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Comparing densities


What has density to do with fluctuation?

To try to answer this question, let's calculate the density of water. Before that, we need to experimentally determine the volume and mass of one or more samples of this liquid.

Examples of results of these determinations are:

  • 1 cm3 of water has mass 1 g;
  • 2 cm3 of water have mass 2 g;
  • 100 cm3 of water have mass 100 g;
  • 1,000 cm3 of water have mass 1,000 g.

With this data we come to:

Thus, comparing the density values:

DWater = 1 g / cm3 dcork = 0.32g / cm3 dlead = 11.3 g / cm3

We conclude that: dcork <dWater<dlead

Density and fluctuation

The result we got (dcork <dWater <dlead) suggests that cork floats on water as it is less dense than cork; and the lead sinks. because it is denser than this liquid. Indeed, much experimental evidence has allowed scientists to conclude that this statement is true.

The combination of densities allows you to predict whether a body will sink or float in a certain liquid. Imagine, for example, that a marble (d = 2.7 g / cm3) and a piece of Styrofoam (d = 0.03 g / cm3) are placed in a bottle of olive oil (d = 0,92g / cm3). What can we predict?

The piece of Styrofoam, less dense than olive oil, will float on it. And the marble, denser than it, will sink.

Some factors affecting density

The density depends first on the material in question and secondly on the temperature of this material. A heating, for example, causes material swelling (swelling), which interferes with the density value.

In the case of gases, whose volume is very sensitive to pressure variation, the density, besides being temperature dependent, also depends on the pressure. Therefore, if there are changes in the physical state of a substance, changes in the density of this substance will also occur. The fact that liquid water, for example, has a density of 1 g / cm3, and solid water (ice), 0.92 g / cm3, lets us understand why ice floats on water.