These very high densities represent the ultimate limits of how much mass can be packed into a given volume. The following chart will give you some feeling for the values of density found in nature generally top , in common solids middle , and in gases and liquids bottom.
In general, gases have the lowest densities, but these densities are highly dependent on the pressure and temperature which must always be specified. To the extent that a gas exhibits ideal behavior low pressure, high temperature , the density of a gas is directly proportional to the masses of its component atoms, and thus to its molecular weight.
Measurement of the density of a gas is a simple experimental way of estimating its molecular weight. Liquids encompass an intermediate range of densities. Mercury, being a liquid metal, is something of an outlier. Liquid densities are largely independent of pressure, but they are somewhat temperature-sensitive.
The density range of solids is quite wide. Metals, whose atoms pack together quite compactly, have the highest densities, although that of lithium, the highest metallic element, is quite low. Composite materials such as wood and high-density polyurethane foam contain void spaces which reduce the average density.
All substances tend to expand as they are heated, causing the same mass to occupy a greater volume, and thus lowering the density. For most solids, this expansion is relatively small, but it is far from negligible; for liquids, it is greater. The volumes of gases, as you may already know, are highly temperature-sensitive, and so, of course, are their densities. The density of a substance is the ratio of its mass to its volume.
At constant temperature and pressure, the density of a substance is constant. Thus, density can be used to identify an unknown pure substance if a list of reference densities is available, and the experimenter can choose a convenient amount of substance to work with when measuring density. To measure the density of a sample of a substance, it is necessary to measure its mass and volume. Mass is typically measured using an analytical balance, a precise instrument that relies on the force exerted by the sample due to gravity.
The container to hold the sample also used to measure volume is weighed and tared, so only the sample mass appears on the balance display when the sample is added to the container. For liquids, this container is typically a volumetric flask, which has one marking that corresponds to a specific volume. The container is filled to the line with the liquid sample and weighed again after the empty flask has been tared. The measured density is the ratio of the measured mass to the volume indicated on the flask.
Most solid substances are irregularly shaped, which complicates volume determination. It is inaccurate, for example, to determine the volume of a powder by measuring its dimensions. Instead of directly measuring dimensions or using glassware like a volumetric flask, it is necessary to make use of a liquid displacement method to measure the volume of an irregularly shaped solid.
A graduated cylinder containing a known volume of liquid in which the solid is insoluble is tared. The solid is added to the cylinder, and the total mass is weighed again to determine the mass of the solid. Addition of the solid causes an upward displacement of the liquid, resulting in a new volume reading.
The volume of the solid is equal to the change in volume due to liquid displacement i. As for liquids, the measured density of a solid sample is the ratio of the measured mass to the measured volume. Subscription Required. Please recommend JoVE to your librarian. Density, defined as a substance's mass per unit volume, is an important physical property for characterizing a material or chemical system.
To obtain the density of a substance, its mass and volume are determined by measurement. This video will introduce the principles of density determination, the procedures for calculating the density of both solid and liquid substances, and some applications of density in scientific research.
However, the volume of space occupied by the same mass is different for different substances, depending on their respective density. For example, a ton of bricks has the same mass as a ton of feathers, but occupies considerably less volume.
Density is obtained by dividing mass by volume. Mass can be measured with scales or balances, and is expressed in grams or kilograms. By convention, the volume of liquids and gases is often expressed in units of liters or milliliters, measured with glassware.
The dimensions of regularly shaped solids can be measured directly with rulers or calipers, which have linear units, giving volumes in units such as cubic centimeters.
One milliliter is equivalent to one cubic centimeter. The dimensions of irregularly shaped solid samples cannot be easily measured. Instead, their volumes can be determined by submerging the solid in a liquid. The volume of the submerged solid is equal to the volume of liquid displaced. Now that you understand the concept of density, let's take a look at two protocols for accurately determining the density of a liquid and a solid. To begin this procedure, place a clean and dry mL volumetric flask on an analytical balance.
After the measurement has stabilized, tare the balance. The balance should read zero. Use a funnel to add approximately 45 mL of liquid to the flask. Do not fill to the calibration mark. Use a Pasteur pipette to carefully add the final 5 mL of liquid, just until the bottom of the liquid's meniscus touches the line on the flask. Weigh the flask again and record the mass of the liquid. Repeat the measurements at least twice to obtain additional values to calculate an average density.
The results are shown in this table. The average measured density was 0. To determine the density of an irregular solid in pellet form, add approximately 40 mL of water to a clean and dry mL graduated cylinder. Record the exact volume. Place the cylinder on an analytical balance and tare. Add approximately 10 pellets, and record the new volume after the addition.
Weigh the cylinder, water, and pellets. The mass is only the pellets, as the rest have been tared. Make at least two additional sets of mass and volume measurements to calculate an average value of the density. The density for zinc was measured for three different samples. It was found to be 6. For more facts and fundamentals, read through these examples of physical properties and examples of scientific notation.
All rights reserved. Oil and vinegar being poured into a bowl as examples of density. Equal to 20 degrees C 68 F and 1 atmosphere of pressure. Our examples below are all at NTP. Equal to 0 degrees C 32 F and 1 atmosphere of pressure. Everyday Density Examples Examples of relative density, or different densities in different substances, occur throughout everyday life. In an oil spill in the ocean, the oil rises to the top because it is less dense than water, creating an oil slick on the surface of the ocean.
A Styrofoam cup is less dense than a ceramic cup, so the Styrofoam cup will float in water and the ceramic cup will sink. Wood generally floats on water because it is less dense than water. Rocks, generally being denser than water, usually sink. This obvious example illustrates the power of science in real life.
Many widely used hardwoods, such as ebony , mahogany and lignum vitae , are dense enough to sink in water, and a few rocks, such as pumice, are light enough to float. All that matters is the relative densities of the substances. Helium balloons rise because helium is less dense than the surrounding air. Over time, the helium escapes the balloon and is replaced by air, causing it to sink.
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