21 '14 at 16:05
  • 1
    $\begingroup$ You have to take all such statements with a grain of salt. Yes water doesn't dissolve an appreciable amount of silica in the lab. But heat and pressure will allow groundwater to dissolve much more. Quartz crystals in the earth are grown from such solutions cooling and depositing the supersaturated silica. $\endgroup$ – MaxW Feb 1 '17 at 5:36

It really does react: \begin{align} \ce{4HF + SiO2 &-> SiF4 + 2 H2O}\\ \ce{SiF4 + 2 HF &-> H2[SiF6]}\\ \ce{SiO2 + 2 NaOH &-> Na2SiO3 + H2O}\\ \ce{SiO2 + 2 F2 &-> SiF4 + O2}\\ \ce{SiO2 + 2 C &-> Si + 2 CO}\\ \ce{(2,1)Na2O + (1,1)SiO2 &-> Na4SiO4/Na2SiO3}\\ \ce{SiO2 + Si &-> 2 SiO} \end{align}

  • $\begingroup$ Will sio2 with 6hf directly give h2[sif6] or not? $\endgroup$ – Fawad Oct 2 '18 at 11:17

$\ce{SiO2}$ definitely reacts with strong bases like $\ce{NaOH}$ or $\ce{KOH}$ which is why such solutions should not be stored in glass bottles.

Concentrated and hot aqueous solution of $\ce{NaOH}$ can damage glassware in the course of several days. $\ce{KOH}$ or $\ce{NaOH}$ in methanol is even more potent. Reaction is obviously faster with finely divided $\ce{SiO2}$

$\ce{4NaOH + SiO2 -> Na4SiO4 + 2H2O}$


Now, I realize that silicon dioxide is a very stable compound. However, there is an exception which is it will react react with a thing- Hydrofluoric acid ($\ce{HF}$)

The reaction is undergo like the equation below:

$$\ce{4HF(g) + SiO2(s) -> SiF4(g) + 2H2O(l)}$$

Only $\ce{HF}$ can react with sand ($\ce{SiO2}$), others chemical can't as sand is really that stable.


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