HSK 4
空
kōng形（副）
empty; air; sky; in vain
这个盒子是空的里面没东西。
Zhè ge hé zi shì kōng de lǐ miàn méi dōng xi.
This box is empty; there is nothing inside.
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Other HSK words sharing a character
Also free:
Appears across levels (4（5）) in the HSK 3.0 word list.