When blending hot liquids, the steam from the soup can expand and has the potential of creating enough pressure to push the lid off the top. There are certain brands of blenders, which come with vented lids to release steam and heat-proof bowls that are designed to blend hot liquids such as the Vitamix for example. They generally do not recommend to add anything that exceeds 170°F / 75°C. To be safe, let the soup cool off for a couple of minutes. Always blend in batches only filling your blender up to 1/3 or 1/2.