Comma or semi-colon?

If he misses his appointment _____ he'll have to wait another six months to see the dentist.

There are many negative side effects from smoking ____ there are no positive side effects from smoking.

The artist preferred to paint in oils ____ he did not like watercolours.

I'm going to leave early today ____ unless the boss comes back from the meeting.

The challenges are not impossible ____ but they are very hard to clear.

Italy is my favourite foreign country ____ in fact, I plan on staying there for three weeks this summer.

In the first place, it was snowing too hard to see the road ___ secondly, we had no chains.

The computer can perform many calculations at once ___ however, it cannot reason at all.

Even when the house looks clean ___ there is usually a bunch of dirt swept under the rug.