Science Lab Techniques

Measuring the volume of a liquid with a graduated cylindar:

The surface of a liquid confined in a cylindar curves to form what is known as a meniscus. The meniscus of most liquids curves up the sides of the container, making the center of the curve appear lower than the edges. Mercury is one of very few exceptions - it curves down at the edges.link to a local picture Since reading the meniscus at the top or at the bottom of the curve will make a difference in the volume measured, it is generally agreed to always read the bottom of the curve. The smaller the container, the greater the curve of the meniscus. Pictured below is the meniscus in a 10 ml graduated cylindar. To gain experience in reading liquid volumes, click on the picture to enlarge it and read the volume in the following way: the meniscus in a graduated cylindar

  1. The largest graduations on this graduated cylindar are numbered, representing milliliters. What whole number of milliliters are represented in the picture? answer
  2. There are five graduations from one major line to the next in this picture. In other words, each milliliter is divided into tenths and each small graduation represents two tenths. What volume is represented in the picture, to the nearest tenth of a milliliter? answer
  3. Both whole milliliters and tenths of milliliters can be read from the graduations in the picture. If divisions between the tenths graduations are estimated, the volume can be read to hundredths of a milliliter. What is the volume of liquid in the picture to the nearest hundredth of a milliliter? answer
As the diameter of the cylindar increases, the curve of the meniscus flattens out. See a picture of the meniscus in a 100 ml graduated cylindar.link to a local picture While the curve is not as pronounced, because of its thickness, we must still read the bottom of the meniscus. What volume of liquid is represented in the picture, to three significant digits? answer

You can see that the lines drawn on the answer pictures help identify the location of the bottom of the meniscus. If reading volumes in a cylindar is going to be regularly done, making a buret card link to a local picture might be worthwhile. This is a small index card with a very thick horizontal line drawn on it with a magic marker. By holding the card behind the cylindar, and immediately below the bottom of the meniscus, the volume can be easily read.

Measuring the volume of a liquid with a pipet:

Pipets are much more accurate than graduated cylindars. Reading the volume of liquid in a pipet is just like reading a graduated cylindar, however there is one additional technique needed with a pipet. The diameter does not allow a liquid to be poured into a pipet - the liquid must be drawn into the pipet. regular bulb on top and safety bulb on bottom This picture shows two of several types of pipet bulbs used to draw a liquid into a pipet. The red bulb is known as a standard pipet bulb. The black bulb is known as a safety pipet bulb. Much of the "professional" lab work today is done with automatic pipets. These are expensive little gadgets that come in many different volumes, each delivering exactly its assigned volume with one click of a button. High school chemistry students need to know the basic techniques of pipetting, so you will be using glass pipets and the standard pipet bulb.

The standard pipet bulb requires manual dexterity that is improved by repetitive use. You may have to practice using the bulb with a pipet before you are able to accurately transfer a measured volume of liquid. The technique goes something like this:

Measuring mass with an electronic balance:

The electronic balance has many advantages over other types of balance. The most obvious is the ease with which a measurement is obtained. All that is needed is to place an object on the balance pan and the measurement can be read on the display to hundredths of a gram. an electronic balance A second advantage, using the ReZero button on the front of the balance, is less recognized by beginning science students. Because one must never place a chemical directly on the balance pan, some container must be used. Place the container on the balance and the mass of the container will be displayed. By pressing the ReZero button at this point, the balance will reset to zero and ignore the mass of the container. You may now place the substance to be weighed into the container and the balance will show only the mass of the substance. This saves calculation time and effort. However, when the container is removed from the balance, the display will go into negative numbers until the ReZero button is pressed again.

Our electronic balances also have a Mode button on the front. Pressing this button will change the units being measured. Since we have very few times when we need something other than metric units, you should not have to change the mode on the balance. Because of the Mode and ReZero buttons, there are two things you must always do before placing objects onto an electric balance to be measured:

  1. See that the display is reading 0.00
  2. See that the unit sign in the upper right of the display shows g

Measuring mass with a triple-beam balance:

The triple-beam balance was once the "standard" balance in the general chemistry lab. While the electronic balance has replaced it in many cases, good science students should be familiar with the triple-beam balance and how to read one. a triple-beam balance The balance is named for its three "beams". An object is placed on the pan of the balance and tares on the beams are moved to balance the mass. As you face the balance, the back beam is graduated in 10 gram steps and the middle beam is graduated in 100 gram steps. It is very important that the tares on these two beams are in the notch for the whole number of grams and not in between notches. The front beam is a sliding scale graduated in grams. The tare on this beam can be positioned anywhere on the scale. Masses on a triple-beam balance can be read to tenths of a gram, and estimated to hundredths. Clicking on the balance picture will give you a close view of the position of the tares on the three beams. What mass is represented, to five significant digits? answer

Using a laboratory burner:

Many chemistry experiments require something to be heated. This is done with one of several types of laboratory burners. The lab burners at DVHS use propane gas delivered through the gas outlets at student lab stations. a proper lab burner flame Before attempting to light any lab burner, check to see that the jet hole between the base and the burner tube is free of obstruction. If chemicals have covered this jet, the burner will not operate properly. After attaching the hose to the gas outlet, turn the handle on the outlet parallel to the nozzle to open the gas valve. The gas valve is turned off by turning the handle 90 degrees in either direction. Carefully check to see that you hear gas escaping from the mouth of the burner tube. When you are sure that you have gas, bring the head of the striker over the burner and squeeze the striker handle. The spark produced will ignite the gas and your burner is lit. Adjust the air control vent so that the flame has the proper color pictured here. A yellow flame is an indication of a lack of oxygen, meaning that the air vent needs to be opened. The hottest part of the burner flame is just at the top of the bright blue inner cone. Normal heating is done with an object at the top of the light blue outer cone, while strong heating is done with an object at the top of the bright blue inner cone. To heat a container gently, move the container back and forth through the outer cone.

Filtering a precipitate from a solution:

Filtering a solid out of a liquid is done using filter paper and a filter funnel. The filter funnel is supported by the ring on a ring stand. Lay a clay triangle across the ring, then place the filter funnel into the triangle.

filter paper To prepare the filter paper, fold the paper in half link to a local picture, then fold it in half again.link to a local picture When you look at the open edge of the folded paper you will see four edges of paper. With thumb and finger, catch three of these edges. Squeeze the sides of the folded paper and a cone will form with three thicknesses of paper on one side and one thickness of paper on the other. Place this cone of paper into the filter funnel. Place a "catch container" under the stem of the filter funnel and adjust the height of the ring on the ring stand until the tip of the stem is below the mouth of the container.

Use a wash bottle and wet down the inside of the filter paper. This will help it stick to the funnel. You are now ready to filter. Carefully pour the liquid to be filtered into the mouth of the funnel. Do not let the liquid rise to the top of the filter paper. If any liquid goes over and around the paper, your procedure is ruined. Be patient, it will take time for the liquid to move through the pores of the paper. When all the original liquid has been poured into the funnel, use the wash bottle to rinse any remaining precipitate out of the original container. Do not touch or try to stir the liquid inside the filter paper. The wet paper is easily torn, which will ruin your procedure.

If the objective of your filtration is the solid-free liquid, throw the filter paper and its contents into the trash. If your objective is the solid, carefully remove the filter paper and set it in a secure place to dry.

Titration is a neutralization reaction between acid and base:

two burets supported with a double buret clamp The classic lab apparatus for a titration uses a set of double burets - pictured here. One buret holds the acid, the other the base. The common procedure is to place a measured volume of the acid into a flask, then add two drops of phenolphthalein indicator. This indicator is colorless in an acid, but turns dark pink in a base. Small amounts of the base are added to the flask as it is being slowly twirled to mix the solution. At the end-point, or "neutralization", the phenolphthalein will be barely pink when viewed over a white background. For more about titration, go to this webpage.link to a local webpage

Using litmus paper to determine acid or base:

red and blue litmus paper An acid turns blue litmus paper red and a base turns red litmus paper blue. While even grade-school students know this, there is one mistake commonly made when using either litmus or pH paper strips. You should never dip the test paper into the solution being tested. While the degree to which this contaminates the solution is not great, good chemistry students know not to do it. Always use a glass stirring rod. Dip a clean stirring rod into the solution, then touch the wet stirring rod to the paper.

Using a magnetic stirrer:

A magnetic stirrer is helpful for dissolving solids in liquids. While there are several different styles, all will at least have a base with a speed-controlled spinning magnet inside and an external stirring bar. The stirring bar is placed into a flask or beaker by gently sliding it along the wall of the container. a magnetic stirrer To prevent breakage, do not drop the bar onto the bottom of the container. Place the container on the stirrer base and turn the speed control knob to its lowest setting. Use just enough speed to start the bar turning in the container. The picture here shows the "vortex" that forms inside the continer. Be patient, the bar might "hop" at excess speeds, causing splashing.

If using exact volumes, as with a volumetric flask, be sure take the measurements before adding the stirring bar.

When stirring is completed, keep the stirring bar in the container by "decanting" the liquid into another container. Be sure to carefully wash the original container and the stirring bar.

Distillation: one type of distillation apparatus

In general chemistry, distillation can be used to remove dissolved substances from a liquid or to separate a mixture of liquids that have different boiling points.

To successful separate a mixture of liquids, you must know the boiling point of each liquid involved and be able to measure temperature changes as heat is applied. Heat is added to the mixture until it reaches the first boiling point. The temperature must be kept at this temperature until all the first liquid is removed. The temperature is then allowed to rise to the next boiling point and the second liquid removed. The closer these boiling points are together, the more difficult it is collect pure liquids from the mixture.

One distillation apparatus is pictured at right. The picture here shows an apparatus that can be used. No matter how the apparatus is set up, the following things must be accomplished:

  • The original liquid is heated.
  • The temperature is measured.
  • The vapor is collected and condensed back into a liquid.
  • The new liquid is collected.

Generating and collecting gases:

gas generating bottle A gas generating bottle is used to produce gases from a chemical reaction. The solid is placed into the bottle, then a solution is poured down the funnel. As the gas builds, it will move into the collecting tube at the top of the bottle. The picture here shows the collection of a gas that is heavier than air. This is indicated by the gas going "down" into the upright collecting container. If the gas were lighter than air, it would not go down into the container. Gases that are lighter than air must be collected by water displacement.

For water displacement, the container is filled with water then inverted into a water trough. The gas is bubbled up under the container and pushes the water out. This is a very common procedure in general chemistry labs and has only one draw-back. If the gas being generated is highly soluble in water, precise measurements of the gas produced will not be possible.

Fume Hood: fume hood

The fume hood is a safety glass-front cabinet with an exhaust fan. It is used for experiments known to produce noxious fumes or smoke. Do the following to perform an experiment in the hood:

  1. Raise the door of the hood
  2. Turn on the light and set up the apparatus
  3. When all material for the experiment is ready, turn on the fan
  4. Pull the hood door at least 1/3 way down
  5. Perform the experiment
  6. When finished, pull the door all the way down until all smoke and fumes are removed
  7. Turn off the fan and light, then remove the equipment to a regular lab station for cleaning
  8. Leave the hood clean

Hotwater Bath:

A hotwater bath is used in the general chemistry lab to heat something slowly and evenly. Many different types of glassware can be used to set up a hotwater bath. What glassware you use will depend on the quantity of the substance you wish to heat. All hotwater baths put the substance to be heated in a container then place that container into a container of water. Heat is then applied to the water container. The substance is heated by the water, not the burner flame.

In addition to heating slowly and evenly, water baths are also safer than direct heat. For this reason, highly volatile substances should only be heated using hotwater baths.

Calorimeter:

coffee-cup calorimeter The specific heats of metals can be determined by calorimetry. Calorimetry involves heating the metal to a known temperature, placing it into a measured amount of cold water in an insulated container called a calorimeter and measuring the resultant rise in temperature of the water in the calorimeter. The temperature of the water in the calorimeter will rise rapidly as heat is transferred from the hot metal to the cold water. After the water reaches a maximum temperature, it will slowly decrease.

While there are very expensive calorimeters that will absorb no heat from the reaction inside, the heat absorbed by the "coffee-cup" calorimeter shown here is negligible within the range of our measuring instruments.

An equation used in calorimetry calculations:

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