Chem Mystery

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Unit Objectives

  1. KNOW and EMPLOY safe laboratory practices.
  2. DISTINGUISH between accuracy and precision.
  3. Accurately DETERMINE physical measurements with the correct number of significant digits.
  4. APPLY the concepts of area, volume, mass, temperature and density.
  5. In a laboratory setting, CALCULATE the density of solid objects.
  6. CALCULATE the percentage error from given data.
  7. CONVERT metric units of mass, volume and length.
  8. INFER from demonstrations the Kinetic Theory of matter.
  9. IDENTIFY examples of Brownian motion and diffusion.
  10. IDENTIFY the major parts of scientific method: stating a problem, obtaining information, forming a hypothesis, conducting an experiment and stating conclusions from the results.
  11. APPLY knowledge of experimental design by correctly IDENTIFYING and DIAGRAMING the independent variable(s), dependent variable(s), constant(s), control group and the number of repeated trials.
  12. DESIGN, CONDUCT and EVALUATE a scientific investigation.

 

Measurement Again

Accuracy is how close a measurement comes to the actual dimension or true value of whatever is measured. Precision is concerned with the reproducibility of the measurement. To illustrate this we can use darts and a dartboard.

 

Let the bull's-eye of the dartboard represent the true value or accepted value of what we are measuring. The closer we are to the bull's-eye the more accurate our measurement. Let's look at each board separately.

  1. Has all the darts stuck in the bull's-eye. Each dart in the bull's-eye represents an accurate measurement of the true value. Thus, our measurements are very accurate. All of the darts are also closely grouped together, therefore, our measurements are very reproducible and very precise.
  2. All the darts are closely grouped, but they are not in the bull's-eye. Our precision is good because we could reproduce the same measurement. We have missed the true value (bull's-eye) thus, we are very inaccurate.
  3. The darts are spread randomly around the target. Our measurements are neither accurate nor precise. We have missed the true value and failed to reproduce our measurements.

 

In science we report measurements in significant figures. The significant figures in a measurement include all the digits that can be known accurately plus a last digit that must be estimated. When taking a measurement the number of digits that our known depends on the accuracy of the instrument. For example, if you were taking a temperature with a thermometer, which is marked in increments of 1° , the accurately known digits would be in 1° increments. You can then estimate to one more digit, which would be to the nearest tenth of a degree. So a measurement with this thermometer could not be more accurate than one decimal place (i.e. 38.3° ). Practice with the following.

 

 

 

 

 

 

 

 

 

 

 

When calculations are done with scientific measurements, we sometimes end up with an answer that has more digits than can be justified as significant. An answer cannot be more precise than the least precise measurement and must be rounded. Rounding must be done by deciding how many significant digits our answer should have, and then look at the digit immediately following the last significant digit. If this digit is less than 5 round down and if the digit is 5 or greater round up.

When doing scientific experiments there is bound to be some error in our measurements. These errors have many sources such as human error, instrumentation, spills, and many others that you will soon realize. Because of the possibility of errors scientist use a specific formula to calculate their experimental error. The experimental error is calculated by subtracting the observed value (your measurement) from the accepted value (usually from a standard or book). Another calculation can be done to determine a percentage error. This is done to compare the experimental error to the accepted value expressed as a percentage. This can be expressed as a positive percentage (the experimental result is too large) or as a negative percentage (the experimental result is too small). The formula for this is as follows:

 

% Error = Observed- Accepted X 100

Accepted

 

 

UNITS -- The Language Of Science

 

A measurement depends on a standard for reference. The standards of measurement used by scientists are those of the metric system. The metric system was originally established in France in 1790. The International System of Units (SI) is a revised version of the metric system. It is possible to report all measured quantities in SI units, however, non-SI units are preferred for convenience or practical reasons.

 

When a measurement is made, the numerical value must be assigned the correct units. Without these units it would be impossible to communicate the meaning of a measurement. It would be somewhat confusing if you were given the instructions to heat the solution for 20. Does this mean 20 minutes, seconds, hours, or days? To avoid such confusion it is required that all our measurements include units.

In the metric system the basic unit of length, or linear measure, is the meter (m). There are 100 centimeters (cm) in a meter. Area is the measurement of the number of squares (i.e. cm2) that fit into a two-dimensional object. Volume is a measurement of the number of cubes (i.e. cm3) that fit into a three-dimensional object. The liter (L) is the SI measurement of volume. There are 1,000 millimeters (mL) in a liter. A cm3 is the same as a mL.

Matter is anything that occupies space. Mass is the quantity of matter an object contains. Mass is not weight. Weight is a force. The SI unit of mass is the kilogram (Kg) which is defined as the mass of 1,000 cm3 (mL) of water at 4° C. There are 1000 grams (g) in one kilogram.

The statement "Lead is heavier than wood" has no precise meaning. Most people take it to mean tat if pieces of lead and wood of the same volume are weighed, the lead has a greater mass than the wood. It would take a much larger volume of wood to equal the mass of the lead. What all this brings us to is the idea that there is an important relationship between an object’s mass and its volume. This relationship is called density. Density is the ratio of the mass of an object to its volume. Density measures the amount of matter in a given unit of volume. The units for density are g/mL or g/cm3. Density is defined as mass/volume. Water has a density of 1 g/mL. If an object is more dense than water, it will sink. A sunken object displaces its volume in water. If an object is less dense than water it will float, a floating object will displace its mass in water.

Other Units of measurements in the metric system are as follows:

Temperature = Celsius

Heat = calorie or joule.

Make-up questions

Kinetic Theory of Matter

All matter is made up of extremely small particles. In between these particles is empty space. The word kinetic means motion. The kinetic theory says that the tiny particles in all forms of matter are in constant random motion. These particles may be atoms, ions, or molecules of gases, liquids, or solids. Motion suggests energy. The energy an object has because of its motion is kinetic energy. In a collection of molecules there will be a wide range of kinetic energies from very low to very high. Most molecules will have speeds in between. When discussing the kinetic energy of a substance, scientists use the average kinetic energy of the particles. The average kinetic energy of the particles of a substance is proportional to the temperature of the substance. We can measure the average kinetic energy of the particles in a substance by measuring the temperature. The units for temperature are oF, oC, oK. By increasing the temperature of a substance the average kinetic energy is also being increased. The temperature at which the motion of particles ceases is known as absolute zero.

Brownian motion is the random movement of very small particles suspended in a fluid that results from collisions with molecules.

Diffusion is the ability of one substance to penetrate into a mass of particles of another substance.

Make up questions

 

Scientific Method

  1. Stating a problem - something is considered a problem if its solution is not obvious. Some crucial information is missing. Solving the problem involves finding this missing information.
  2. Collecting information on the problem -- the more you know about the problem the more precisely you can state the problem and the less time you will waste looking for solutions.

3. Making a hypothesis.

    1. Use what you know about the problem to predict a solution and try it.
    2. Look for patterns that will help you make predictions about the problem.
    3. Make a model or a representation, of what you’re working with.

d. Break the problem down into smaller, simpler problems.

  1. Performing an experiment - design an experiment that will provide a means for you to make a solid conclusion on your hypothesis.
  2. Make a conclusion - a solid conclusion is related to the hypothesis and based on the results of a well designed experiment.

 

EXPERIMENTAL DESIGN CONCEPTS

A science experiment is designed so that only one variable is being tested at a time. A variable is something that is changed to study how this change affects the time being studied. By changing only one variable, when you make your conclusion you can be assured that it is only that one variable that is causing the effect.

  1. Independent variable (IV) - the variable that is purposely changed by the experimenter.
  2. Dependent variable (DV) - the variable that responds and is the variable measured.
  3. Constant (C) - all factors that are kept the same during the experiment.
  4. Control - the standard to compare the experimental effect against.
  5. Repeated trials - the number of objects/organisms undergoing treatment for each value of the independent variable, or the number of times the experiment is repeated.

Make up questions

Based on the above information complete the following scenarios by filling in the experimental design table with the appropriate information and then state what could be improved. The first one is done for you.

 

Scenario One: The amount of vitamin C in orange juice.

Description: Erica’s chemistry class was studying a unit on acids and bases. They investigated the amount of absorbic acid (vitamin C) in a solution. Erica’s laboratory group decided to experiment with the amount of vitamin C in types of orange juice. The types of orange juice they experimented with were fresh, frozen, and bottled. They collected 100 mL of fresh squeezed juice. They mixed up 100 mL of frozen juice as directed on the package. They purchased 100 mL of orange juice from the vending machine in the cafeteria. Using an established procedure, the students measured the milligrams of absorbic acid in each sample of juice.

Title : Vitamin C in Orange Juice

IV: Types of orange juice

Fresh

Frozen

Bottled

1 Trial

1 Trial

1 Trial

DV : Amount of vitamin C (mg/100 mLs)

C's : amt. of OJ, analysis procedure

 

Ways that the experiment could have been improved?

There needs to be at least 3 trials of each treatment. Erica has only done one. She also needs to have a control. A control could be a vitamin C tablet that has a know amount of absorbic acid.

Scenario Two: The effectiveness of water sealants

Description: Jake’s family had recently built a new deck for a hot tub. He wondered which brand of water sealant would best protect the wood decking from absorbing water splashed from the hot tub. Jake performed a test by cutting small blocks from the decking material. He dipped two blocks in each of one of three major brands of sealant and allowed it to dry for two days. He massed each block. Jake placed the blocks in hot tub water overnight. He massed each block again, finding the difference in the masses (grams). He concluded the difference was the amount of water absorbed.

Title :

IV:

DV :

C's:

 

Ways that the experiment could have been improved?

 

Scenario Three: The rate of a metal reaction is affected by the concentration of acid.

Description: Susan found that magnesium reacts with hydrochloric acid. She observed that the metal disappears as the acid reacts with it. She found that many bubbles were released as the reaction happened. She wondered if changing the concentration (strength) of the acid would affect how fast the reaction occurred. Susan prepared five concentrations of hydrochloric acid, .25M, .5M, 1M, 2M, 4M. She placed a piece of magnesium in each concentration and described the intensity of the reaction as fast, moderate, or slow. She also recorded color changes in the metal as the reaction occurred. She repeated the procedures four times for each concentration of acid.

Title :

IV:

DV :

C's:

 

Ways that the experiment could have been improved?

 

 

 

Scenario Four: What kind of metals give the most voltage in a battery.

Description: Tyler was studying batteries. He discovered that a battery works as electrons flow between two dissimilar metals through a solution. He wanted to find out what common metal combinations would give him the highest voltage measurements. He decided to test copper-iron, copper-aluminum, copper-zinc, iron-aluminum, iron-zinc. He cut out strips of various sizes of the metals. Tyler prepared three different weak salt solutions. He placed each pair of metals into 50 mL of the solutions and measured the voltage produced, using a voltmeter. Tyler conducted three trials in each of the three different solutions with each pair of metals.

Title :

IV:

DV :

C's:

 

Ways that the experiment could have been improved?

 

 

 

 

 

Scenario Five: The effect of salt on the freezing point of water.

Description: Amy was performing a special project in chemistry dealing with salt and the freezing point of water. Amy prepared .5%, 2.5%, 6% and 10% salt solutions. She put some ice in a Styrofoam container. She placed a test tube of each solution one at time into the ice and began to make it freeze by turning it around and around in the ice. When the solution in the tube turned to slush Amy took the temperature with a thermometer.

Title :

IV:

DV :

C's:

 

Ways that the experiment could have been improved?