Let's look at your sentence: "The Sahara is the largest and the hottest desert on Earth. But scorching during the day, it gets really chilly at night."
There are two small points to fix so the sentence sounds just right:
- The phrase "But scorching during the day" is missing a verb. We can say "But it is scorching during the day," or replace "But" with "Although" for the meaning to be clearer.
- Putting it together, you can write:
The Sahara is the largest and hottest desert on Earth. Although it is scorching during the day, it gets really chilly at night.
This means the Sahara Desert is big and very hot in daytime, but at night, it becomes cold. Great job describing it!