What is the colour of lead oxide when it is hot and when it is cold?

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Detailed Solution

Lead(II) oxide (PbO) exists in two crystalline forms—tetragonal (litharge) and orthorhombic (massicot)—and its colour changes depending on temperature:

When cold (at room temperature): PbO appears as a red or orange solid, corresponding to the stable tetragonal (litharge) form.
When heated (hot): It transitions into a yellow, metastable form (orthorhombic, massicot), and may also exhibit a brownish hue at elevated temperatures.

Temperature State	Colour of Lead Oxide (PbO)	Form / Notes
Cold / Room Temp	Red or orange	Tetragonal (litharge), stable
Hot / Heated	Yellow; may appear brownish	Orthorhombic (massicot), metastable

At room temperature, lead(II) oxide is typically red or orange. When heated, it transforms into a yellow (sometimes brownish) form.

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