Final answer:
Generally, samples should not be placed directly on dry ice due to the potential damage from rapid freezing, though exceptions exist based on the experiment's needs. Controlled cooling with proper containers is typically preferred for most laboratory procedures. Adhering to safety guidelines is crucial when working with dry ice.
Step-by-step explanation:
The statement that no sample should EVER be placed directly on dry ice is generally true, but it may depend on the nature of the sample and the context of the experiment. Dry ice is solid carbon dioxide (CO2), which sublimates at -78.5°C (-109.3°F). Placing a sample directly on dry ice can cause it to freeze rapidly which might be useful in certain applications like preserving biological samples or flash freezing materials. However, for most laboratory procedures that require controlled cooling or freezing of samples, placing them directly on dry ice may be too abrupt and could potentially damage the sample, especially if it involves delicate structures or active biological components.
Therefore, it's important to use proper containers, such as dry ice buckets or insulated foam containers that are designed to hold samples above the dry ice, allowing for controlled cooling without direct contact. In some cases, a sample might need to be cooled quickly and thus directly placing on dry ice is appropriate, but in many cases, indirect cooling methods should be used to manage temperature more precisely and avoid thermal shock to samples. Always follow safety guidelines and experiment protocols when handling dry ice and samples.