Chemistry Lab/Physical Properties

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Physical Properties is a topic for Chemistry Lab for the 2018 and 2019 season.

Physical properties of an object are properties that can be observed and measured without changing the composition of that object, just as physical changes occur without affecting the chemical composition of matter. Physical properties can range from a wide variety of topics; thus the event and 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 an engineering standpoint of physical properties rather than a 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. Note that flammability is only observed when an object is burned. Corrosivity is another chemical property, describing how readily a material reacts with oxygen and rusts.

Physical properties, however, can be observed based on a material itself, without changing the object's chemical composition. 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 as more matter results in more mass, while density is intensive as more mass does not change the density.

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 classified as 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 distance between molecules. Thermal expansion thus decreases density and increases specific volume. In solids, this expansion is small but certainly not negligible, for example, in the expansion of concrete during the hot summer months. In liquids, thermal expansion is greater. Finally, in gases, thermal expansion has the most drastic effects on volume and density.

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.

From the results of this procedure, the density can be calculated:

  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.

Magnetism

Magnetism is a phenomenon based on the behavior of magnetic fields. It is commonly understood based on attraction and repulsion between traditional magnets, but it is actually much more complex. Magnetism should instead be thought of as the result of the motion of electric charges. From the perspective of physics, magnetism is usually viewed as a result of electric current or the motion of charged particles. However, the focus of magnetism in chemistry will likely be the motion of electrons in atomic orbitals.

Electron spins in a ferromagnetic material.

Ferromagnetism

Ferromagnetism is the most commonly known form of magnetism. It occurs in a limited number of substances, including iron, nickel, and cobalt. Ferromagnets exhibit long range order where unpaired electrons have parallel spins. However, this long range order is usually limited to small domains of a material. In their natural form, ferromagnetic materials usually are fairly disordered at the macroscopic scale, and magnetic fields from domains cancel out, rendering the material relatively nonmagnetic. However, these domains can be aligned by a small external field to magnetize the object.

Hysteresis

In general, hysteresis is the tendency of a physical property to have a delayed response to an external force. Hysteresis is notable, however, for its effects in ferromagnetism. Ferromagnets have the tendency to stay magnetized even once an external magnetic field is removed. This allows ferromagnets to produce fairly permanent magnetic fields.

Paramagnetism

Paramagnetism is also caused by the behavior of unpaired electrons; it occurs in any material with unpaired electron orbitals. In paramagnetism, unpaired electrons align in the presence of a magnetic field. This magnetization occurs in the direction of the applied field, causing attraction. However, paramagnets do not maintain their aligned state, causing them to be much weaker than ferromagnets.

Electron spins in an antiferromagnetic material.
Electron spins in a ferrimagnetic material.

Diamagnetism

Diamagnetism is caused by the presence of unpaired electron orbitals. It weakly opposes applied magnetic fields. In a material with both unpaired and paired electrons, the attractive force from paramagnetism dominates the repulsive force from diamagnetism.

Antiferromagnetism

Antiferromagnetism is another ordered form of magnetism. In antiferromagnetism, neighboring unpaired electrons have opposite spins, known as an antiparallel arrangement. When no external field is applied, this leads to the complete absence of magnetic properties.

The Neel temperature is specific to each antiferromagnetic material. Above this temperature, thermal disorder overcomes antiferromagnetic order, typically causing a material to become paramagnetic.

Ferrimagnetism

Ferrimagnetism is another type of magnetism involving a lattice of two opposing magnetic fields. However, in ferrimagnetism, one field is stronger than the other, leading to a net magnetic field. An external field can induce ferrimagnetic behavior in an antiferromagnet.

Electrical Resistance

Electrical resistance is the resistance of an object to the flow of current. Even conductive materials naturally resist the flow of electric current. A voltage difference across a resistor provides a driving force for current. In many ways, this parallels the behavior of fluids, where the change in pressure causes fluid flow.

Ohm's law describes the relationship between voltage, current, and resistance:

[math]V = IR[/math]

Most substances have a constant resistance such that voltage and current are always directly proportional; these materials are described as ohmic.

4 resistors in series.
4 resistors in parallel.

Resistors

It is important to note that resistance is actually an extrinsic property, depending on the size and shape of a material as well as the qualities of the material itself. This property can be easily seen through the behavior of resistors in circuits.

For resistors in series, the resistance increases.

[math]R_{total} = R_1 + R_2 + R_3 + R_4[/math]

For resistors in parallel, however, the resistance decreases.

[math]\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \frac{1}{R_4}[/math]

In other words, as the total length of resistors increases, the resistance increases; as the total area of resistors increases, the resistance decreases.

Resistivity

This relationship leads to a critical property of resistance. While resistance is an extrinsic property, it can be calculated from the intrinsic property of resistivity.

[math]R = \frac{\rho *l}{A}[/math]

Resistivity is constant regardless of the amount of material; it only depends on the temperature.

Conductivity

Conductivity is the inverse of resistivity.

Elasticity

Elasticity is the ability of an object to undergo reversible deformation. When a force is applied to a material, it will deform; elastic materials will return to their original state once the force is removed. This contrasts with plastic deformation, which is permanent. It is important to note that perfect elasticity is only a theoretical concept, and thus all materials experience some plastic deformation.

A typical stress-strain curve.

Young's Modulus

The process of deformation can be measured by two characteristics: stress and strain.

Stress is a measure of force per unit area, which allows the applied force to be compared across different experimental setups. It is measured in units of Pascals, or more typically, MPa and GPa.

[math]\sigma = F/A[/math]

Stress describes the change in length of a material. It can be expressed as a dimensionless quantity or a percentage.

[math]\epsilon = \Delta l/l_0[/math]

Stress and strain are often plotted against each other on a stress-strain curve. This curve demonstrates the the changes in stress as a material expands or contracts. In the elastic region, stress increases linearly with strain. This behavior follows Hooke's Law for a spring, as the force increases linearly with change in length. Young's Modulus is the slope of this region.

[math]E = \sigma /\epsilon[/math]

A higher Young's Modulus means a material resists deformation to a greater extent. Thus, for the purposes of physics or chemistry, a stiffer material has a higher elasticity. Note that this is the direct opposite of the common definition of elasticity.

Solubility

Solubility is the property of a substance to dissolve in another substance. The substance that is dissolving is called the solute while the substance the solute is dissolving in is called the solvent. For the purpose of this event, this will likely involve a solid solute dissolving into a liquid solvent. A substance is described as miscible if it fully dissolves at any concentration. Meanwhile, if a substance is very poorly soluble, it is often described as insoluble.

A common rule of solubility is "like dissolves like". Substances with similar structures are more likely to be soluble. For example, nonpolar substances are often soluble in hexane, polar substances are often soluble in water, and aromatic substances are often soluble in benzene.

Solubility Rules

While most questions about solubility will be fairly generalized, it is important to know the behavior of common ionic solids in water. The following rules can be used to determine solubility, with earlier rules taking precedence.

  1. Alkali metal or ammonium (NH4+) salts are soluble.
  2. Nitrate (NO3-) or acetate (CH3COO-) salts are soluble.
  3. Chloride, bromide, or iodide salts are soluble.
    1. Halide salts of silver, lead or mercury are insoluble.
  4. In general, silver salts are insoluble.
  5. Sulfate salts are soluble.
    1. Exceptions: CaSO4, BaSO4, PbSO4, Ag2SO4 and SrSO4.
  6. Hydroxide (OH-) salts are insoluble.
    1. Alkaline earth metal hydroxides are slightly soluble.
    2. Alkali metal or ammonium hydroxides are soluble.
  7. Inorganic acids are soluble.
  8. Sulfides of transition metals are insoluble. Also, arsenic, antimony, bismuth, and lead sulfides are insoluble.
  9. Carbonates, chromates, phosphates, and fluorides are frequently insoluble.

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. Various substances absorb different wavelengths of light. The light that is reflected determines the color of the substance.

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.

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 or 1 Cal/g*K.

Concentration

A solution has a concentration, which is defined as the ratio of the amount of solute to the amount of solvent or solution. Concentration measures include: molarity, molality, mole ratio.

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

Energy includes:

  • Kinetic Energy
  • Potential Energy (Gravity, Spring, Electromagnetic, etc.)
  • Internal Energy (Thermal, Chemical, Nuclear, etc.)

The total amount of energy in a substance is related proportionally to its mass, so it is an extensive property.

Enthalpy

Enthalpy is related to internal energy by the formula H = U + pV. It is related to the total amount of internal energy and work that a gas can do.

Entropy

Entropy is defined as the total disorder of a substance. It is measured in J/molK. Its symbol is S.

Definitions of entropy include:

  • The product of Boltzmann's Constant and the logarithm of the number of equiprobable microstates for a given macrostate.
  • The change in entropy equals the reversible change in energy divided by temperature.

Gibbs Free Energy

Gibbs Free Energy is defined in relation to the system: G = H - TS. Gibbs Free Energy is defined in relation to the universe: G = -TS.

Gibbs Free Energy will always decrease for a spontaneous occurrence.

Four Laws of Thermodynamics

Not exactly physical properties, but relates the concepts described above.

0. Thermal equilibrium is an equivalence relation.

That Is: All systems are in thermal equilibrium with themselves. If a system is in thermal equilibrium with another, the latter system is in the thermal equilibrium with the first. If two systems are in thermal equilibrium with a third, they are in thermal equilibrium with each other.

Note: The last statement is the official statement of the Zeroth Law. Other statements are unstated assumptions.

1. Energy is conserved.

That Is: If a substance gains a certain amount of energy, the universe loses that amount of energy, and vice versa.

2. Entropy can never decrease.

That Is: The universe tends toward the state with the most disorder. The law involving Gibbs Free Energy is a corollary of this law.

3. A perfect crystal at absolute zero has no entropy.

That Is: Low thermal motion and ordered arrangement are the two fundamental components of entropic order.

Heat Capacity

Heat capacity is the ratio of energy added to temperature increased. It is the extensive property equivalent of specific heat.