Overview
This lecture covers density, its calculation, applications as a conversion factor, and the concept of specific gravity, with example problems.
Density: Definition and Properties
- Density is defined as mass divided by volume (Density = mass/volume).
- It is usually reported in grams per milliliter (g/mL).
- Density helps identify substances and can be used as a conversion factor between mass and volume.
Specific Gravity
- Specific gravity is the ratio of the density of a substance to the density of water.
- The density of water is 1.00 g/mL.
- Specific gravity is unitless because the units cancel when dividing by water's density.
Example Problems
- To find specific gravity, divide the sample's density by that of water (e.g., 1.02 g/mL ÷ 1.00 g/mL = 1.02).
- When converting mass to density, ensure mass is in grams and volume in milliliters before dividing.
- For substances reported with specific gravity, multiply by water's density to get actual density.
- Convert volume units where necessary (e.g., liters to milliliters) before using density as a conversion factor.
- Rearranging the density formula:
- mass = density × volume
- volume = mass ÷ density
Medical Application Example
- To check for conditions like diabetes, compare a urine sample's specific gravity to the normal range (1.003–1.030).
- Calculate the urine's density and then divide by water's density for specific gravity.
- A result within the normal range suggests no unusual concentration, lying on the lower end may indicate type 2 diabetes risk.
Key Terms & Definitions
- Density — mass of a substance divided by its volume, typically in g/mL.
- Specific Gravity — density of a substance divided by the density of water; a unitless value.
Action Items / Next Steps
- Review the PHET simulation for density for practical experience in measuring and comparing densities.