升华是由化合物受热气化为蒸气,蒸气直接冷凝为固体的过程。
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“茶叶中提取咖啡碱”实验,若实验操作顺利的话,可在滤纸上看到3~4mm长的白色略带黄色的针状结晶。
A. 对
B. 错
Traditionally, travel groups often hire ____to conduct their journeys, especially when the journeys are not too long.
Trains are ____to planes in that railway terminals are often in the centre of the destination while airports are often located 20 or 30 kilometers away from the centre.
The load of ____flights (of 90% or more) is usually higher than that of scheduled ones (which can be as low as 20%).