1A.6: Physical Properties

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Learning Objectives

• Define physical properties and give examples of common physical properties used to identify a substance
• Differentiate between extensive and intensive properties and give examples of each
• Define density as a way to relate the mass of a substance to its volume
• Apply the concept of density to environmental hazard such as plastics in bodies of water

Introduction

Physical properties are are typically observable properties that describe the physical state of matter. In contrast, chemical properties describe the chemical arrangement, composition and reactivity of matter.

Physical Properties can be Extensive or Intensive.

What are Extensive Properties?

Extensive Physical Properties are those that depend on the "extent" of the system. Volume and mass are extensive, and two gallons of water at 20 deg C have twice the volume and mass as one gallon of water at 20 deg C.

What are Intensive Properties?

Intensive physical properties do not depend on the "extent" of the system. Density and temperature are intensive, when you combine 2 gallons of water the temperature stays at 20 deg (it does not become 40) and the density stays at approximately 1g/ml. Intensive properties are often constants and can be used to identify a substance.

Table$$\PageIndex{1}$$: Common Physical Properties

 Texture Color Temperature Shape Luster Malleability Ductility Density Viscosity Solubility Mass Volume

Description of Some Physical Properties

Luster

The luster of an element is defined as the way it reacts to light in a glowing sort of reflective manner. Luster is a quality of a metal. Almost all of the metals, transition metals, and metalloids are lustrous. The non-metals and gases are not lustrous. Metallic luster can be attributed to the fact that metals have lots of loosely bound electrons that can interact with light particles (photons) of many different energies.

Malleability

Malleability is also a quality of metals. Metals are said to be malleable. This means that the metals can deform under an amount of stress. For example, if you can hit a metal with a mallet and it deforms, it is malleable. Also, a paperclip can be shaped with bare hands. In contrast a toothpick is not malleable and would snap if you applied a stress to it. This is also related to the metallic bond and its loose electrons, that allow for bonds to form in many different directions. Non-metals tend to have bonds defined by specific geometric orientations, and thus they break if the atoms are moved out of those orientations.

Ductility: Ability to be drawn into a thin wire

In materials science the ability to be drawn into a thin wire is called ductility. For example, raw copper can wrapped into a cord. Once again, this property is characteristic of mainly metals, nonmetals do not possess this quality.

Video$$\PageIndex{1}$$: 0.29 min youtube uploaded by Chemistry Channel explaining ductility and malleability, https://youtu.be/oVqnigxQ6eM

Density ($$d=\frac{m}{v}$$)

The density of an object is its mass divided by its volume. A substance will have a higher density if it has more mass in a fixed amount of volume. For example, take a ball of metal, roughly the size of a baseball, compressed from raw metal. Compare this to a baseball made of paper. The baseball made of metal has a much greater weight to it in the same amount of volume. Therefore the baseball made out of metal has a much higher density. The density of an object will also determine whether it will sink or float in a particular liquid. Water for example has a density of 1g/cm3. Any substance with a density lower than that will float, while any substance with a density above that will sink.

Figure$$\PageIndex{1}$$: Iceberg floating on water. Note the ice is less dense and so it floats. Image courtesy of Wikimedia commons

Note that much of the ice is below the water, but because ice is less dense then water if floats and does not sink. The mass of the water displaced is equal to the mass of the iceberg, and since the iceberg is less dense and has a larger volume, some if it floats above the surface and therefor it does not sink to the bottom. In the winter the ice on a lake acts as an insulator and protects the deep water from freezing, if water was more dense than ice and lakes froze from the bottom up, they could solidify in the winter and not support aquatic life as we know it.

Table $$\PageIndex{1}$$: Density of Several Substances at 20°C.
Substance Density / g cm-3
Helium gas 0.000 16
Dry air 0.001 185
Gasoline 0.66-0.69 (varies)
Kerosene 0.82
Benzene 0.880
Water 1.000
Carbon tetrachloride 1.595
Magnesium 1.74
Salt 2.16
Aluminum 2.70
Iron 7.87
Copper 8.96
Silver 10.5
Uranium 19.05
Gold 19.32

Therefore, a column in a table or the axis of a graph is conveniently labeled in the following form:

As of 2015 there was an estimated 5.25 trillion pieces of plastics debris in the ocean, which represents a grave threat to the health of the ocean and earth's ecosystems. Of that, 269,000 tons floats while there are huge amounts (4 billion plastic microfibers/sq kilometer) that sink.

Video$$\PageIndex{2}$$: Video show plastics problem in waterways.

The fact that some plastics sink and some float can easily be understood by looking at a table of densities. Note, although a PETE 1 water bottle may initially float (because it is full of air), it will eventual degrade (physical change) to very small particles (micro-particles) and sink.

Table$$\PageIndex{2}$$: Density of common plastics
Plastic Symbol name Density/g/cm3 Uses
Polypropylene 0.90 - 0.92 yogurt cups, plasticware
Low Density Polyethylene 0.91 - 0.93 squeezable bottles
High Density Polyethylene 0.94 - 0.96 milk bottles, bags
Polystyrene 1.03 - 1.06 egg cartons, packing peanuts
Polyethylene Terephthalate 1.35 - 1.38 water bottles
Polyvinyl Chloride 1.32 - 1.42 juice bottles, cling wrap
Often Polycarbonate or
Acrylonitrile butadiene styrene (ABS)
1.3
1.06
bottles
auto body parts

In fact, we can sort plastic particles by placing them into different solvents of different densities, as shown in this Youtube. Note, these solvents must not mix with each other (so one phase must be like an oil, and the next like water, as oil and water do not mix), and the particles will aggregate in the solvent whose densities are closest to them (they sink in lighter density solvents and float in heavier density solvents).

Video$$\PageIndex{3}$$:The above 4:03 Youtube uploaded by Hegelrast shows how you can use density to separate plastics.

Viscosity

Viscosity is defined as the resistance to flow. The substance on the left has a lower viscosity and quickly flows to the walls of the table, while the substance on the right flows slower. Water has a lower viscosity than honey or magma. Viscosity is influenced by temperature.

Figure $$\PageIndex{2}$$: Viscosity demonstration. The fluid on the left has a lower viscosity than the fluid on the right. Image used with permission (CC SA-BY 4.0; Synapticrelay).

Q and A

Physical Properties describe the physical state of matter. Examples include the state of matter, odor, color, volume, denisty, melting point, boiling point, temperature, electrical conductivity,...

What is the Difference Between Extensive and Intensive Properties ?

Extensive properties depend on the amount of matter (extent of the system), while Intensive do not depend on the amount of system. Mass and Volume are extensive, while temperature and density are intensive. If you add one gallon of gasoline at 250C to a car's gas tank that contains a gallon of gasoline at 250C, you double the mass, volume and distance you could drive (energy of the gas), but the temperature does not double, and the density is the same.

Can intensive properties be used to identify what a substance is?

Not always, but often they can. For example, if a substance is incompressible (solid and liquid) the density is considered constant at a given temperature and pressure, and can be used to identify a substance (gold has a density of 19.3g/ml, and if a gold ring does not have that density, it is not pure gold (note the inverse is not necessarily true, just because the density is 19.3g/ml does not mean it must be gold, but that it could be gold). But a Gas is compressible, and fills the container, so density alone can not indicate what the gas is.

Can extensive properties be used to identify what a substance is?

Not directly, as they change as the system changes..

Do you measure intensive of extensive properties?

You usually measure extensive properties. To measure the density of something you measure it's mass and volume, and then take the ration (d=m/v).

Doesn't a thermometer measure temperature, an intensive property?

No, you actually measure something else that changes as the temperature changes. For example, if you know how the density of mercury (or alcohol) changes as a function of temperature, you can construct a thermometer where you measure the height of the column. What you are really measuring is the change in volume as the tube has a constant cross area V=Area time height, and you mark the height in units of temperature. So in reality, you are measuring the change in volume, an extensive property

How about an electric thermometer like a thermocouple, doesn't that measure the change in voltage across a junction as a function of the temperature change, and isn't voltage an intensive property? Yes, voltage is an intensive property, but the device that measures the voltage, like the voltmeter, is not really measuring voltage, but current across a fixed resistor. It uses Ohm's Law where the voltage = current times resistance (V=IR), and the current is an extensive property. This can be done with where a needle is deflected by the magnetic field produced by the current as it goes through a coil, and since the resistance the device can be calibrated in units of voltage. But it is really the current that is changing.

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