Measurement and Safety Chapter One Section 4

What You Will Learn Explain the importance of the International System of Units. Determine appropriate units to use for particular measurements. Identify lab safety symbols, and determine what they mean.

At one time, standardized units were based on parts of the body. Long ago, in England, the standard for an inch was three grains of barley. Using such units was not a very accurate way to measure things because they were based on objects that varied in size. Recognizing the need for a global measurement system, the French Academy of Sciences developed a system in the late 1700s. Over the next 200 years, the metric system, now called the International System of Units (SI), was refined.

Using the International System of Units Today, most scientists and other people in almost all countries use the International System of Units. One advantage of using SI measurements is that all scientists can share and compare their observations and results. Another advantage of the SI is that all units are based on the number 10, which is a number that is easy to use in calculations

Length To measure length, a scientist uses meters (m). A meter is the basic SI unit of length. You may remember that SI units are based on the number 10. If you divide 1 m into 100 parts, for example, each part equals 1 cm. In other words, a centimeter equals one-hundredth of a meter. Some objects are so tiny that smaller units must be used.

Length Continued To describe the length of microscopic objects, scientists use micrometers (µm) or nanometers (nm). To describe the length of larger objects, scientists use kilometers (km).

Volume Imagine that you are a scientist who needs to move some fossils to a museum. How many fossils will fit into a crate? The answer depends on the volume of the crate and the volume of each fossil. Volume is the measure of the size of a body or region in three-dimensional space.

Volume Continued The volume of a liquid is often given in liters (L). Liters are based on the meter. A cubic meter (1 m 3 ) is equal to 1,000 L. So, 1,000 L will fit into a box measuring 1 m on each side. The volume of a large, solid object is given in cubic meters. The volumes of smaller objects can be given in cubic centimeters (cm 3 ) or cubic millimeters (mm 3 ). To calculate the volume of a box-shaped object, multiply the object’s length by its width and then multiply by its height.

The length, height, and width of irregularly shaped objects, such as rocks and fossils, are difficult to measure accurately. However, the volume of an irregularly shaped object can be determined by measuring the volume of liquid that the object displaces. The student in Figure 1 is using a graduated cylinder to measure the volume of water a rock displaces.

Mass A measure of the amount of matter in an object is mass. The kilogram (kg) is the basic unit for mass. The kilogram is used to describe the mass of things such as boulders. Grams (g) are used to describe the mass of smaller objects, such as an apple. One thousand grams equals 1 kg. The mass of large objects, such as an elephant, is given in metric tons. A metric ton equals 1,000 kg.

Temperature Temperature is the measure of how hot (or cold) something is. You are probably used to describing temperature with degrees Fahrenheit (ºF). For example, if your body temperature is 101ºF, you have a fever. Scientists, however, usually use degrees Celsius (ºC). The thermometer in Figure 2 shows the relationship between degrees Fahrenheit and degrees Celsius. The kelvin, the SI base unit for temperature, is also used in science.

Area How much paper would you need to cover the top of your desk? T o answer this question, you must find the area of the desk. Area is a measure of how much surface an object has. area = length × width

Density The ratio of the mass of a substance to the volume of the substance is the substance’s density. Because density is the ratio of mass to volume, units often used for density are grams per milliliter (g/mL) and grams per cubic centimeter (g/cm 3 ). You can calculate density by using the following equation:

Section Summary The SI is the standard system of measurement used by scientists around the world. The basic SI units of measurement for length, volume, mass, and temperature are the meter, liter, kilogram, and kelvin, respectively. Safety rules must be followed at all times during a scientific investigation.