Density Lab

Density Lab, also known by the names Buoyancy Lab and Buoy Oh Buoy, is a new event rotating in for the 2019 season. It was previously a trial event at the 2018 National Tournament. This events consists of two parts - a written test on density, buoyancy, concentrations, and the behavior of gases, and one or more hands-on tasks relating to those concepts.

Test Description
A Density Lab test consists of two parts, a written test and at least one hands-on task. The written test is 50% to 75% of the total event score, while the hands-on portion contributes the remaining 25% to 50%. The event requires a basic knowledge of chemistry and physics as they pertain to density, concentrations, gas behavior, and buoyancy.

Students must use metric units with correct significant figures, unless instructed otherwise.

Possible Hands-On Tasks
Students may be required to: Most hands-on tasks will require students to make measurements using provided instruments. For this reason, it is recommended that students practice the hands-on tasks using a variety of instrumentation and techniques.
 * measure or calculate the mass density of a given solid
 * collect a volume of gas and calculate the volume, mass, and mass density
 * determine the number density of multiple objects, such as a bag of brown M&M's
 * determine the mass that a given helium balloon can lift
 * determine the depth to which an object may sink in water
 * determine the density of a material at different temperatures

Density
In general, density is a description of the amount of a substance per unit volume or area. There are several common expressions of density, including mass density, number density, and area density. Usually the term density is used to refer to mass density.

Densities are expressed in a variety of different units, depending on what type of density it is and the units used to express the mass, number, area, and volume of the substance. All densities, however, are rates and thus their units follow the general form [Unit of mass or number] per [Unit of volume or area]. For example, the mass density of gold is approximately 19.3 grams per cubic centimeter.

In general, the density of a material is greatest when it is a solid and decreases as it changes phase to a liquid and from a liquid to a gas. This is not always the case, however; water is less dense as a solid than liquid. Likewise, within a single phase, density usually decreases with increasing temperature, a phenomenon known as thermal expansion. Conversely, substances within a single phase usually increase in density with decreasing temperature, known as thermal contraction.

Mass Density
Mass density, usually just called density, is the mass of a substance per unit volume. It is usually represented by the lowercase Greek letter rho ([math]\rho[/math]).

The formula for calculating density is shown in the image to the right.

Number Density
Number density is the number of countable objects per unit volume. It is given by the formula [math]\rho_{N} = \frac{N}{V}[/math] where [math]\rho_{N}[/math] is the number density of the objects, [math]N[/math] is the number of the objects, and [math]V[/math] is the volume of space being examined.

Number density can be calculated for any objects, though it is often used for the density of individual atoms or molecules. It is important to specify the object the number density refers to when writing the unit.

Area Density
Area density is the mass of an object per unit of surface area. It is commonly used for expressing density of objects such as paper or fabrics, whose thickness is not easily found using typical measuring instruments.

The formula for area density is [math]\rho_A = \frac{m}{A}[/math] where [math]\rho_{A}[/math] is the area density, [math]m[/math] is the mass of the object, and [math]A[/math] is the surface area over which the density is expressed.

Concentrations
A concentration is an expression of the amount of one substance that is present in a given amount of a mixture of substances. Similar to density, concentrations may be expressed in multiple ways, including mass per unit mass ("mass/mass"), mass per unit volume ("mass/volume"), and volume per unit volume ("volume/volume").

Similar to density, the units used to express concentration will depend on the units of mass or volume used to measure the substance and mixture, but are rates and follow the general form [Unit of mass or volume] per [Unit of mass or volume]. For example, the concentration of salts in seawater is approximately 3.5 grams per liter.

When a concentration is mass/mass or volume/volume and the mass or volume of the substance and mixture are measured in the same units, the concentration may also be expressed as a proportion in percent, parts-per-million, or parts-per-billion notation. For example, if one gram of red dye was added to 100 grams of water, the concentration may be expressed as 0.01 grams (of dye) per gram (of water), or 1%. If expressing a concentration as a proportion, it is necessary to specify whether the proportion refers to a mass/mass or volume/volume concentration by stating "by mass" or "by volume" afterward. In the previous example, this would be done by stating the concentration as "1% by mass".

Gas Laws
Gas laws describe the relationship between key variables of gases. These variables include: When using gas laws that involve temperature, it is necessary to only use units of absolute temperature. Absolute temperature scales express absolute zero, the lowest theoretically-possible temperature, as zero on the scale. Therefore, doubling the temperature on an absolute scale represents a double in the average kinetic energy of the gas particles. The most common scale of absolute temperature is the Kelvin scale. In contrast, Fahrenheit and Celsius temperatures are not absolute and must be converted to an absolute temperature before being used for calculation.
 * Temperature (T) - a measure of the average kinetic energy of gas molecules; usually given in units of kelvin (K)
 * Pressure (P) - a measure of force per unit area resulting from collisions of gas molecules; usually given in units of pascals (Pa), equivalent to one newton per square meter
 * Volume (V) - the amount of space occupied by the gas; usually given in units of cubic meters (m³)
 * Amount of substance (n) - the number of atoms or molecules of the gas; usually given in units of moles (mol), equivalent to 6.022×1023 atoms or molecules

Archimedes' Principle
Archimedes' Principle will be one of the most important concepts to learn for this event.

Archimedes' Principle: any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid displaced by the body. The volume of displaced fluid is equivalent to the volume of an object fully immersed in a fluid or to that fraction of the volume below the surface for an object partially submerged in a liquid. The weight of the displaced portion of the fluid is equivalent to the magnitude of the buoyant force. The buoyant force on a body floating in a liquid or gas is also equivalent in magnitude to the weight of the floating object and is opposite in direction; the object neither rises nor sinks.

Additional Links

 * 2018 Trial Event rules
 * 2012 NYS Trial rules
 * [[Media:Significant Figures.pptx|Significant Figures Tutorial (PPTX)]]