Final answer:
When taking a second melting point, it is necessary to use a fresh sample to prevent contamination.
Step-by-step explanation:
When taking a second melting point, it is necessary to use a fresh sample to prevent contamination. Contamination can occur if the previous sample was exposed to impurities or substances that could affect the melting point. By using a fresh sample, we can ensure that the melting point measurement is accurate and reliable.