Chemistry Lab/Physical Properties

Physical Properties is a topic for Chemistry Lab for the 2018 season. It is expected to be a topic for the 2019 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; the event and thus this wiki will focus on only a subset of these topics.

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.

Density

Density is one of the most commonly tested physical properties. It is fairly simple in nature and can easily be measured, but questions on density can still discuss complex lab procedures. Density is an intensive property.

Density follows the following basic formula:

[math]\rho = \frac{m}{V}[/math]

It is critical to understand conversions between common units of density.

[math]1 g/mL = 1 kg/L = 10^3 kg/m^3[/math]

Specific Volume

The specific volume is closely tied to density.

[math]v = \frac{V}{m} = \rho^-1[/math]

However, specific volume is most commonly used in the field of thermodynamics, not in traditional chemistry.

Specific Gravity

The specific gravity, or relative density, compares the density of a substance to the density of a reference. This reference density is typically water at 3.98 degrees C, which is approximately 1 g/mL.

[math]SG = \frac{\rho _{substance}}{\rho _{reference}}[/math]

Thermal Expansion

Substances tend to expand as they are heated. This occurs because increases in thermal energy cause molecules to vibrate more vigorously, increasing the average between molecules. Thermal expansion thus decreases density and increases specific volume. In solids, this expansion is small but certainly not negligible. In liquids, thermal expansion is greater. Finally, in gases, thermal expansion has drastic effects on volume and density.

Measurement

Gases

The density of a gas can be calculated based on the basic relationship between mass and volume. However, is often difficult to measure either of these properties of a gas. Instead, density can be calculated from other, more easily measured properties thanks to the ideal gas law.

The most common form of the ideal gas law, including the effects of temperature and thermal expansion: [math]PV = nRT[/math]

A modified form of the ideal gas law, using specific volume and molar mass: [math]Pv = MRT[/math]

A further modified form of the ideal gas law, utilizing the relationship between specific volume and density: [math]P = \rho MRT[/math]

From these equations, the direct relationship between volume and temperature and the inverse relationship between density and temperature are both apparent. It is important to note that all of these equations are only valid for gases, and should not be used for calculations with solids or liquids.

A commercial hydrometer with schematic representation.

Liquids

The density of liquids can be measured directly through the use of a hydrometer. The hydrometer is constructed of a glass tube with a bulb at the end; the bulb is weighted with mercury, lead, or another dense substance. If the hydrometer is placed in a liquid of unknown density, it will float upright at a certain height. At that height, the mass of the hydrometer divided by the volume submerged within the liquid is equivalent to the density of the liquid. The hydrometer will usually contain a calibrated scale, so that this equivalent density or specific gravity can be easily read.

The hydrometer uses Archimedes' Principle of buoyancy. For a solid submerged within a liquid, the solid experiences a buoyant force equal to the weight of the displaced fluid. The solid will reach equilibrium if its own weight is equal to the buoyant force, or in other words if its own density is equal to the density of the fluid.

[math]F_b = V_{solid}*\rho _{liquid}*g[/math]

[math]w = V_{solid}*\rho _{solid}*g[/math]

If weight and buoyant force are equal: [math]\rho _{solid} = \rho _{liquid}[/math]

Solids

Since the masses of solids are easily obtained, calculating the density of a solid largely relies on measuring its volume. This volume can be obtained by a number of procedures:

For a regular solid, the important dimensions of the solid can be measured with a ruler or calipers. For example, the volume of a regular cylinder could be calculated from its height and diameter.

For irregular, continuous solids, the volume can be measured by volume displacement.

1. A graduated cylinder is partially filled with water.
2. The initial volume of the water is measured.
3. The solid is carefully placed in the graduated cylinder.
4. The final volume of the water and solid is measured.
5. The volume of the solid can be calculated from the difference between the initial and final volumes.

Note that this procedure also works for regular solids.

For powdered, granular, or porous solids, a pycnometer is used. The pycnometer is simply a container with a precisely calibrated volume.

1. The initial mass of the empty pycnometer is measured.
2. A relatively small amount of the solid is placed into the empty pycnometer.
3. The mass of the pycnometer and solid is measured.
4. The pycnometer is filled with a liquid of known density. Note that the solid must be completely insoluble in the liquid.
5. The final mass of the pycnometer, solid, and liquid is measured.

Calculations:

1. The mass of the liquid can be calculated from the difference in mass between steps 3 and 5.
2. The volume of the liquid can be calculated from the mass and known density.
3. The mass of the solid can be calculated from the difference in mass between steps 1 and 3.
4. The volume of the solid can be calculated from the difference between the calibrated volume and the volume of the liquid.
5. By definition, the density of the solid can be calculated from its mass and volume.

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.

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.