To determine which molecule could be the stable product of the reaction, we should assess the stability of the given compounds based on the principles of chemical bonding and the octet rule.
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SiO₂ (Silicon Dioxide): Silicon can form stable bonds with oxygen, and in SiO₂, each silicon atom is surrounded by four oxygen atoms in a tetrahedral structure, achieving a stable configuration. SiO₂ is a very stable compound.
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PH₅ (Phosphorus Pentahydride): Phosphorus typically forms three bonds, and PH₅ would involve phosphorus having five bonds. This exceeds the typical octet rule for stable configurations, making PH₅ generally unstable or not commonly found.
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KCl₃ (Potassium Trichloride): Potassium typically forms one bond and does not typically bond with three chlorines. Thus, KCl₃ is not a stable compound and is not likely to exist.
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CaO₂ (Calcium Peroxide): Calcium typically forms two bonds, and in CaO₂, calcium is bonded to two oxygen atoms, which shares some characteristics of stability. However, peroxides can sometimes be reactive and not always classified as stable.
Considering the above evaluations, the only molecule that consistently behaves as a stable product under normal conditions is SiO₂ (Silicon Dioxide).
Therefore, the correct answer is SiO₂.