图为小芳在“探究口腔中的化学消化”实验的装置。

（1）在进行“探究口腔中的化学消化”实验时，向1号试管滴加2ml水，应向2号试管滴加2ml ____ 。
（2）实验中1号试管的作用是____ 。
（3）小芳实验中滴加碘酒后，两支试管都变成了蓝色。你能分析原因吗？____ 

 用所学知识回答．

（1）加热冷水，当温度尚未达到沸点时，水中常有气泡冒出，这说明气体的溶解度随温度的升高而____ ．

（2）如图是a、b、c三种固体物质的溶解度曲线，根据此图回答问题：

①t₂℃时，a、b、c三种固体的溶解度由大到小的排列顺序为____ （用“=”“＞”“＜”符号连接）．

②a中含有少量的b，若要提纯a，可采用____的方法．

③若c为氢氧化钙，在室温时，向c的饱和溶液中加入适量的生石灰，恢复到室温后，饱和溶液中溶质的质量____ （填“变大”或“变小”或“不变”）．

    People from all over the world make friends____ writing letters. One way to find a pen friend is to ____ a club. International Pen Friends started in 1967 and has over 300,000members(成员) in different countries. Their____ member is eight years old, and their oldest member is over 80. Some people write to find out about life in another country, ____ want to practice their English, or enjoy their hobbies. Stamp collectors, for example, write to people in different countries ____ that they can collect the stamps on the letters they receive. Most people just enjoy making friends, and they invite their pen friends to visit them.