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Q1: How is density calculated and what units are used?
Density is calculated by dividing a substance's mass by its volume, expressed mathematically as ρ = mass/volume. The Greek symbol ρ denotes density. Mass is measured in grams or kilograms using scales or balances. Liquid volume is expressed in liters or milliliters, while solid volume is in cubic centimeters, with one milliliter equivalent to one cubic centimeter.
Q2: Why is density considered an intensive property?
Density is an intensive property because it does not depend on the amount of substance present. Whether you measure one gram or one kilogram of a material, the ratio of mass to volume remains constant. This makes density useful for identifying and characterizing pure materials regardless of sample size.
Q3: What is the water displacement method for measuring solid volume?
The water displacement method determines irregular solid volume by submerging the solid in a liquid, typically water in a graduated cylinder. The volume of liquid displaced equals the solid's volume. This technique works for irregularly shaped solids whose dimensions cannot be easily measured directly with rulers or calipers.
Q4: How do you measure the density of a liquid in the laboratory?
To measure liquid density, use a volumetric flask and analytical balance. Tare the balance with a clean, dry flask, then add liquid carefully using a Pasteur pipette until the meniscus touches the calibration mark. Weigh the flask again and record the mass. Repeat measurements at least twice to calculate average density for greater accuracy.
Q5: How can density be used to test the purity of materials?
Density serves as a quick purity test because pure substances have characteristic, known densities. For example, gold has a density of 19.3 g/cm³. If a gold coin's calculated density differs significantly from this value, it indicates the coin is not pure gold. This method works for any material with a known reference density.
Q6: What is density gradient centrifugation and how does it separate mixtures?
Density gradient centrifugation separates mixture components by layering decreasing concentrations of sucrose or polymers to create a density gradient. The sample is added on top, then centrifuged at high speed. Components migrate to positions along the gradient where their density matches, effectively separating different substances based on their individual densities.
Q7: Why do regularly and irregularly shaped solids require different volume measurement methods?
Regularly shaped solids have measurable dimensions, so volume is calculated from length, width, and height measurements using rulers or calipers. Irregularly shaped solids lack uniform dimensions, making direct measurement impractical. Instead, their volume is determined by water displacement, where the volume of liquid displaced equals the solid's volume.