Difference between revisions of "Chemistry Lab/Physical Properties"

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Revision as of 21:28, 12 April 2018

Physical Properties is a topic for Chemistry Lab, expected to rotate in for the 2018 season.

Physical properties can be observed and measured without changing the composition of matter, just as physical changes occur without affecting the chemical composition of matter. Physical properties can range from a wide variety of topics; this wiki attempts to explain the most important physical properties.

The Materials Science page may also be useful for this topic, but many of the Materials Science topics are focused on the engineering standpoint of physical properties rather than the chemical standpoint.

Types of Properties

Chemical properties of matter describe its ability to undergo various reactions or chemical changes. Chemical properties require experimentation to discover, and are not readily apparent based on a material itself. For example, flammability is a chemical property, which describes how readily a material burns. Corrosivity is another chemical property, describing how readily a material reacts with oxygen and rusts.

Physical properties, meanwhile, can be observed based on a material itself. They do not require a reaction to be tested. Physical properties can further be divided into extensive properties, which depend on the amount of matter in a system, and intensive properties, which depend only on the material itself. For example, mass is an extensive property, while density is intensive.

Intensive Properties

As described above, intensive properties do not depend on the amount of a material.

Some basic intensive properties include temperature, pressure, and color.

Other more detailed intensive properties are described below.

Color

Color is the property of a substance when light is reflected by it and is often used to describe substances.

The various types of colors are represented at different wavelengths in the visible light spectrum.

An easy acronym to help remember the order of the colors from longest wavelength to shortest is ROYGBV.

Conductivity

Thermal conductivity is the property of a substance to conduct heat and is measured in watts per meter-kelvin (W/(m·K)) in SI units.

Objects with a high thermal conductivity have a higher rate of heat transfer, and object with a low thermal conductivity have a lower rate of heat transfer. The objects with low conductivity are often used for thermal insulation.

There are two ways to measure conductivity: steady-state and non-steady-state/transient methods. The steady-state method is used when the substance is at a constant temperature; techniques include the divided bar (for rocks samples) and Searle's bar method (for good heat conductors). The transient method is used when the material is experiencing an increase in temperature and can be performed faster than the steady-state methods.

Density

Density is an important property, especially when used to distinguish substances.

In the past, some Materials Science tests have included a density lab, which may also appear in future Chem Lab tests.

The formula for density is as follows: [math]D = m/V[/math]

Density, therefore, requires two measurements - mass, and volume.

Mass is fairly simple to measure, as most labs will provide fairly precise balances.

Volume may be calculated a number of ways. In some cases, a ruler or calipers can be used to measure the dimensions of fairly regular objects, such as rectangular prisms, cylinders, and spheres. Irregular objects can also be measured using the water displacement method, where the object is dropped into a known volume of water and the volume is measured from the change of water volume.

Competitors should also be able to use this equation in other directions, calculating either mass or volume from a known density and other value.

Melting/Boiling Points

Solid green represents normal substances. Dotted green represents water.

Many substances are described by their melting and boiling points. The melting point is the temperature at which a substance undergoes phase changes between liquid and solid. The boiling point, meanwhile, is the temperature at which a substance undergoes phase changes between liquid and gas. As melting point and boiling point are based on temperature, an intensive property, melting and boiling points are also intensive.

Note that melting point and boiling point are measured at a standard pressure of 1 atm, or 103.25 kPa.

This makes melting points and boiling points a fairly simplistic view of phase changes. As viewed on the right, all substances in fact have a much more complex system of phase changes. For most materials, high pressure favors the solid phase, while high temperature favors the gas phase. However, water behaves differently, as ice is lower density than water. High pressure in substances like water favors the liquid phase, instead of the solid phase.

Each line represents an equilibrium, where multiple phases can coexist and where phase transitions occur. This includes the traditional melting points and boiling points. Also, at certain pressures and temperatures, substances are able to sublimate directly from the solid to the gas phase. Some substances, like iodine, do this in standard conditions. Past the critical temperature or critical point, gases and liquids have the same density making them indistinguishable. Finally, at the triple point, the solid, gas, and liquid point of a substance all exist.

All aspects of a phase diagram are intensive properties, not just melting point and boiling points.

Specific Heat Capacity

The specific heat capacity is the amount of energy required to raise a given mass of a substance a given temperature. In the SI system, this is given in J/g*K. Specific heat is based off of heat capacity, but is not mass-sensitive and this is an intensive and not an extensive property. The molar heat capacity is also intensive, but is based on heat capacity per unit amount instead of mass.

Specific heat is often used in calorimetry (See: Chem Lab/Thermodynamics) to calculate heat transfer.

[math]q = m\cdot \Delta T\cdot C_p[/math], where q represents heat flow.

A notable specific heat is that of water, 4.184 J/g*K.

Concentration

Extensive Properties

Extensive properties are affected by the amount of material.

Some basic extensive properties include mass and volume.

Other more detailed extensive properties are described below.

Energy

Enthalpy

Entropy

Gibbs Free Energy

Heat Capacity