Lesson 1: Measurement in Chemistry
Objectives:
Define chemistry and consider the occupations that use chemistry
Examine the scientific method
Review scientific notation
Convert numbers to and from scientific notation
Understand the rules of significant digits
Solve problems that involve density
Take time to consider what is chemistry?
Lab Activity: Massing Small Objects
Safety First and Always
Safety is important in all activities, but the chemistry laboratory is an area where safety must come first. Always have an adult present to supervise laboratory activities; never work alone when dealing with hazardous materials. Familiarize yourself in advance with the hazards found in each experiment, so that you will know the precautions to take with each chemical you will use in an experiment. Know the location and proper use of all safety equipment. Use common sense and ask a teacher or your mentor for assistance if you are in doubt about the safety of any procedure. Always notify an adult of any problems you encounter.
Some specific rules will help you remember how to proceed:
Choose a safe place to work.
Clear your work area of unnecessary materials such as books and papers before you start. Never taste laboratory materials: no food, drinks, or gum are allowed when working with chemicals.
Wear safety goggles at all times.
Follow the instructions for each experiment; do not perform unauthorized experiments.
Tie back long hair. Avoid wearing draping sleeves or other clothing or jewelry that could catch on equipment.
Familiarize yourself with equipment in advance. Always know how equipment works before beginning an experiment.
Avoid awkward transfers. Pour from large containers into wide-mouthed beakers or containers, not test tubes or other small-necked flasks.
Carry chemicals defensively: be sure there are no obstacles in your path and that you have a suitable surface on which to place your container.
Glass and metal stay hot longer than they look hot. Let all hot containers cool before handling. Use hot pads to protect counters and surfaces where needed.
Label your chemicals with the substance name, the date, and your name. The substance name must be complete and legible.
Wash your hands thoroughly after each experiment.
Lock chemicals away out of reach of children.
Clean up after each experiment to minimize hazards.
Treat all chemicals with respect and caution to minimize hazards and maximize your fun in lab.
Safety Equipment
Fire extinguishers should be used according to instructions. You will need to read the procedures on your fire extinguisher in advance so that if an emergency does arise, you will be prepared. Fire extinguishers are found in almost all chemistry laboratories. Your local fire department or fire marshal’s office can provide more information.
Fire blankets are designed to be mounted vertically on a wall in a laboratory. A fire blanket can help to smother flames that may be on your back or other areas that are difficult to reach. To use a fire blanket, grab the handle and pull away from the wall as you roll your body towards the wall.
Laboratory aprons and coats are a useful layer of protection over your clothing. They serve to minimize exposure to hazardous chemicals that may spill on you. They should be easy to remove. Protective clothing should be made of flame retardant materials that are tightly woven to prevent seepage of liquids onto your clothing.
Safety showers are designed to provide a large volume of water to rinse your entire body quickly in the event of a large spill or splash. They are particularly useful if spills or splashes cause your head and body to be covered in hazardous substances. A safety shower should provide a large volume of water and should rinse you from the top down, just as your shower at home does.
Eye Wash stations provide gently running water that you get close to, so that your eyes can be rinsed with water for 15 minutes. Eye wash stations often resemble your kitchen or bathroom sink.
Cardboard boxes are kept nearby for safe disposal of sharp items, such as broken glass. Trash bags and other soft containers are easily punctured by broken glass or sharp metal points or edges. Be sure that you have a disposable container of cardboard, thick plastic, or other material that is safe from puncture by broken glass.
First aid kits may be actual boxed kits or simply your collection of first aid supplies that are assembled and ready to use in an easily accessible location. You should include band-aids, adhesive tape, headache remedies, sterile eye wash, burn cream for heat burns, and other useful items such as tweezers, sterile gauze, small scissors.
What is Chemistry?
Chemistry is often called the central science because it is the study of all substances and the changes they undergo. Chemistry is going on all around us and in our bodies all the time. The processes that help us breathe, think, laugh, and eat all involve chemistry. The processes that allow us to have automobiles and telephones, space shuttles and televisions, all involve chemical processes. Without chemistry we would not have foods packaged and preserved safely for storage on convenient supermarket shelves or for shipping to locations far from where the food was produced. The wide variety we enjoy in our diets, the safety and convenience of home refrigerators, and many other items are available to us because of our understanding and application of chemistry. We could not exist without chemistry.
So, what does this mean to you when you meet someone who is a chemist? The chemist typically has a bachelor’s degree in chemistry or a closely-related field, or perhaps a Master’s degree or Ph.D. in chemistry or chemical engineering. Sometimes the job title “chemist” does not necessarily mean specialized degrees, but may refer to job functions that include monitoring chemical mixing in an industrial setting.
Jobs for chemists most often include several activities: research, analysis, writing scientific papers and other communications, monitoring a process, or changing a process to make it better. Research can take many forms, but it usually refers to the investigation of new ideas or of applying known ideas in new ways. Analysis often means using some type of instrument to measure the physical and chemical properties of substances. Writing and communicating ideas to other people is an important function of chemists and other scientists that people sometimes overlook. It can be fun and challenging to try to explain a complicated new idea in a way that gets everyone excited about it!
Many occupations require some knowledge of chemistry even though the job title might not be “chemist." Biologists must know the chemical processes at work in cells to understand the functioning of living organisms. A wetlands ecologist is a type of biologist dealing with all types of creatures found in or near bodies of water. The substances in the water and their possible toxic effects on wildlife are of particular concern to the wetlands ecologist. Archeologists must understand how materials are preserved, how to determine the age or materials, and how different materials change over time. Firefighters must know the proper use of fire retardant materials as well as the type of fire they are fighting so that the appropriate extinguishing substance, water or chemical foam, may be used. Engineers must understand the properties of the materials they use to fabricate bridges, roads, buildings, and intricate electronic devices. A hairstylist must understand the properties of hair care products designed to clean, condition, color, and style hair, and all of these substances must be handled appropriately to insure safe and happy customers. A “permanent” that only last a few days isn’t very permanent!
Those are just a few of the many occupations that require a special knowledge of chemistry. For example, it is not obvious to others that chemistry is important for candy manufacturers who must know how to effectively handle sugar solutions of varying concentrations and densities to produce the best treats. A chef must know the importance of proper cooking times and temperatures to insure that raw food is cooked safely. Chemical changes that occur during cooking are important both for proper taste and safe food. Photographers often develop their own photographs, which requires a knowledge of silver bromide and other developing materials. A mortician must understand the properties and usage of embalming fluids. A swimming pool maintenance worker needs knowledge of acids and bases and pH balance. A pilot needs to understand changes in atmospheric pressure. A jeweler must understand properties of crystals and metals to achieve results that are pleasing to the eye, as well as suitable for different applications. An architect must understand the properties of different woods and metals so that they can be used in building applications. Laboratory technicians in many fields (including medical labs) also need knowledge of chemistry, especially acids, bases, and concentrations. So get ready, as we explore these topics and more.
Units of Measure
Units of measure allow us to make sense out of the numerical measurements we encounter. Standard measurements for length, mass, and volume are familiar to us. Both the traditional (English) system and the SI (systeme international) system of measurement have standard units for measuring these quantities. We will deal with the SI system here, which is most commonly used for scientific applications. The advantages of the SI system include the easy division of all quantities by ten or some power of ten (which also makes estimation easier and more practical) and world-wide acceptance of this system as a standard. Even Great Britain, home of the English system, has recently begun converting their national measurement system to the SI system. Only the US still maintains the English system as a national standard.
Name of Standard Units
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Units of Measure |
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Physical Quantity |
Symbol |
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Mass |
kg |
kilogram |
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Length (distance) |
m |
meter |
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Volume |
L |
liter |
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Time |
s (or sec) |
second |
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Electric current |
A |
ampere |
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Temperature |
K |
Kelvin |
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Amount of Substance |
mol |
mole |
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Luminous intensity |
cd |
candela |
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Common Prefixes |
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Prefix |
Symbol |
Explanation |
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kilo- |
103 |
kilometer means 103 meters, which equals 1,000 meters (1 km= 1,000 m) |
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centi- |
10-2 |
centimeter means 10-2 meters, which equals 0.01 meter (1 cm = .01 meter and 100 cm = 1 m) |
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milli- |
10-3 |
millimeter means 10-3 meters (1mm = 0.001 m and 1,000 mm = 1 m) |
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The prefixes are used to indicate a larger or smaller unit than the standard unit. For example, a kilometer is 103 times one meter, meaning that 1 km = 1000 m. A km is a LARGER unit than a meter. Let’s look at an example that indicates a smaller unit. A milliliter, mL, is a very common unit of measure; in fact, you may have noticed that most soda and juice bottles give the volumes in milliliters as well as ounces. A milliliter is 10-3 times one liter, meaning that 1000 milliliters = 1 liter. The liter is the larger unit in this case.
Now let’s consider temperature. We know temperature to be the measure of how cold or hot something is. Temperature scales are typically based on some readily observable property or event. The boiling point (temperature at which a liquid vaporizes to a gas) and melting point (temperature at which a solid becomes a liquid) of water have been used as these reference points. A scale of temperature measurement used primarily in the United States is the Fahrenheit scale The Fahrenheit scale's reference points range from 32 to 212 degrees. In brief, 32ºF is freezing and 212ºF is boiling.
Most measurements worldwide are in Celsius, the centigrade temperature scale, indicated by C. The Celsius system was devised by a an eighteenth century Swedish astronomer named Andres Celsius. The boiling point of water on this scale is 100ºC, and the freezing point of water is 0ºC.
The third temperature scale we will consider is the Kelvin scale. This is called the absolute temperature scale since it is not dependent on the observation of known properties but is instead based on the energy associated with matter on an atomic level. The idea of absolute zero comes from the Kelvin temperature scale (0 K = absolute zero). Notice that there is no degree sign before the K symbol. |
Precision, Accuracy, and Significant Digits
Precision and accuracy are terms that are often used to mean the same thing in everyday conversation, but, in fact, they are not really the same. Precision is the ability of a measuring instrument to give the same measurement over and over again. Precision is often called “repeatability.” Accuracy, on the other hand, refers to the closeness of a measurement to the accepted value. Accuracy refers to correctness of a measurement
Keep in mind that all measurement involves a person reading a scale of some sort. The object being measured may fall exactly on a specific measurement or may fall between specific readings on the scale. The person making the measurement determines exactly what reading is correct and often estimates the measurement. The estimates that are made may be very good, but they always involve some degree of uncertainty. The other factor that contributes to uncertainty in measurement is the physical limitations of the measuring device. No device is absolutely perfect, though some devices may be extremely accurate and precise. Thus, a measurement could be repeatable, but not correct (precise but not accurate). Or a measurement could be correct once, but not repeatable (accurate but not precise); it might be neither of these or both.
It is useful to have a way to indicate how precise and how accurate measured values are for any quantity we measure. We use significant figures to help us do this. Significant figures in a measured number include all the digits known for certain, plus one digit that is uncertain (the estimated digit would be the uncertain digit in a measurement). Even electronic instruments make an estimate in the last digit. The precision of any instrument is ultimately determined by how well the instrument is constructed, as well as how well the instrument is maintained. To determine which digits are significant in a correctly written measurement, use these rules and examples to guide you. Remember that all non-zero digits are significant; that is, they provide information for us to some degree of certainty
For instance, the measurement 5.2 cm has 2 significant digits, while 5.23 cm has 3 significant digits. The digits for which we need rules are the zeros. Zeros are sometimes significant, and sometimes are not. Zeros between two nonzero numbers are significant. In the measurement 3.005, there are 4 significant figures. Zeros at the end of a number are significant if they are to the right of a decimal point. In 7.0, there are 2 significant figures (the zero is significant; it would not be written if it were not). Zeros to the left are not significant, as in 0.045 (there are 2 significant figures here). There is one situation in which it is difficult to decide if a zero is significant; if a zero is to the right of nonzero numbers, but there is no decimal. In 3500, there is nothing to indicate if one or both of the zeros is significant or not; there may be 2, 3 or 4 significant figures. If both are meant to be significant, the number could be written with a decimal at the end (3500.). The decimal would then indicate that both zeros are significant. This is a bit awkward, so we typically use scientific notation to correctly express significant figures for these types of cases: 4.200 x 103 would correctly show 4 significant figures, 4.20 x 103 would show 3, and 4.2 x 103 would clearly indicate only 2 significant figures.
Rules for Calculations Involving Significant Figures:
For addition and subtraction, the answer should have the same number of digits to the right of the decimal place as are found in the number having the fewest digits to the right of the decimal place. For example, when adding 56.01234 g and 4.29 g, your answer will be correctly expressed as 60.30 g.
DISCUSSION
Chemistry affects our lives in so many ways. You have to understand chemistry to really understand how lasers work, why cells produce CO2, or why detergents clean your clothes. What in the world isn't chemistry?
Lab Activity: Massing Small Objects
Mass is measured in the laboratory with a balance. To complete this lab, you will need a balance and mass set. Consult your teacher for this equipment. You may be worried that the balance included in your kit is not as accurate as more modern-looking electronic balances. Actually, it should have the same precision as the electronic balances which are used in most classrooms. Your balance will be precise enough to find the difference in the mass of two quarters.
In order to take advantage of the accuracy of your balance, special procedures must be used. You will use the balance often to determine the mass of chemicals involved in chemical reactions. Learn how to use the balance properly and your laboratory results will be much better (which will bear upon your grade in the class).
Massing an Object
Always use some type of container to hold the chemicals you will be massing. Examples: milk bottle caps, short Styrofoam cups, etc. Remember that the container will have some mass of its own.
Place the container on the balance and tare it.
Place the object to be massed in the container. Wait until the numbers do not change anymore.
Record the mass in your notebook.
When writing the mass of the object, always include the units as part of the answer.
