How does lime addition as an ingredient prevent soda lime glass from dissolving in water? Does it form calcium silicate in the process of melting?


The addition of lime does indeed form calcium silicate, but it doesn't serve the purpose you mentioned. Silica of itself isn't water soluble, nor soda glass.

The difference is in the crystalline structure. Silica ($\ce{SiO2}$) consists of silicon atoms bridged by oxygen atoms, forming a periodic crystal with no net charges on any atom.

However when some alkali is introduced ($\ce{Na2O}$), the additional oxygen atoms remain charged, and cannot link silicon atoms together; those form silicate anions. The silica breaks and depolimerizes, and the remaining $\ce{Na+}$ fill holes in the structure to maintain charge balance. Thus the solid melts at lower temperature with reduced viscosity, it's "more fluid".

Lime contains calcium, which bonds more strongly to silicate anions than sodium. It also decreases viscosity of molten glass because of the non bridging oxygen, but in addition increases hardness and tensile strength of the glass in solid state.

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  • $\begingroup$ Thank you for clarifying, it seems to be a common misconception that soda glass is soluble enough for it to be a concern. May I ask you an additional question here in the comments, since I do not want to spam the forum? Since neither calcium oxide or calcium carbonate is soluble in the melt (or am I wrong again?), how come the resulting glass is clear, instead of being cloudy due to a suspenion of solid CaO in it? $\endgroup$ – Francis L. Jan 27 '19 at 18:48

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