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help sheet: How to write a successful science investigation

Investigating the factors that affect rusting

Plan

Key Factors that could affect rusting:

  1. Type of metal
  2. Contact of metal with air (oxygen)
  3. Contact of metal with salty air
  4. Contact of metal with water
  5. Contact of metal with salty water
  6. Length of time in contact with air, water, salt
  7. Coating metal with paint or grease

Choice of Factor

  1. Type of metal
    Rusting is an example of a corrosion reaction. All metals will corrode, but only iron takes part in rusting. I will use iron as the metal, as my investigation is looking at rusting.
  2. What's in contact with the iron?
    To make the investigation fair, I need to have at least 5 different variables or observations. To find out which factor or factors affect rusting the most, I will put the metal in air, salty air, water, salty water and no air.
  3. Length of time in contact with air, water, salt
    To make the investigation a fair test, I will only change what the metal is in contact with. I will make the length of time the same for each piece of iron.

Preliminary Work

To help me write my plan I carried out a practice experiment. From the practice I found out:

  1. I will use iron nails as my metal, as these are all the same size and this will make the experiment a fair test, as I will only be changing what the metal is in contact with.
  2. I need 2 cm3 of water to completely cover my iron nail.
  3. I will use grease to prevent any air getting in contact with one nail.
  4. I will use 0.5g of salt. I will weigh this out each time, so I can repeat my experiment to get reliable results. I will not use a spatula, as this would not be as accurate as using a balance.
  5. I will use distilled water and tap water, to extend the number of variables I have to draw a conclusion from.

Apparatus

  • 14 x 1cm iron nails
  • balance
  • cake case
  • spatula
  • salt - sodium chloride
  • 10 cm3 measuring cylinder
  • distilled water
  • grease 14 test tubes
  • 14 rubber bungs
  • test tube rack or large beaker to store experiment in

Observations

I plan to change the substance that is in contact with the iron. I will measure the mass change of the iron, by weighing the nail before and after the experiment.

Method

  1. Take 7 x 1cm iron nails and weigh each nail, accurately recording the starting mass of each.
  2. Take 7 test tubes.
  3. Coat one nail completely with grease, put in a test tube and securely bung the top. It is important to bung the test tube as this will make each experiment a fair test. No other substances can then come in contact with the iron. Label the test tube as grease.
  4. Take a second nail and put it in a test tube and securely bung the top. Label the test tube as air.
  5. Put a third nail in a test tube, add 2 cm3of tap water and securely bung the top. Measure the water using a 10 cm3 measuring cylinder. Label the test tube as water.
  6. Repeat step 5 with distilled water. Label the test tube as distilled water.
  7. Repeat step 5 and 6, but this time add 0.5g of salt. Weigh the salt using a balance and a cake case. Make sure that the balance is at zero before you weigh the salt. Press the tare button to zero the balance. Label the test tubes as tap water + salt and distilled water + salt. Securely bung both test tubes.
  8. Put a seventh nail in a test tube and add 0.5g of salt, weighed out in the same was as step 7. Securely bung the test tube and label as air + salt.
  9. Repeat the whole investigation so that you have 2 sets of results to draw a firm conclusion from. This will help make the experiment a fair test.
  10. Leave the experiment for 1 week to allow the rusting process to occur. This length of time may have to be longer, if no rust has appeared. Remove each nail from the test tube and weigh the new mass. Be careful not to dislodge any rust from the nail.

Prediction

I predict that the iron nail that is in salty water will rust at a faster rate. This nail will weigh the most after one week, as it will be covered in the most iron oxide (rust).

Scientific Explanation

Iron rusts when it is in contact with oxygen and water. Rust is iron oxide. The chemical formula for iron oxide is Fe2O3. Rusting is an oxidation reaction. This means a reaction with oxygen to form an oxide.

A word equation for rusting is:

iron + oxygen = iron oxide

A balanced symbol reaction for rusting is:

4Fe + 3O2 = 2Fe2O3

Oxidation also means loss of electrons. The iron loses electrons. An ionic half equation for this reaction is:

Fe (s) = Fe2+ (aq) + 2e-

Water is needed for rusting to occur because the electrons that the iron loses travel through the water. If salt is present, rusting happens quicker because the salt makes the water more conductive. It allows the electrons to travel through the water faster.

Obtaining Results

  Test tube one     Test tube two    
Test Tube Mass before in grams Mass after in grams Mass change in grams Mass before in grams Mass after in grams Mass change in grams
air 0.50 0.60 0.10

0.50

 0.61 0.11
air + salt 0.50 0.69 0.19 0.50 0.73 0.23
tap water 0.50 0.73 0.23 0.50 0.75 0.25
tap water + salt 0.50 0.90 0.40 0.50 0.93

0.43
distilled water 0.50 0.71 0.21 0.50

0.69

 0.19
distilled water + salt 0.50 0.92 0.42 0.50 0.89 0.39
no air 0.50 0.51 0.01 0.50 0.90 0.40

 

Test Tube Initial Average Mass Change in grams Final Average Mass Change in grams
air 0.105 0.11
air + salt 0.21 0.21
tap water 0.24 0.24
tap water + salt 0.415 0.42
distilled water 0.20 0.20
distilled water + salt 0.405 0.41
no air 0.205 0.01

Analysis

Numerical Analysis

The average mass changes, which were calculated to 3 decimal places, have been rounded to 2 decimal places. This is because the balance only read to 2 decimal places. If I gave the average to 3 decimal places I would be trying to make the answer seem more accurate than it really is. I also ignored the second mass change result for the iron nail in no air. This result of 0.40g seems anomalous, as it is much higher than the result from the first experiment. I only expected the iron in no air to increase in mass slightly, as iron needs oxygen and water for rusting to occur.

Conclusion

From my results I found that the biggest change in mass happened in the test tubes with water and salt. There seems to be no difference if the water is tap or distilled water. The higher the mass gain, the more iron oxide (rust) is added to the iron. This means the conditions for rusting are best if oxygen, water and salt are present.

Comparing with my prediction

My results confirm my prediction when I said, "I predict that the iron nail that is in salty water will rust at a faster rate. This nail will weigh the most after one week, as it will be covered in the most iron oxide (rust)." From my results you can see that the largest mass change occurred with water, oxygen and salt present. The mass changes were 0.42 and 0.41g. This is nearly twice the increase in mass compared to water alone.

Science

My original scientific ideas made in my prediction seem to be true. In my prediction I found out that:

Iron rusts when it is in contact with oxygen and water. Rust is iron oxide. The chemical formula for iron oxide is Fe2O3. Rusting is an oxidation reaction. This means a reaction with oxygen to form an oxide. A word equation for rusting is:

iron + oxygen = iron oxide

A balanced symbol reaction for rusting is:

4Fe + 3O2 = 2Fe2O3

Oxidation also means loss of electrons. The iron loses electrons.

An ionic half equation for this reaction is:

Fe (s) = Fe2+ (aq) + 2e-

Water is needed for rusting to occur because the electrons that the iron loses travel through the water. If salt is present, rusting happens quicker because the salt makes the water more conductive. It allows the electrons to travel through the water faster.

Evaluation

What I needed to change from my plan

My plan worked well and I was able to obtain two sets of results. However, I did have to leave the whole experiment for 3 weeks before any noticeable rust formed on the nail.

Quality of my data

I am confident that my results are of a good quality because both sets of results are similar in value. E.g., the values for metal in tap water and salt are 0.40g and 0.43g, only 0.03g difference. I also compared my results to another student's and although our mass changes were different, the pattern of the results is the same.

Another student's results

Test Tube Final Average Mass Change in grams
air 0.26
air + salt 0.45
tap water 0.48
tap water + salt 0.85
distilled water 0.52
distilled water + salt 0.92
no air 0.05

Anomalous results

I have identified an odd result for the second experiment with no air. The mass change was very high and the nail looked very rusted. This is probably due to air and water coming into contact with the iron. The bung may not have been securely placed in the test tube.

Improvements

To improve the experiment I would use a chemical that absorbs water to make sure that no water got in contact with the nails in just air or no air. I would also weigh the nail whilst in the test tubes, as this might prevent any iron oxide (rust) from being lost when you take the nail out.

Further Work

To investigate this experiment further I would try using bottled water to control the chemical content of the water. I would also use rain water, as this is often the water that comes in contact with iron. I would also find out if grit used to cover roads when it's frosty has an affect on rusting rates. I would investigate how the surface of the iron affects rusting. I would look at a smooth nail, a scratched nail and a painted nail. The corrosion of other metals could also be investigated.

Click here for a science investigation help sheet.

Click here to plan your own investigation on water hardness.

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