This is the 7237th most frequent Chinese word.
纯净
"Pure" or "clean."
In this sentence, '纯净' describes the physical purity and clarity of the lake water.
这片湖水非常纯净，几乎可以直接喝。
This lake is very pure, almost drinkable.
Here, '纯净' metaphorically illustrates the innocence and undisturbed state of the person's mind.
他的心灵如同孩子一样纯净，没有杂念。
His mind is as pure as a child's, free from distracting thoughts.
In this usage, '纯净' refers to the cleanliness and absence of pollutants in the air.