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Why would glass containing too much sodium oxide be attacked by water?

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Glass containing high levels of sodium oxide is susceptible to attack by water because sodium acts as a flux that disrupts the stable silica network.1

In the glass structure, alkali ions like sodium are mobile and can be exchanged with hydrogen or hydronium ions from water or atmospheric humidity through an ion exchange reaction. This process, known as leaching, extracts the alkali modifiers from the silica network. As these ions are removed, the glass structure is left under stress, leading to microfractures, cloudiness (crizzling), and the formation of surface deposits as the leached alkali reacts with carbon dioxide and water to form carbonates.23

While adding lime can help render the glass insoluble, compositions with excessive sodium oxide remain highly soluble, forming solutions often referred to as water glass. To improve chemical durability, manufacturers aim to make the glass surface as silica-rich as possible, as glasses lower in alkali offer significantly better resistance to weathering.12