求英语高手帮忙分析下面的句子,
black substance made from coal tar,turpentine or petroleum which is sticky and
semi-liquid when hot,and hard when cold,and is used to fill in cracks or
spaces,eg between the planks of a floor or of a ship's deck,to make roofs
waterproof,etc .
句中的 when hot 和 when cold 这样的用法感觉好奇怪。另外，望高手帮忙划分出句子成分。