Thinking Critically

Observing and Inferring

Imagine that you have just finished a volleyball game. At home, you open the refrigerator and see a jug of orange juice on the back of the top shelf. The jug feels cold as you grasp it. Then you drink the juice, smell the oranges, and enjoy the tart taste in your mouth.

As you imagined yourself in the story, you used your senses to make observations. You used your sense of sight to find the jug in the refrigerator, your sense of touch when you felt the coldness of the jug, your sense of hearing to listen as the liquid filled the glass, and your senses of smell and taste to enjoy the odor and tartness of the juice. The basis of all scientific investigation is observation.

Scientists try to make careful and accurate observations. When possible, they use instruments such as microscopes and thermometers or balances to make observations. Measurements with a balance or thermometer provide numerical data that can be checked and repeated.

When you make observations in science, you'll find it helpful to examine the entire object or situation first. Then, look carefully for details. Write down everything you observe.

Scientists often make inferences based on their observations. An inference is an attempt to explain or interpret observations or to say what caused what you observed. For example, if you observed a CLOSED sign in a store window around noon, you might infer the owner is taking a lunch break. But, it's also possible that the owner has a doctor's appointment or has taken the day off to go fishing. The only way to be sure your inference is correct is to investigate further.

When making an inference, be certain to use accurate data and observations. Analyze all of the data that you've collected. Then, based on everything you know, explain or interpret what you've observed.

Comparing and Contrasting

Observations can be analyzed by noting the similarities and differences between two or more objects or events that you observe. When you look at objects or events to see how they are similar, you are comparing them. Contrasting is looking for differences in similar objects or events.

Suppose you were asked to compare and contrast the planets Venus and Earth. You would start by looking at what is known about these planets. Arrange this information in a table, like the one below.

Comparing of
Venus and Earth

Similarities you might point out are that both planets are similar in size, shape, and mass. Differences include Venus having a hotter surface temperature that reflects more sunlight than Earth reflects. Also, Venus lacks a moon.

Recognizing Cause and Effect

Have you ever watched something happen and then made suggestions as to why it happened? If so, you have observed an effect and inferred a cause. The event is an effect, and the reason for the event is the cause.

Suppose that every time your teacher fed the fish in a classroom aquarium, she or he tapped the food container on the edge of the aquarium. Then, one day your teacher just happened to tap the edge of the aquarium with a pencil while making a point about an ecology lesson. You observed the fish swim to the surface of the aquarium to feed. What is the effect, and what would you infer to be the cause? The effect is the fish swimming to the surface of the aquarium. You might infer the cause to be the teacher tapping on the edge of the aquarium. In determining cause and effect, you have made a logical inference based on your observations.

Perhaps the fish swam to the surface because they reacted to the teacher's waving hand or for some other reason. When scientists are unsure of the cause of a certain event, they design controlled experiments to determine what causes the event. Although you have made a logical conclusion about the behavior of the fish, you would have to perform an experiment to be certain that it was the tapping that caused the effect you observed.

Measuring in SI

The metric system is a system of measurement developed by a group of scientists in 1795. It helps scientists avoid problems by providing standard measurements that all scientists around the world can understand. A modern form of the metric system, called the International System, or SI, was adopted for worldwide use in 1960.

Metric Prefixes

The metric system is convenient because unit sizes vary by multiples of 10. When changing from smaller units to larger units, divide by 10. When changing from larger units to smaller, you multiply by 10. For example, to convert millimeters to centimeters, divide the millimeters by 10. To convert 30 millimeters to centimeters, divide 30 by 10 (30 millimeters equals 3 centimeters).

Prefixes are used to name units. Look at the table for some common metric prefixes and their meanings. Do you see how the prefix kilo- attached to the unit gram is kilogram, or 1000 grams? The prefix deci- attached to the unit meter is decimeter, or one-tenth (0.1) of a meter.

Length

You have probably measured lengths or distances many times. The meter is the SI unit used to measure length. A baseball bat is about one meter long. When measuring smaller lengths, the meter is divided into smaller units called centimeters and millimeters. A centimeter is one-hundredth (0.01) of a meter, which is about the size of the width of the fingernail on your ring finger. A millimeter is one-thousandth of a meter (0.001), about the thickness of a dime.

Most metric rulers have lines indicating centimeters and millimeters. The centimeter lines are the longer, numbered lines, and the shorter lines are millimeter lines. When using a metric ruler, line up the 0-centimeter mark with the end of the object being measured, and read the number of the unit where the object ends, in this instance 7.5 cm.

Surface Area

Units of length are also used to measure surface area. The standard unit of area is the square meter (m2). A square that's one meter long on each side has a surface area of one square meter. Similarly, a square centimeter (cm2) is one centimeter long on each side. The surface area of an object is determined by multiplying the length times the width.

Volume

The volume of a rectangular solid is also calculated using units of length. The cubic meter (m3) is the standard SI unit of volume. A cubic meter is a cube one meter on each side. You can determine the volume of rectangular solids by multiplying length times width times height.

Liquid Volume

During science activities, you will measure liquids using beakers and graduated cylinders marked in milliliters. A graduated cylinder is a cylindrical container marked with lines from bottom to top.

Liquid volume is measured using a unit called a liter. A liter has the volume of 1000 cubic centimeters. Because the prefix milli- means thousandth (0.001), a milliliter equals one cubic centimeter. One milliliter of liquid would completely fill a cube measuring one centimeter on each side.

Mass

Scientists use balances to find the mass of objects in grams. You will use a beam balance similar to the one illustrated. Notice that on one side of the balance is a pan and on the other side is a set of beams. Each beam has an object of a known mass called a rider that slides on the beam.

Before you find the mass of an object, set the balance to zero by sliding all the riders back to the zero point. Check the pointer on the right to make sure it swings an equal distance above and below the zero point on the scale. If the swing is unequal, find and turn the adjusting screw until you have an equal swing.

Place an object on the pan. Slide the rider with the largest mass along its beam until the pointer drops below zero. Then move it back one notch. Repeat the process on each beam until the pointer swings an equal distance above and below the zero point. Add the masses on each beam to find the mass of the object.

You should never place a hot object or pour chemicals directly onto the pan. Instead, find the mass of a clean beaker or a glass jar. Place the dry or liquid chemicals in the container. Then find the combined mass of the container and the chemicals. Calculate the mass of the chemicals by subtracting the mass of the empty container from the combined mass.