问题 问答题

小华用热水配制了大半烧杯硝酸钾的饱和溶液,让一个较大的塑料块浮在液面上,然后小华让溶液冷却至室温(已知溶液的温度约为60℃,室温为20℃).请你说出小华会看到的两个现象,并用学过的理化知识解释.

答案

小华会看到有KNO3晶体析出.因为物质的溶解度与温度有关,温度越低,KNO3的溶解度越小,所以随着温度降低,会有KNO3晶体析出.小华还会看到塑料块浸入溶液中的体积增大.因为冷却后有KNO3晶体析出,故冷却后烧杯内溶液密度减小,塑料块会逐渐的下沉,塑料块浸入溶液中的体积会增大.

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根据短文内容回答问题。

                                                   No more chores, please!

     Sam hated doing chores. And he had many chores. For example, on Mondays, Wednesdays

and Fridays he took out the trash. And on the weekend he always washed his father's car. He

also cleaned his own room once a week. What a chore that was ! Sometimes he even had to look

after his little sister and brother.

     Sam had a foreign friend at school. His name was Kumar. Doing chores in Kumar's family

was very different. In the house only the girls did chores. His sisters made all the beds and

cleaned all the rooms too, even his bedroom. His mother always cooked the meals, and his sisters

helped their mother. So when Kumar talked to his friend Sam about chores, he felt very lucky.

One day, Sam had an idea. He asked Kumar,"Could I borrow your sisters?" He wanted

them to help him clean his room, but they refused.  Instead, they asked Sam to teach their brother

how to do chores, so he could make his own bed.

     Sam and Kumar are very good at science. They are going to be scientists after university.

They want to make a robot do the chores. Then everyone-mothers and fathers, sons and daughters

will be happy !

1. When did Sam take out the trash?

    ____________________________________________

2. How often did he clean his room?

    ____________________________________________

3. What did Kumar's sisters do in the house?

    ____________________________________________

4. Why did Sam want to borrow Kumar's sisters?

    ____________________________________________

5. What did Sam and Kumar want to do to help with the chores?

   ____________________________________________

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