Missing Adjective Forms

Compare the objects according to the given model.

1. A lemon is sourer than an orange. An orange is not so sour as a lemon.
2. The Black Sea is more beautiful than the Baltic Sea. The Baltic Sea is not so beautiful as the Black Sea.
3. The Chinese language is more difficult than the French language. The French language is not so difficult as the Chinese language.
4. A lane is narrower than a street. A street is not so narrow as a lane.
5. Copper wire is more flexible than iron wire. Iron wire is not so flexible as copper wire.