Laboratory Resource Packet 2013/2014
TO THE STUDENT
Chemistry is exciting! Each day in the laboratory you are given the opportunity to confront the unknown, and to understand it. Each experiment holds many secrets. Look hard and you may see them. Work hard and you can solve them. The word science comes from the Latin word scire, which means “to know.” The goal of all science is knowledge. Scientists are men and women who devote their lives to the pursuit of knowledge.
In this class, you are given the opportunity to do what scientists do. You can wonder how things work, ask why and how, and then think of ways to answer your own questions. You are given the chance to understand what is unknown to you and to many other people. It is a great opportunity. Do not waste it by being lazy or careless. Work hard. Master the scientist’s skills of observation and experiment. These skills are tools to solve the secrets of the unknown.
Chemistry is a laboratory science. As part of your laboratory experience you will handle many chemical substances and manipulate specialized laboratory equipment. Many of these substances pose a health risk if handled improperly, while some of the laboratory equipment can cause severe injury if used improperly. This section is a guide to the safe laboratory practices you will use throughout this course.
Preparation and Safety
To get the most out of your laboratory experience, you must be well prepared for each experiment. This means that you must read the experiment thoroughly before coming to the laboratory. Make sure you have a clear idea of what the experiment is about. Be sure that you understand each step of the procedure. If you are unsure of any part of the experiment, ask your teacher for help before the laboratory begins.
Preparation is important not only to understanding, but also to safety. If you are well prepared for the laboratory, it is much less likely that an accident will occur. In the laboratory, you are responsible not only for your safety, but also for the safety of your classmates. If an accident happens because you are not prepared, it can also affect your friends. This is all the more reason for you to take the time and make the effort to prepare for the laboratory.
Be sure to note the safety warnings listed in the Safety section of each experiment. Note that these warnings are emphasized by symbols appearing in the margins. The symbols mark those parts of the procedure that may be hazardous. In addition, be sure to observe the general safety precautions described in the safety section at the beginning of the manual. Finally, remember the most important safety advice of all: Always wear safety goggles in the chemistry laboratory!
You should be aware of possible hazards in the laboratory and take the appropriate safety precautions. By doing so, you can minimize the risks of doing chemistry. This safety section is intended to acquaint you with the hazards that exist in the laboratory and to indicate how you can avoid these hazards. In addition, information is provided on what to do if an accident should occur.
A thermal burn can occur if you touch hot equipment or come too close to an open flame. You can prevent thermal burns by being aware that hot and cold equipment look the same. If a gas burner or hot plate has been used, some of the equipment nearby may be hot. Hold your hand near an item to feel for heat before touching it.
Treat a thermal burn by immediately running cold water over the burned area. Continue applying the cold water until the pain is reduced. This usually takes several minutes. In addition to reducing pain, cooling the burned area also serves to speed the healing process. Greases and oils should not be used to treat burns because they tend to trap heat. Medical assistance should be sought for any serious burn. Notify your teacher immediately if you are burned.
A chemical burn occurs when the skin or a mucous membrane is damaged by contact with a substance. The Materials section of each exercise indicates which substances can cause chemical burns. c stands for corrosive. It indicates that the chemical can cause severe burns. I stands for irritant. It indicates that the chemical can irritate the skin and the membranes of the eye, nose, throat, and lungs. Chemicals that are marked corrosive should be treated with special care. Chemical burns can be severe. Permanent damage to mucous membranes can occur despite the best efforts to rinse a chemical from the affected area.
The best defense against chemical burns is prevention. Without exception, wear safety goggles during all phases of the laboratory period— even during cleanup. Should any chemical splash in your eye, immediately use a continuous flow of running water to flush your eye for a period of 20 minutes. Call for help. If you wear contact lenses, remove them immediately. This is especially crucial if the chemical involved is an acid or base. It can concentrate under the lens and cause extensive damage. Wear a laboratory apron and close-toed shoes (no sandals) to protect other areas of your body. If corrosive chemicals should contact your exposed skin, wash the affected area with water for several minutes.
An additional burn hazard exists when concentrated acids or bases are mixed with water. The heat released in mixing these chemicals with water can cause the mixture to boil, spattering corrosive chemical. The heat can also cause non-Pyrex containers to break, spilling corrosive chemical.
To avoid these hazards, follow these instructions: Always add acid or base to water, very slowly while stirring; never the reverse. One way to remember this critical advice is to think of the phrase “Pouring acid into water is doing what you ought-er.”
Estimates for the time required for permanent corneal damage to occur following exposure to 1M NaOH are in the range of 30 seconds. Concentrated sulfuric acid causes thermal burns because it reacts with water in the skin, releasing substantial amounts of heat. Nitric acid does not produce thermal burns, but reacts with the proteins in the skin, destroying tissue. Nitric acid burns are very slow to heal.
Cuts from Glass
Cuts occur most often when thermometers or pieces of glass tubing are forced into rubber stoppers. Prevent cuts by using the correct technique for this procedure. The hole should be lubricated with glycerol or water to facilitate the movement of the glass tubing. The glass should not be gripped directly with the hands, but rather by means of cloth towels. The towels will protect your hands if the glass should break. Use a gentle twisting motion to move the tube smoothly into the stopper. Avoid cuts from other sources by discarding chipped and cracked glassware according to your teacher’s instructions. If you should receive a minor cut, allow it to bleed for a short time. Wash the injured area under cold running water, and notify your teacher. Serious cuts and deep puncture wounds require immediate medical help. Notify your teacher immediately. While waiting for assistance, control the bleeding by applying pressure with the fingertips or by firmly pressing with a clean towel or sterile gauze.
A fire may occur if chemicals are mixed improperly or if flammable materials come too close to a burner flame or hot plate. When using lab equipment, prevent fires by tying back long hair and loose-fitting clothing. Do not use a burner when flammable chemicals are present. Flammable chemicals are designated with the symbol f in the Materials section for each exercise. Use a hot plate as a heat source instead of a burner when flammable chemicals are present. If hair or clothing should catch fire, do not run, because running fans a fire. Drop to the floor and roll slowly to smother the flames. Shout for help. If another person is the victim, get a fire blanket to smother the flames. If a shower is nearby, help the victim to use it. In case of a fire on a laboratory bench, turn off all accessible gas outlets and unplug all accessible appliances. A fire in a container may be put out by covering the container with a nonflammable object. It could also be smothered by covering the burning object with a damp cloth. If not, call for a fire extinguisher. Spray the base of the fire with foam from the extinguisher. CAUTION: Never direct the jet of a fire extinguisher into a person’s face. Use a fire blanket instead. If a fire is not extinguished quickly, leave the laboratory. Crawl to the door if necessary to avoid the
smoke. Do not return to the laboratory.
Many of the chemicals used in this manual are toxic. Toxic chemicals are identified in the Materials sections with the symbol T. You should do several things to prevent poisoning. Never eat, chew gum, or drink in the laboratory. Do not touch chemicals. Clean up spills.
Keep your hands away from your face. In this way you will prevent chemicals from reaching your hands, mouth, nose, or eyes. In some cases, the detection of an odor is used to indicate that a chemical reaction has taken place. It is important to note, however, that many gases are toxic when inhaled. If you must detect an odor, use your hand to waft some of the gas toward your nose. Sniff the gas instead of taking a deep breath. This will minimize the amount of gas sampled. Glycerol should not be used in the presence of powerful oxidizers such as sulfuric acid, nitric acid, dichromates, or permanganates.
Carbon dioxide fire extinguishers have the advantage of being very effective in fires involving flammable liquids or electricity. However, carbon dioxide extinguishers are not useful in fires involving wood or embers. Carbon dioxide does have the advantage of not leaving any residue should discharge occur. ABC fire extinguishers discharge a powder which is effective on all fires except those that involve metals. ABC extinguishers do leave a residue, which must be removed. Aim the discharge at the base of the fire. Post the telephone number of your local Poison Control Center in the classroom, or have the number readily available.
Working in the chemistry laboratory, you will be handling potentially dangerous substances and performing unfamiliar tasks. This section provides you with a guide to the safe laboratory techniques needed in
this course. While performing experiments throughout the year, refer back to this section any time you are unsure of proper laboratory techniques.
• Always read the label on a reagent bottle before using its contents.
• Always wear safety goggles when handling chemicals.
• Never touch chemicals with your hands.
• Never return unused chemicals to their original containers. To avoid waste, do not take excessive amounts of reagents.
1. Use the back of your fingers to remove the stopper from a reagent bottle. Hold the stopper between your fingers until the transfer of liquid is complete. Do not place the stopper on your workbench.
2. Grasp the container from which you are pouring with the palm of your hand covering the label.
3a. When you are transferring a liquid to a test tube or measuring cylinder, the container should be held at eye level. Pour the liquid slowly, until the correct volume has been transferred.
3b. When you are pouring a liquid from a reagent bottle into a beaker, the reagent should be poured slowly down a glass stirring rod. When you are transferring a liquid from one beaker to another, you can hold the stirring rod and beaker in one hand.
Filtering a Mixture
Sometimes it is necessary to separate a solid from a liquid. The most common method of separating such a mixture is filtration.
1. Fold a filter paper circle in half and then quarters. Open the folded paper to form a cone, with one thickness of paper on one side and three thicknesses on the other.
2. Put the paper cone in a filter funnel. Place the funnel in an iron ring clamped to a ring stand. Moisten the filter paper with a small volume of distilled water, and gently press the paper against the sides of the funnel to achieve a good fit. (If the correct size of filter paper has been used, the top edge of the cone will be just below the rim of the filter funnel.)
3. Place a beaker beneath the funnel to collect the filtrate. The tip of the funnel should touch the inside surface of the beaker and extend about one inch below the rim. Guide flow of liquid with a glass rod Mixture being filtered Filtrate Solid collects on filter paper Stem touches side of beaker.
4. Decant the liquid from the solid by pouring it down a glass stirring rod into the funnel. Be careful to keep the liquid below the top edge of the cone of filter paper at all times; the liquid must not overflow. Finally, use a jet of distilled water from a wash bottle to wash the solid into the filter.
5. When the filtration is complete, wash the solid residue on the filter paper with distilled water to remove traces of solvent. Dry the solid.
6. If the filtrate contains a dissolved salt, it may be recovered by evaporation if desired.
Using a Gas Burner
Laboratory gas burners produce various kinds of flames when different mixtures of gas and air are burned. The two most common models are the Bunsen burner and the Tirrell burner. Both have adjustable air vents;
the Tirrell burner has a gas control valve in its base.
1. Examine your laboratory burner. Determine which model you have.
2. Connect the burner to the gas supply with rubber tubing.
3. Close the air vents. If your model is a Tirrell burner, also close the gas control valve at the base of the burner.
4. Hold a lighted match at the top of the burner tube and turn on the gas supply. Do this by opening the main gas supply valve located on top of the nozzle to which you attached the rubber tubing. (If your model is a Tirrell burner, first open the main gas supply valve, then open the gas control valve at the base approximately onehalf- turn.) You should get a yellow, or luminous, flame. When a Tirrell burner is used, the main gas supply valve should be opened fully and the gas flow regulated by the gas control valve. Gas supply to a Bunsen burner is controlled by the main gas valve.
5. Open the air vents slowly, to admit more air into the flame, to produce a light blue (nonluminous) cone-shaped flame. If the flame “blows out” after lighting, the gas supply should be reduced.
6. Adjust the air vents and gas supply to produce the desired size of flame. For most laboratory work, the blue inner cone of the flame should be about 1 inch high and free of yellow color. If you want a smaller flame, close the air vent slightly and reduce the gas supply. You will learn how to control the burner flame by trial and error.
7. Turn the burner off at the main gas supply valve when done. CAUTION: Confine long hair and loose clothing when using a gas burner. Do not reach over a burner. Ensure that flammables are not being used when a burner is lit. Never leave a lit burner unattended. Know the location of fire extinguishers, the fire blanket, and safety
Heating a Liquid in a Test Tube
The correct procedure for heating liquids in the laboratory is important to laboratory safety.
1. Adjust your gas burner to produce a gentle blue flame.
2. Fill a test tube one-third full with the liquid to be heated.
3. Grasp the test tube with a test-tube holder, near the upper end of the tube.
4. Hold the test tube in a slanting position in the flame, and gently heat the tube a short distance below the surface of the liquid.
5. Shake the tube gently as it is being heated, until the liquid boils or reaches the desired temperature.
CAUTION: Never point the open end of a test tube you are heating either toward yourself or anyone working nearby. Never heat the bottom of the test tube.
Heating a Liquid in a Beaker
Many laboratory experiments require the use of a hot water or boiling water bath. This procedure describes how to assemble a water bath.
1. Fasten an iron ring securely to a ring stand so that it is 2–4 cm above the top of a gas burner placed on the ring stand base.
2. Place a 250-mL beaker one-half-filled with water on a wire gauze resting on the iron ring.
3. Light your gas burner and adjust it to produce a hot flame.
4. Place the burner beneath the wire gauze. For a slower rate of heating, reduce the intensity of the burner flame.
CAUTION: Never heat plastic beakers or graduated glassware in a burner flame. Never let a boiling water bath boil dry; add water to it as necessary.
Inserting Glass Tubing
In many experimental procedures, you are required to insert a thermometer or a length of glass tubing into a hole in a rubber stopper. It is essential that you know the correct way to do this. Otherwise, serious injury may result.
1. Lubricate the end of the glass tubing with a few drops of water, washing-up liquid, glycerol, or vegetable oil.
2. Hold the glass tubing close to where it enters the hole in the rubber stopper. Protect your hands with work gloves or pieces of cloth.
3. Ease the tubing into the hole with a gentle twisting motion. Push the tubing through the hole as far as is required. Do not use force!
4. Wipe excess lubricating material from the tubing before continuing with the experiment.
5. If the glass tubing is to be removed from the stopper, it should be done immediately after the experiment is completed.
CAUTION: The end of the glass tubing should be fire-polished or smoothed with emery cloth before being inserted into a rubber stopper. Do not try to bend the glass tubing—it will break. Ensure that the palm of the hand holding the rubber stopper is not in line with the emerging glass tube.
In many experiments you are required to determine the mass of a chemical used or produced in a reaction. An object’s mass is determined by measuring it on a balance. When you determine the mass of an object, you are comparing its mass with a known mass. In the SI, the base unit of mass is the kilogram.
• Check the balance before you start. The balance pan should be empty and clean, and all masses (or dials) should be set on zero. The balance must be level. Check the bubble level on the base. See your
teacher if you need assistance with checking your balance.
• Objects to be placed directly on the balance pan must be clean, dry, and at room temperature. Solid chemicals and liquids must never be put directly on the balance pan. Liquid samples should be placed in beakers or sealed containers. Solid chemicals can be conveniently placed in beakers, disposable plastic weighing boats, or on 10-cm squares made of glossy paper.
• The balance is a precision instrument that must be handled with care. To avoid damaging it, always be sure that the balance is in an arrested position when objects are placed on or removed from the pan. Always turn all dials slowly.
• Never move or jar either a balance or the balance table.
• If you spill a chemical on or near the balance, clean it up immediately. If in doubt, inform your teacher. A camel-hair brush is usually provided to wipe minute traces of solid from the balance pan before you use it.
• Never attempt to measure an object with a mass greater than the maximum capacity of the balance.
• When you are done, return all the masses to zero, and make sure the balance pan is clean.
Do not attempt to use a balance until your teacher has demonstrated the proper technique.
Volume measurements are important in many experimental procedures. Sometimes volume measurements must be accurate; other times they can be approximate. Most volume measures in the laboratory are made using equipment calibrated in milliliters. Although some beakers have graduation marks, these marks are designed only for quick, rough estimates of volume. Accurate volumes must be measured with pipets, burets, or volumetric flasks.
Using a Graduated Cylinder
Half-fill a 100-mL graduated cylinder with water, and set the cylinder on your laboratory bench. Examine the surface of the water. Notice how the surface curves upward where the water contacts the cylinder walls. This
curved surface is called a meniscus.
A volume measurement is always read at the bottom of the meniscus, with your eye at the same level as the liquid surface. To make the meniscus more visible, you can place your finger or a dark piece of paper behind and just below the meniscus while making the reading.
Graduated cylinders are available in many capacities. The 100-mL cylinder is marked in 1-mL divisions, and volumes can be estimated to the nearest 0.1 mL. The last digit in these measurements is therefore significant
Using a Pipet
A pipet is used to accurately measure and deliver volumes of liquids. Two types are in common use: volumetric pipets and graduated, or measuring, pipets. The use of a volumetric pipet will be described. A volumetric pipet has a single calibration mark and delivers the volume printed on the bulb of the pipet at the temperature specified. (A graduated pipet has calibrations along the length of the pipet.) Volumes can be measured more accurately with a volumetric pipet than with a graduated pipet.
1. Place the tip of the pipet below the surface of the liquid to be dispensed.
2. Compress a pipet bulb and press the hole in the bulb against the upper end of the pipet. CAUTION: Never fill a pipet by applying suction with your mouth. Never push the pipet bulb over the end of the pipet.
3. Slowly release pressure on the bulb so that liquid is drawn into the pipet to a level about 2 cm above the calibration mark.
4. Remove the bulb and simultaneously place your index finger over the end of the pipet. If you are right-handed, you should hold the pipet in your right hand and the pipet bulb in your left.
5. Keep your index finger pressed firmly against the end. Withdraw the pipet from the liquid, and carefully wipe the outside of the stem with a paper towel.
6. Slowly reduce the pressure on your finger to allow the excess liquid to drain into a waste receiver, until the bottom of the meniscus is at the calibration mark.
7. Now, deliver the remaining liquid in the pipet into the designated receiver. When releasing liquid from a volumetric pipet, let it drain completely. Wait 20 seconds, then touch the pipet tip to the side of the flask or surface of the liquid. This action will remove some, but not all, of the liquid in the tip. The pipet delivers the stated volume when this procedure is followed. A small amount of liquid remains in the tip. Do not blow this out into your receiver.
Cutting and Fire Polishing
1. Place the glass tubing or glass rod on a flat surface (such as the laboratory bench).
2. Hold the glass tightly with one hand close to the area to be cut.
3. Using a firm stroke, make a single deep scratch with a triangular file.
CAUTION: Do not use a sawing motion or repeated scratching.
4. Grasp the glass in both hands with the scratch facing away from you and both thumbs directly behind the scratch.
5. Push firmly with the thumbs and pull with your fingers. The glass should snap with a clean break.
CAUTION: Be careful with the cut ends of the glass. They may be sharp and jagged. Do not attempt to break glass tubing having an outside diameter greater than 6 mm.
6. The cut ends of the glass tubing should be fire-polished to make the tubing safe to handle. Rotate one end of the glass tube in the hottest part of a burner flame, until the sharp edges have softened and become rounded.
CAUTION: Do not hold the tubing in the flame too long. If you do, the hole in the tube will close.
7. Place the hot glass on a wire gauze square to cool.
CAUTION: Hot glass and cold glass look alike. Make sure one end of a piece of glass has cooled before you attempt to fire-polish the other end.
Bending Glass Tubing
1. Put a wing top or flame spreader on your gas burner.
2. Light the burner and adjust the flame to produce an even blue (hot) flame across the wing top.
3. Grasp a length of glass tubing that has been fire-polished at both ends. Hold the center of it lengthwise in the flame, just at the top of the blue region. This is the hottest part of the flame.
4. Rotate the tubing in the flame to heat approximately a 5-cm section uniformly, until it becomes soft and just begins to sag.
5. Remove the tubing from the flame and bend it to the desired shape in one movement.
6. When it has hardened, put the glass tubing on a wire gauze to cool.
CAUTION: Hot and cold glass look alike.