The sink-float method is often used to identify the type of glass material found at crime scenes by determining its density. Several different types of glass of known density are placed into solutions of varying densities. Determine whether each glass piece will sink, float, or do neither when immersed in the given solution. Glass that will sink Glass that will float Glass that will not sink or float Alkali zinc borosilicate with a density of 2.57 g/mL solution with density of 2.42 g/mL Polish sodium lead with density of 3.05 g/mL solution with density of 1.69 g/mL Alkali strontium with density of 2.26 g/mL in solution with density of 2.32 g/mL Red glass with density of 2.27 g/mL solution with density of 0.26 g/mL Polish borosilicate with density of 2.16 g/mL in solution with density of 0.216 g/mL
The Correct Answer and Explanation is:
Correct Classification:
Glass that will sink:
- Alkali zinc borosilicate (2.57 g/mL) in solution (2.42 g/mL)
- Polish sodium lead (3.05 g/mL) in solution (1.69 g/mL)
- Red glass (2.27 g/mL) in solution (0.26 g/mL)
- Polish borosilicate (2.16 g/mL) in solution (0.216 g/mL)
Glass that will float:
- Alkali strontium (2.26 g/mL) in solution (2.32 g/mL)
Glass that will not sink or float:
- None
Explanation:
The sink-float method is a forensic technique used to help identify materials—commonly glass—based on their density. The principle behind this method is straightforward: when an object is placed in a liquid, it will:
- Sink if its density is greater than the liquid’s density,
- Float if its density is less than the liquid’s density, and
- Remain suspended (neither sink nor float) if its density is equal to the liquid’s density.
In forensic investigations, comparing the density of unknown glass fragments to reference glasses or immersion liquids can help match a fragment to a possible source.
Let’s analyze the given samples:
- Alkali zinc borosilicate (2.57 g/mL) vs. solution (2.42 g/mL):
The glass is denser, so it will sink. - Polish sodium lead (3.05 g/mL) vs. solution (1.69 g/mL):
The glass is much denser, so it will sink. - Alkali strontium (2.26 g/mL) vs. solution (2.32 g/mL):
The glass is less dense than the solution, so it will float. - Red glass (2.27 g/mL) vs. solution (0.26 g/mL):
The glass is significantly denser, so it will sink. - Polish borosilicate (2.16 g/mL) vs. solution (0.216 g/mL):
The glass is much denser, so it will sink.
None of the glass samples have the exact same density as the corresponding solutions, so none will “not sink or float.” Only the alkali strontium glass floats; the rest are denser and sink.
