Overview
This section covers the concepts of density and specific gravity, focusing on their definitions, uses as conversion factors, and methods for application in problem-solving.
Density
- Density is a physical property measuring mass per unit volume (density = mass/volume).
- Typical units for density are grams per milliliter (g/mL) or grams per cubic centimeter (g/cm³), but other unit combinations are possible.
- Density can be used as a conversion factor to relate mass and volume in calculations.
- When you see "per" in units (e.g., 1.05 g/mL), interpret it as "equals" (1.05 g = 1 mL) for conversion purposes.
- Use fractions to set up conversion problems; align units so they cancel appropriately.
- Avoid memorizing formulas; focus on converting between mass and volume using density as an equality.
- Always consider significant figures when performing calculations in chemistry.
Specific Gravity
- Specific gravity compares the density of a substance to the density of water at the same temperature.
- It is unitless because the units in the ratio cancel (e.g., g/mL ÷ g/mL).
- For accurate comparison, ensure both densities are in the same units.
- If specific gravity > 1, the substance sinks in water; if < 1, it floats; if = 1, it has the same density as water.
- Specific gravity is a practical, real-world measure to quickly predict if something will float or sink in water.
Key Terms & Definitions
- Density — Mass of a substance divided by its volume, commonly in g/mL or g/cm³.
- Conversion Factor — A ratio that allows conversion between different units (e.g., using density to convert grams to milliliters).
- Significant Figures (Sig Figs) — Digits in a measurement that are meaningful for precision and accuracy.
- Specific Gravity — Ratio of the density of a substance to the density of water (unitless).
Action Items / Next Steps
- Practice using density as a conversion factor in homework problems.
- Ensure answers have the correct number of significant figures.
- Convert all densities to the same unit before calculating specific gravity.
- Prepare for in-class practice on these concepts.