Sodium oxide is a white solid. This color is due to the absence of any significant absorption or emission of light in the visible spectrum by the compound. The white color is consistent with the typical appearance of inorganic compounds that are composed of sodium and oxygen.

The color of sodium oxide is attributed to its crystal structure and electronic configuration. Sodium oxide has a cubic crystal structure, which is common for many ionic compounds. The sodium ions (Na+) are arranged in a face-centered cubic lattice, while the oxide ions (O2-) are located at the corners of the cube. This arrangement does not involve any electronic transitions that would result in the absorption or emission of visible light.

According to the study by Kopp et al. (2004), the color of inorganic compounds is often determined by the electronic transitions within the molecules or crystals. In the case of sodium oxide, the lack of such transitions in the visible region is what contributes to its white color. Additionally, the reflectance and transmittance properties of sodium oxide are also consistent with its white appearance, as reported by the same study.

In summary, sodium oxide is a white solid due to its cubic crystal structure and the absence of electronic transitions in the visible spectrum. This is supported by scientific research that examines the electronic properties and optical behavior of inorganic compounds.