What should be mixed fresh for a lead test at the crime scene?

Sodium rhodizonate is the preferred choice for a fresh mixture in lead testing at a crime scene due to its specific chemical properties that allow for the effective detection of lead residues. When used in a test kit, sodium rhodizonate reacts with lead ions to produce a colored complex that indicates the presence of lead. This color change allows investigators to visually identify and confirm lead contamination on surfaces or items collected at a crime scene.

Using freshly mixed reagents is crucial because the reactivity of the chemicals can diminish over time, leading to inaccurate results. Sodium rhodizonate's reaction provides clear and immediate feedback, making it highly suitable for on-site analysis where quick assessments are often necessary to guide the investigation.

In contrast, the other substances listed do not serve the same purpose in lead detection. Sodium iodide and sodium bicarbonate are not capable of indicating lead presence in the same effective manner as sodium rhodizonate. Calcium carbonate is typically used as a buffering agent and does not have the necessary chemical properties for lead testing. Therefore, sodium rhodizonate stands out as the effective choice for conducting lead tests in crime scene investigations.