Anhydrous sodium peroxide has a remarkable yellow color. Based on the color of sodium ions and peroxide ions one would expect a white solid. Sodium peroxide is a very reactive and dangerous chemical. Organic matter like paper and cotton, mixed with sodium peroxide inflames when it is wetted somewhat.

The sample shown here is around 2 grams. The usual form in which sodium peroxide appears is shown here, small spheres of 0.3 to 0.5 mm diameter.

When sodium peroxide is kept in contact with air, then it quickly turns white due to absorption of water vapor and carbon dioxide. While doing so it decomposes, giving sodium hydroxide, sodium carbonate and oxygen.