Chemistry and Measurements: Density and Specific Gravity

Density

  • Definition: Density compares the mass of an object to its volume and is the mass of a substance divided by its volume.
    • Expressed as: Density=massvolume\text{Density} = \frac{\text{mass}}{\text{volume}}
  • Particle Arrangement:
    • Higher density: Particles are closely packed.
    • Lower density: Particles are farther apart.
  • Examples:
    • Metals like gold and lead have higher densities because their atoms are closely packed.

Units of Density

  • Densities are expressed in different units based on the state of matter in the metric system.
    • Solids and Liquids: grams per cubic centimeter (g/cm3)(g/cm^3) or grams per milliliter (g/mL)(g/mL)
    • Gases: grams per liter (g/L)(g/L)

Densities of Common Substances

  • Table of densities at 25°C:
    • Solids (g/mL):
      • Cork: 0.26
      • Body fat: 0.909
      • Ice (at 0°C): 0.92
      • Muscle: 1.06
      • Sugar: 1.59
      • Bone: 1.80
      • Salt (NaCl): 2.16
      • Aluminum: 2.70
      • Iron: 7.86
      • Copper: 8.92
      • Silver: 10.5
      • Lead: 11.3
      • Gold: 19.3
    • Liquids (g/mL):
      • Gasoline: 0.74
      • Ethanol: 0.79
      • Olive oil: 0.92
      • Water (at 4°C): 1.00
      • Urine: 1.003–1.030
      • Plasma (blood): 1.03
      • Milk: 1.04
      • Blood: 1.06
      • Mercury: 13.6
    • Gases (g/L):
      • Hydrogen: 0.090
      • Helium: 0.179
      • Methane: 0.714
      • Neon: 0.902
      • Nitrogen: 1.25
      • Air (dry): 1.29
      • Oxygen: 1.43
      • Carbon dioxide: 1.96

Chemistry Link to Health: Bone Density

  • Osteoporosis: A condition characterized by severe thinning of bone, leading to loss of bone minerals.
  • Bone Density Measurement:
    • Low-dose X-rays are passed through the narrow part of the femur (hip) and the spine.
    • High-density bones block more X-rays than less dense bones.

Density of Solids Using Volume Displacement

  • A solid submerged in water displaces its own volume of water.
    • The volume of the solid is equal to the increase in water volume when the solid is submerged.

Problem Solving Using Density

  • If volume and density are known, mass can be calculated using density as a conversion factor.

  • Example 1:

    • An unknown liquid has a density of 1.32gmL1.32 \frac{g}{mL}. What is the volume (mL) of a 14.7-g sample?
      1. Given: 14.7 g, Need: milliliters, Connect: density (1.32gmL)(1.32 \frac{g}{mL})
      2. Plan: mass →\rightarrow volume

  • Example 2:

    • John took 2.0 teaspoons (tsp) of cough syrup. The syrup's density is 1.20gmL1.20 \frac{g}{mL}, and 1 tsp = 5.0 mL. What is the mass in grams?
      1. Given: 2.0 tsp, Need: grams, Connect: conversion factors (mL/tsp, density of syrup)
      2. Plan: teaspoons →\rightarrow milliliters →\rightarrow grams

Specific Gravity

  • Definition: Specific gravity (sp gr) is the ratio of a substance's density to the density of water.

  • Calculation: Specific gravity=Density of a substanceDensity of water\text{Specific gravity} = \frac{\text{Density of a substance}}{\text{Density of water}}

    • The density of water is approximately 1.00gmL1.00 \frac{g}{mL} at 25°C.
    • A substance with a specific gravity of 1.00 has the same density as water.

  • Measurement:

    • A hydrometer is used to measure the specific gravity of urine.
    • Normal range for urine: 1.003 to 1.030.
      • Decreased specific gravity may indicate type 2 diabetes or kidney disease.
      • Increased specific gravity may indicate dehydration, kidney infection, or liver disease.
    • Dipsticks with chemical pads are used in clinics/hospitals to evaluate specific gravity.