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A white solid leaves a residue upon dry heating that is yellow when hot and white when cold. Upon heQualitative and Quantitative Analysis Chemistry Question

Question

A white solid leaves a residue upon dry heating that is yellow when hot and white when cold. Upon heating this white residue with a drop of cobalt nitrate in a charcoal cavity, a distinct green mass is formed. The white solid originally tested is likely:

Answer: C

💡 Solution & Explanation

Zinc salts decompose to leave a white residue of $ZnO$, which is famously yellow when hot and white when cold. When $ZnO$ is heated with cobalt nitrate, it forms a double oxide $ZnO \cdot CoO$, known as Rinmann's green. Thus, the original solid contains $Zn^{2+}$.

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