Why are the caps loosened on a tube of agar when heating it?

• A. To allow air to escape from the tube as it is heated.
• B. To speed up heating by increasing air flow.
• C. To prevent contamination by air exposure.
• D. To vent steam away from the sample.

Answer: (A)

When you heat a tube of agar, the air inside and the water in the agar turn into gas and expand. If the cap is tight, that expanding gas has nowhere to go and pressure builds up, which can cause the cap to pop or the tube to crack and even spray molten agar. Loosening the cap provides a safe outlet for the expanding air and steam, preventing dangerous pressure buildup. This is about pressure relief during heating, not about speeding heating or preventing contamination, though anything left loosely capped should be handled carefully to avoid exposure.