A(n) __________ shows how hot or cold something is relative to two reference temperatures.

Daniel Gabriel Fahrenheit (1686-1736) was a German physicist who is credited with the invention of the alcohol thermometer in 1709 and the mercury thermometer in 1714. The Fahrenheit temperature scale was developed in 1724. Fahrenheit originally established a scale in which the temperature of an ice-water-salt mixture was set at 0 degrees. The temperature of an ice-water (no salt) mixture was set at 30 degrees and the temperature of the human body was set at 96 degrees. Using this scale, Fahrenheit measured the temperature of boiling water as 212°F on his scale. He later adjusted the freezing point of water from 30°F to 32°F, thus making the interval between the freezing and boiling points of water an even 180 degrees (and making body temperature the familiar 98.6°F). The Fahrenheit scale is still commonly used in the United States.

Anders Celsius (1701-1744) was a Swedish astronomer credited with the invention of the centigrade scale in 1742. Celsius chose the melting point of ice and the boiling point of water as his two reference temperatures to provide for a simple and consistent method of thermometer calibration. Celsius divided the difference in temperature between the freezing and boiling points of water into 100 degrees (thus the name centi, meaning one hundred, and grade, meaning degrees). After Celsius's death, the centigrade scale was renamed the Celsius scale and the freezing point of water was set at 0°C and the boiling point of water at 100°C. The Celsius scale takes precedence over the Fahrenheit scale in scientific research because it is more compatible with the base ten format of the International System (SI) of metric measurement (see our module on The Metric System). In addition, the Celsius temperature scale is commonly used in most countries of the world other than the United States.

Comprehension Checkpoint

Which temperature scale is used more in science?

Lord William Kelvin (1824-1907) was a Scottish physicist who devised the Kelvin (K) scale in 1854. The Kelvin scale is based on the idea of absolute zero, the theoretical temperature at which all molecular motion stops and no discernible energy can be detected (see our States of Matter module for more information). In theory, the zero point on the Kelvin scale is the lowest possible temperature that exists in the universe: -273.15ºC. The Kelvin scale uses the same unit of division as the Celsius scale; however, it resets the zero point to absolute zero: -273.15ºC. The freezing point of water is therefore 273.15 Kelvins (graduations are called Kelvins on the scale and neither the term "degree" nor the symbol º is used), and 373.15 K is the boiling point of water. The Kelvin scale, like the Celsius scale, is a standard SI unit of measurement used commonly in scientific measurements. Since there are no negative numbers on the Kelvin scale (because theoretically nothing can be colder than absolute zero), it is very convenient to use Kelvins when measuring extremely low temperatures in scientific research. (The three scales are compared in Figure 1.)

Comprehension Checkpoint

Temperatures below absolute zero on the Kelvin scale

Although it may seem confusing, each of the three temperature scales discussed allows us to measure heat energy in a slightly different way. A temperature measurement in any of the three scales can be easily converted to another scale using the simple formulas below.

This module provides an introduction to the relationship between energy, heat, and temperature. The principle behind thermometers is explained, beginning with Galileo’s thermoscope in 1597. The module compares the three major temperature scales: Fahrenheit, Celsius, and Kelvin. It discusses how the different systems use different references to quantify heat energy.

Key Concepts

• There are three different systems for measuring heat energy (temperature): Fahrenheit, Celsius, and Kelvin.

• In scientific measures, it is most common to use either the Kelvin or Celsius scale as a unit of temperature measurement.

• Nothing can be colder than absolute zero, which is the point at which all molecular motion ceases.

Archimedes had used the concept of density to expose the fraud. Density is a physical property of matter that expresses a relationship of mass to volume. The more mass an object contains in a given space, the more dense it is. It is important to remember, though, that this relationship is not just about how closely packed together the atoms of an element or the molecules of a compound are. Density is also affected by the atomic mass of an element or compound. Since different substances have different densities, density measurements are a useful means for identifying substances.

For example, how could you distinguish a metric ton of feathers versus a metric ton of bricks, shown in Figure 1, if you could not see them?

One metric ton of either feathers or bricks will have an identical mass of 1,000 kilograms (one metric ton). However, a metric ton of feathers will occupy a volume of almost 400 million cm3 (about the size of four tractor trailer trucks), while a metric ton of bricks will occupy only one-half million cm3 (about the size of a large-screen TV). The bricks are denser than the feathers because their mass is packed into a smaller volume. This relationship between the mass and volume of a substance is what defines the physical property of density:

Density = Mass/Volume

Density is defined as the ratio of an object's mass to its volume, as shown in the equation above. Because it is a ratio, the density of a material remains the same without regard to how much of that material is present. Density is therefore called an intensive property of matter. Mass is the amount of matter contained in an object and is commonly measured in units of grams (g). Volume is the amount of space taken up by a quantity of matter and is commonly expressed in cubic centimeters (cm3) or in milliliters (ml) (1cm3 = 1 ml). Therefore, common units used to express density are grams per milliliters (g/ml) and grams per cubic centimeter (g/cm3).

Let's look at an example. A typical brick has a mass of 2,268 g and occupies a volume of 1,230 cm3. Using the equation above, we can calculate the density of the brick:

Densitybrick = Massbrick/Volumebrick

Densitybrick = 2,268 g/1,230 cm3

Densitybrick = 1.84 g/cm3

Density can sometimes be confused in our minds with weight because the denser of two equal-volume objects will be heavier. Remember, though, that it is the relationship between mass and volume that determines density and not volume or mass alone, or even how closely packed the atoms or molecules are. Look at Table 1 for examples of the density of common substances.

Density of Some Common Substances
Substance	Density (g/cm3)
Air	0.0013
Feathers	0.0025
Pine	0.5
Ice	0.92
Water	1.00
Bricks	1.84
Aluminum	2.70
Steel	7.80
Silver	10.50
Gold	19.30

Comprehension Checkpoint

One metric ton of feathers and one metric ton of bricks have

When Archimedes stepped into his bathing pool, not only did he realize that water spilled over the edges, but he also observed something that we all notice when we go swimming – he felt lighter. The ability of an object to "float" when it is placed in a fluid is called buoyant force, and is related to density. If an object is less dense than the fluid in which it is placed, it will float. If it is more dense than the fluid, it will sink.

This concept explains why some objects float on water while others sink. For example, most types of wood float on water because they are less dense; steel, by comparison, sinks because it is denser than water. How, then, can large steel cruise ships stay afloat? Large ships have a tremendous amount of space in them that is filled with air (think about it: cabins, movie theaters, onboard casinos, etc.). While steel is denser than water, air is a lot less dense than water (see Table 1). Metal ships can float because their total density – steel plus air – is less than that of the water that they float on. When the metal hull of a ship is breached, like when the Titanic struck an iceberg, water rushes in and replaces the air in the ship's hull. As a result, the total density of the ship changes and causes the ship to sink.

The concept of changing density is commonly employed in another type of vessel, a submarine. A submarine has a constant volume but it can vary its mass by taking in water into its ballast tanks. When water is taken into the ballast tanks, the mass (and thus density) of the submarine increases and the submarine attains negative buoyancy that allows it to submerge into the ocean depths. Conversely, when water is released from the ballast tanks the vessel's density decreases allowing it to surface.

Mixing materials of different densities has predictable results. Have you ever noticed what happens to a bottle of oil and vinegar salad dressing when it is allowed to sit still after it has been shaken? The oil will rise to the top and the vinegar will settle to the bottom of the bottle. This happens because oil is less dense than vinegar. When materials of different densities are put in contact with one another, their relative densities will determine how they order themselves. This phenomenon, where materials layer themselves according to their density, is called superposition.

Comprehension Checkpoint

Cruise ships float because

The density of a material is strongly connected to other intensive properties, particularly temperature (see our Temperature module). Many materials expand when they are heated. Because a material that expands takes up a larger volume, its density decreases. This phenomenon occurs in all forms of matter: for example, solids, liquids, and gases. The tightly coupled relationship between density and temperature explains how hot air balloons work. When the air inside of a balloon is heated it expands and its density decreases. The balloon thus gains positive buoyancy with respect to the colder air surrounding it, and it floats into the sky.

Density is a fundamental physical property of matter. It is commonly used as a means of categorizing and identifying different materials. In addition, a thorough understanding of the concept of density is critical for building ships and lighter-than-air craft such as hot air balloons.

Density is a fundamental physical property of matter. This module introduces the concept of density, explains how density is calculated, and lists the densities of common substances. The relationship between density and buoyancy is discussed. The module relates the concept of density to the operation of large ships, submarines, and hot air balloons.

Key Concepts

• Density is a physical property of matter that expresses a relationship of mass to volume.

• When materials of different densities are put in contact with one another, their densities will determine how they order themselves.

• An object's buoyancy is determined by its density in relation to the density of the surrounding liquid.

The simplicity of the metric system stems from the fact that there is only one unit of measurement (or base unit) for each type of quantity measured (length, mass, etc.). The three most common base units in the metric system are the meter, gram, and liter. The meter is a unit of length equal to 3.28 feet; the gram is a unit of mass equal to approximately 0.0022 pounds (about the mass of a paper clip); and the liter is a unit of volume equal to 1.05 quarts. So length, for example, is always measured in meters in the metric system; regardless of whether you are measuring the length of your finger or the length of the Nile River.

To simplify things, very large and very small objects are expressed as multiples of ten of the base unit. For example, rather than saying that the Nile River is 6,650,000 meters long, we can say that it is 6,650 thousand-meters long. This would be done by adding the prefix "kilo" (meaning 1,000) to the base unit "meter" to give us 6,650 kilometers for the length of the Nile River. This is much simpler than the American system of measurement, in which we have to remember inches, feet, miles, and many more units of measurement. Metric prefixes can be used with any base unit. For example, a kilometer is 1,000 meters, a kilogram is 1,000 grams, and a kiloliter is 1,000 liters. Six common prefixes used in the metric system are listed below.

The subunits are used when measuring very large or very small things. It wouldn't make sense to measure your weight in grams for the same reason that you wouldn't measure it in ounces - the unit is too small. You would express your weight in kilograms (each kilogram is equal to 1,000 grams or about 2.2 pounds).

Comprehension Checkpoint

Metric units include

The metric system is a called a decimal-based system because it is based on multiples of ten. Any measurement given in one metric unit (e.g., kilogram) can be converted to another metric unit (e.g., gram) simply by moving the decimal place. For example, let's say a friend told you that he weighed 72,500.0 grams (159.5 lbs). You can convert this to kilograms by moving the decimal three places to the left. In other words, your friend weighs 72.5 kilograms.

Because the metric system is based on multiples of ten, converting within the system is simple. Here's a shortcut: If you are converting from a smaller unit to a larger unit (moving upward in the table shown above), move the decimal place to the left in the number you are converting. If you are converting from a larger unit to a smaller unit (moving down in the table), move the decimal to the right. The number of places you move the decimal corresponds to the number of rows you are crossing in the table. For example, let's say someone told you that you had to walk 8,939.0 millimeters to get to the grocery store. That sounds like a long walk, but let's convert the number into meters to see how long it really is. The base unit, meter, is three rows above the millimeter, so the decimal should be moved three places to the left.

It's less than 9 meters to the grocery store - or about 30 feet. Metric units can be abbreviated for simplicity. Abbreviations for the base units are the first letter of the unit name: m = meter, g = gram, and l = liter. Subunits can be abbreviated using the first letter of the prefix and the first letter of the base unit (all lowercase): mm = millimeter, kg = kilogram, etc.

Comprehension Checkpoint

Moving the decimal place can change

In science, it is common to work with very large and very small numbers. For example, the diameter of a red blood cell is 0.0065 cm, the distance from the Earth to the sun is 150,000,000 km, and the number of molecules in 1 g of water is 33,400,000,000,000,000,000,000. It gets cumbersome to work with such long numbers, so measurements such as these are often written using a shorthand called scientific notation.

Each zero in the numbers above represents a multiple of 10. For example, the number 100 represents 2 multiples of 10 (10 x 10 = 100). In scientific notation, 100 can be written as 1 times 2 multiples of 10:

100 = 1 x 10 x 10 = 1 x 102 (in scientific notation)

Scientific notation is a simple way to represent large numbers because the 10's exponent (2 in the example above) tells you how many places to move the decimal of the coefficient (1 in the example above) to obtain the original number. In our example, the exponent 2 tells us to move the decimal to the right two places to generate the original number:

Scientific notation can be used even when the coefficient is a number other than 1. For example:

This shorthand can also be used with very small numbers. When scientific notation is used with numbers less than one, the exponent on the 10 is negative, and the decimal is moved to the left, rather than the right. For example:

Therefore, using scientific notation, the diameter of a red blood cell is 6.5 x 10-3 cm, the distance from the Earth to the sun is 1.5 x 108 km, and the number of molecules in 1 g of water is 3.34 x 1022.

Also note that in scientific notation, the base numeral is always represented as a single digit followed by decimals if necessary. Therefore, the number 0.0065 is always represented as 6.5 x 10-3, never as 0.65 x 10-2 or 65 x 10-4.

The metric system is the standard system of measurement in science. This module describes the history and basic operation of the metric system, as well as scientific notation. The module explains how the simplicity of the metric system stems from having only one base unit for each type of quantity measured (length, volume, and mass) along with a range of prefixes that indicate multiples of ten.

Key Concepts

• The metric system, also known as the Système international d'unités (SI), was developed in the late 1700s to standardize units of measurement in Europe.

• The metric system is the primary system of measurement used through much of the world and in science.

• Each type of measurement has a base unit to which prefixes are added to indicate multiples of ten.

• Scientific notation is a shorthand for writing very small and very large numbers.

A common misconception in science is that science provides facts or "truth" about a subject. Science is not collection of facts; rather, it is a process of investigation into the natural world and the knowledge generated through that process. This process of investigation is often referred to as the scientific method and it is typically defined in many textbooks and science courses as a linear set of steps through which a scientist moves from observation through experimentation and to a conclusion as shown below:

However, this classic portrayal has a number of problems. Science is not a linear process - it doesn't have to start with an observation or a question, and it commonly does not even involve experiments. Instead, the scientific method is a much more dynamic and robust process. Scientists get their inspiration from the natural world, from reading what others have done, from talking to colleagues, or from experience. They use multiple types of research toward investigating phenomena, including experimentation, description, comparison, and modeling. Some scientific investigations employ one of these methods, but many involve multiple methods, or some studies may even have characteristics of more than one method. Results from one research study may lead in directions not originally anticipated, or even in multiple directions as different scientists pursue areas of interest to them.

Comprehension Checkpoint

Scientists may use different research methods as long as they end up with the results that were predicted.

Given the detail needed to understand this practice, a single paragraph or even chapter will not do. Thus, we have developed a series of 11 modules that convey the practice of science and present the different methodologies used in scientific research. These 11 modules are part of a larger series of modules called our Process of Science modules, and these modules provide a detailed answer to the question "What is science and how does it work?" If you would like to learn more about individual scientific methodologies, please visit our Practice of Science modules:

Scientific investigation is not always a linear process that starts with a hypothesis and ends with a conclusion, as portrayed in the classic “Scientific Method.” The true scientific method is a much more dynamic and much less predictable, and can involve various methods that may overlap. This module serves as an introduction to our Practice of Science series of modules, which describe key scientific methodologies.

Key Concepts

• The scientific method is a process of discovery that does not follow a prescribed, linear pattern of steps.