Introduction: (Initial Observation)
Many steel parts are being galvanized in order to increase their resistance to corrosive factors such as air and moisture.
Galvanization is coating a metal with a layer of zinc as a protective coating. The zinc layer in a galvanized steel will protect the steel from rust and corrosion, that’s why the thickness of this layer is important. Manufacturers often need to test a galvanized metal to ensure sufficient amount of zinc is protecting the under laying metal.
In this project, you will test and measure the thickness of the zinc layer on a piece of galvanized metal. You will design experiments, perform tests and report the results. Thickness of the zinc layer or any other coating can be measured with many different methods, however in this project you will use the chemical method of measuring the thickness of zinc layer.
Find out about the effect of acids on Zinc and Iron. Can you find an acid that reacts with Zinc, but does nothing to Iron?
Read books, magazines or ask professionals who might know in order to learn about the methods that you may calculate the thickness of metal by having its density and size. Keep track of where you got your information from.
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
In this experiment, you will determine the thickness of zinc on a piece of galvanized Iron. You will be introduced to the reactivities of metals with acids and will develop some feeling for the size of an atom.
A specific question for this project is:
Does the thickness of zinc on a galvanized metal vary by the thickness of the metal?
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
For the first part of this project that is measuring the thickness of zinc on a piece of galvanized metal, we do not need to define any variables. For the specific question however, we define variables as follows:
Independent variable (also known as manipulated variable) is the thickness of galvanized sheet metal.
Dependent variable is the thickness of zinc layer.
Controlled variable is temperature
Constants are experiment method and procedures.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Following is a sample hypothesis:
The thickness of zinc coating increases by any increase in the thickness of metal.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1: Measuring the thickness of zinc
Knowing the density of zinc, you can determine the thickness of the zinc coating on the metal by determining the area of the metal covered with zinc and the mass of zinc removed when the metal is reacted with hydrochloric acid. If you are also given the radius of a zinc atom, you can calculate the thickness of the coating in terms of number of zinc atoms.
Active metals such as zinc react with acids to release hydrogen gas according to the following equation:
2 HCl (aq) + Zn(s)—-> H2 (g) + Zn2+(aq) + 2 Cl-(aq)
Iron is much less reactive than zinc and so it reacts with acid only slowly. It is therefore possible to use the rate of bubble formation to signal when removal of the zinc coating is complete.
- Determine the mass of the metal and record.
- Use the ruler to measure the length and width of the metal and record.
- Place the metal in the beaker and cover with 6 M HCl solution. When the gas bubbles stop appearing rapidly, add water; pour off the diluted acid, and rinse and dry the metal that remains.
- Determine the mass of the remaining metal and record.
- Calculate the area of the metal rectangle covered by the zinc. Remember the zinc covers two surfaces of the metal.
- Given that the density of zinc is 7.14 g/cm3, calculate the volume of zinc on the metal.
- Determine the thickness of the zinc coating.
- Given that the radius of the Zn atom is 2.66 × 10-8 cm, calculate the thickness of the zinc coating in terms of zinc atoms.
- Diluted acid solution may be flushed down the drain with water. Remaining metal may be disposed of with solid waste.
Concentrated solutions of HCl will burn skin and damage clothing. Fumes from acid can be caustic and/or irritating; do this experiment In a well-ventilated area (hood, if available). Goggles must be worn throughout this experiment.
Experiment 2: Comparing the thickness of zinc on different galvanized metals
Collect samples of galvanized metal sheets with different thickness.
Repeat the experiment 1 with each rectangular piece separately.
Record your results in a table like this.
|Sheet metal thickness||Thickness of Zinc layer|
Use the above results table to draw a graph.
Just to make sure that you are following basic rules of scientific method in your study, test at least 3 samples of metals in each thickness. Then write the average of your results in the above results table.
You may also want to have a control experiment. Your control experiment is just a piece of galvanized metal that you put aside and do nothing with. You just mass it in the beginning and at the end of your experiments to show that its mass is not changed. This will show that any change of mass on your other samples is caused by the reaction of acid on zinc, not an unknown phenomena.
Materials and Equipment:
6 M HCI (500 mL concentrated HCl solution diluted to 1.00 liter with distilled or deionized water)
3 to 5-cm rectangular piece of galvanized metal
- HCl solution is available from a hardware store as muriatic acid, 28% HCl; this solution is approximately 8 M and may be substituted for 6 M HCl solution.
- Scraps of galvanized metal are available from contractors or a piece of galvanized metal can be purchased from a hardware store.
- A Styrofoam cup or small glass jar may be substituted for the beaker.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
All calculations are described as a part of experiment number 1.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
Have students write down the formulas for the area and volume of the zinc coating and the density of the zinc. By manipulating the equations, students can solve for the thickness of the zinc coating.
* Remind students that the coating covers both sides of the galvanized metal.
Any general chemistry book can be used as a reference for this project.
Toon and Ellis, Laboratory Experiments for Foundations of Chemistry ; Holt / Rinehart & Winston, 1973. A similar experiment is described in this work.