Why do objects of the same size have different weights? We will learn about density, an important indicator for identifying substances.
Basic Knowledge
Density is the mass per unit volume, and it is calculated using density [g/cm3] = mass [g] ÷ volume [cm3]. Density is a value specific to each substance, so it can be used to identify substances.
Examples: water=1.00, iron=7.87, aluminum=2.70, gold=19.3, ice=0.92(g/cm3).
When an object is placed in a liquid, the one with the lower density floats, and the one with the higher density sinks. Ice floats on water because the density of ice (0.92) is lower than that of water (1.00). Volume is measured with a graduated cylinder by measuring the increase in volume when an object is submerged in water.
Density（mass per unit volume. g/cm3）
Mass（the amount of matter in an object. It does not change when the location changes）
Weight（the magnitude of the gravitational force acting on an object. It changes depending on location)
Graduated cylinder（an instrument for measuring liquid volume. Read the lowest point of the liquid surface）
Buoyant force（the upward force acting on an object in a fluid）
Electronic balance（an instrument for measuring mass. Remember to adjust the zero point）
Further Exploration (Background and Significance)
Density is a powerful way to identify substances. For example, whether a “gold-colored metal” is really gold can be determined by measuring its density (gold has a density of 19.3 and is much heavier than other yellow metals). This has long been known as Archimedes’ principle and is connected to the story of determining whether a crown was genuine.
Density also changes with temperature. For most substances, volume increases and density decreases as temperature rises, but water has the unusual property of having its greatest density at 4℃. This is also why the bottoms of lakes are less likely to freeze and fish can survive the winter.
- Density = mass ÷ volume [g/cm3]
- Density is specific to each substance → it can be used for identification
- An object floats if its density is lower than that of the liquid and sinks if it is higher
- The density of water is 1.00 g/cm3(4℃)
- Ice (0.92)＜water (1.00), so ice floats
- Read a graduated cylinder from directly in front of the scale
- Mass and weight are different (mass is constant; weight depends on gravity)
Points to Note (Easily Confused)
① Do not confuse mass (kg, g) with weight (N, newtons). They are often treated using the same units in middle school, but they are different concepts. ② The unit of density is g/cm3 (mass ÷ volume). Do not use its reciprocal. ③ Read the bottom of the liquid surface in a graduated cylinder (the top of the surface for mercury). ④ Ice floats on water because of water’s unusual property; for most substances, the solid has a higher density.
Practice
- Find the density of an object with a mass of 54g and a volume of 20cm3.
- What happens when a substance with a density of 0.92g/cm3 is placed in water (1.00)?
- Explain why density can be used to identify substances.