Get 50% OFF at Our Black Friday Sale View Sale

Hot or Not?

4.6 based on 36 ratings

Updated on Mar 21, 2014



Most people who manually wash their dishes assume that hot water will kill the most microorganisms. The results from this experiment will help people know if hot water really does kill the most microorganisms. If swabs are grown from spoons washed in 10, 25, and 45 degrees Celsius, the least amount of microorganisms will grow from the spoons washed in 10 degrees Celsius.In order to do this the spoons were first soaked with milk to add microorganisms, because the new plastic spoons were sterile, and then swabbed after washed in the three different temperatures. The number of colonies grown after 48, 72, and 96 hours were counted and recorded. This process was repeated three times.After 96 hours the average colonies of the spoons washed in 10 degrees Celsius water was 4 colonies, the average colonies after washed in 30 degrees Celsius was 14 colonies, and the average colonies after washed in 45 degrees Celsius was 11 colonies. The results showed that the least colonies grew from the spoons washed in 10 degrees Celsius followed by the spoons washed in 45 degrees Celsius and lastly 30 degrees Celsius.


This project is in the Life Science category.


This project is appropriate for 5th and 6th graders.


This projects difficulty level was medium.


The cost of this project was approximately $20.

Safety Issues

In this experiment I had to make sure the water wasn’t too hot so it would not burn me. A safety issue in this project was growing microorganisms. To minimize this risk I did not come in contact with the microorganisms.

Time Taken to Complete Project

It took me approximately for me four days to finish this science experiment. I counted the colonies 48, 72, and 96 hours.


This project is about different temperatures of water on the cleanliness of dishes. The goal in this project is to find out what temperature of water kills microorganisms best. Also, if cold water kills just as much or more microorganisms, then if we used cold water we could improve the environment and our heating bills wouldn’t be as high.

Material and Equipment

The materials used in this project was 15 Petri dishes, one 125 milliliter bottle nutrient agar, 15 sterile swabs, a microwave, bleach, masking tape, a sharpie, 15 quart size Zip- Lock bags, a refrigerator, distilled water, stopwatch, ruler, 9 spoons, dawn soap, a sink, a drying rack, and a thermometer. I purchased these items at an online science store and a grocery store. Overall, these items were easy to find and purchase.


People wash their dishes every day. Most people use hot water. Does hot water kill the most microorganisms? Also, if cold water kills just as much or more microorganisms, then if we used cold water we could improve the environment and our heating bills wouldn’t be as high.

In other experiments like mine, cold water was proven to kill the most microorganisms. In order to know if the types of microorganisms grown are harmful, they would have to be cultured. Due to limit of resources I was unable to culture the microorganisms grown.

Throughout our life we will come in contact will many harmful and helpful microorganisms. Microorganisms are all around us; at the pool, school, doctor’s office, church, and our own houses. Some harmful microorganisms cause unpleasant effects like illnesses. Some helpful microorganisms help our bodies in positive ways. Microbiologists, scientists who study microscopic organisms called microorganisms, have found out most of the things that we know about microorganisms. Although some people might think microorganisms are small and insignificant, they play a very important role in our life.

Microbiologists have been able to learn how to control the growing of microorganisms so they can conduct experiments and learn even more by observing the microorganisms. One common way they observe microorganisms is by growing cultures. Cultures are the microbes that grow and reproduce on the Petri dish. This is a circular dish named their inventor, Julius Petri.

Agar is a substance that is used to culture microorganisms on Petri plates. It is a jelly like substance. Agar is made out of certain types of seaweed. Besides being used as a solidifying agent in laboratories, agar is used in jellies, soups, sauces, ice cream, cosmetics, clarifying beverages, and to size fabrics. Blood agar is made out of whole blood and is used for indentifying the typed of microorganism that is grown. Blood agar has been proved to very useful and helpful to microbiologists and others who are interested microorganisms. Blood agar can help determine if the microorganism is harmful or not.At room temperature agar is a gel. It remains a gel until its temperature reaches 65 degrees Celsius. Agar will melt at 85 degrees Celsius and solidifies between 32 degrees Celsius and 40 degrees Celsius.

Most types of microorganisms thrive at temperatures about 35 degrees Celsius. Most refrigerators are about ten degrees Celsius. Refrigerators help keep microorganisms from growing. This helps keep your food clean and safe to eat.

For general experiments when the goal involves all species of microorganisms, a general agar, such as nutrient agar or Luria Bertani Agar should be used. Agar such as Tryptic Soy Agar, XLD Agar, or MacConkey Agar will only grow selective types of microorganisms.

To obtain microorganisms scientists must use a sterile swab to swab the area that they are collecting microorganisms from. It is important that the swabs are sterile so unwanted microorganisms will not enter.

Once the selected area is swabbed, scientists gently glide the swab on the Petri plate in a pattern. This pattern is called an inoculation line.After the microorganisms have started to reproduce, the scientists can see if there were colonies, groups of the same species of microorganisms that grow together, off the inculcation line. If there were, the Petri dish became contaminated.

By learning about microorganisms and how they interact with humans, people can prevent harmful microorganisms and take advantage of the helpful microorganisms. One of the helpful microorganisms is yogurt. Through studies and experiments scientists and everyday people have been able to learn more about microorganisms and new facts that lead to the discovery of new methods of preservation, health, benefits, and much more. Microbiology is a valuable subject that everyone should learn about.

Research Questions

What temperature of water kills the most microorganisms when manually washing dishes

  1. On 15 Petri dishes, divide the bottom part of the dish into quadrants, drawing the lines with the Sharpie.
  2. Loosen the lid on the agar bottle, but do not remove it
  3. Put the agar bottle in the microwave
  4. Microwave the agar on medium heat for a minute
  5. Take the agar out, shake the bottle, and put agar back into the microwave
  6. Repeat steps 4 and 5 until agar is liquid
  7. Open the lid of the Petri dish as little as possible and pour agar on the bottom of the dish to a thickness of 3 mm, measuring agar with the ruler
  8. Close the lid
  9. Let dishes sit until they reach room temperature
  10. Turn each Petri dish upside down and place a single dish in a Zip- Lock bag. Seal each bag.
  11. Place each Zip- Lock bag in the refrigerator, with the Petri dish upside down.
  12. Store each Petri dish upside down in the fridge until ready to conduct the experiment.
  13. Take the Petri plates out of the refrigerator and let them adjust to room temperature
  14. Get three spoon and sterile swab. Swab a spoon for ten seconds, timing with the stopwatch.
  15. Turn the agar plate upside down so the lid is touching the table.
  16. Glide the swab over the surface of the agar in a zigzag pattern, not going in one of the quadrants. The zigzag pattern is your inculcation pattern.
  17. Pour one tablespoon of milk on each spoon.
  18. Let them dry on the drying rack for an 30 minutes.
  19. Get a sterile swab. Swab the same spoon that was swabbed before for ten seconds, timing with the stopwatch for 10 seconds. Then turn the agar plate upside down so the lid is touching the table. Glide the swab over the surface of the agar in you inculcation pattern, not going in one of the quadrants.
  20. Fill the sink with 8 cups of 10 degrees Celsius water determining the temperature by using a thermometer. Put in a tablespoon of dawn soap.
  21. Wash and rinse each spoon in 10 degree Celsius water.
  22. Place each spoon on a drying rack and let it dry for fifteen minutes.
  23. After the spoons had dried, get three sterile swabs and swab the three spoons.
  24. Repeat steps 17-23 but wash and dry in 30 degrees Celsius and 45 degrees Celsius.
  25. Put Petri dishes back in bags and place bags in a warm area, free from drafts, fans, heaters, etc.
  26. Every 24 hours, open the Zip- Lock bags for thirty minutes to allow air- flow. Close Zip- Lock bags after 30 minutes.
  27. After 48 hours count and record the number of visible colonies present along the inoculation line of each Petri dish. If the empty quadrant has more than three colonies, the dish is contaminated and should not be included in the count.
  28. Repeat step 27 after 72 and 96 hours.
  29. After you take the final count, open the Petri dishes and fill them with bleach.
  30. Let the Petri dishes set 24 hours with the bleach.
  31. Tape Petri dishes shut in Zip- Lock bag and dispose of Petri dishes and bags.


The only variable intended to be tested was the temperatures of water used to wash the spoons. The same type of swabs, agar, and Petri dishes were used for each test. The average number of colonies 96 hours after the swab was taken from the spoons washed with 10 degree Celsius was 4 colonies. The average number of colonies after 96 hours after swab was taken from the spoons washed with 30 degree Celsius was 14 colonies. The average number of colonies 96 hours after the swab was taken from the spoons washed with 45 degrees Celsius was 11 colonies.

My hypothesis was proven to be incorrect by this experiment. My hypothesis predicted that the least microbial colonies would grow from the hot water but during the experiment the least microbial colonies grew from the cold water. A limitation in this experiment was not being able to identify the types of colonies to be identified. People need to know what types of colonies there are so they know if the microorganisms are harmful or not.If the colonies aren’t harmful, they shouldn’t influence the count of colonies because they will not hurt the people who eat off the spoons. If this experiment was conducted again with the same procedures there would be similar results. From this experiment it can be concluded that by washing dishes with cold water it would kill the most microorganisms. I could have subtracted the number of colonies grown from each temperature of water from the number of colonies that grew from the milk. Instead I just counted the number of microorganisms that were not killed.Every spoon had the same amount of milk on it so the milk added equal amount of microorganisms to each spoon.

In this experiment I learned how to swab items to find out how many microorganisms are on it. I also learned how to count colonies that are grown from swabbing an item. I learned everything about microorganisms and washing dishes. I also learned how to write up different things with the information and results that I found.

Microorganisms thrive in warm temperatures. The cold water helped make the microorganisms growth decrease. This is the same with refrigerators that are usually about 10 degrees Celsius which is the same as the temperature the spoons were washed in for cold. Washing in cold water is acting like a refrigerator and will help the growth of microorganism decrease. People should use cold water because it will kill the most microorganisms, make the environment better, and lower their heating bill.

In this project there was only one brand of soap used. A future experiment could be testing different brands of dish washing soap to test what brand kills the most microorganisms when manually washing dishes. This experiment would help people who buy soap know what brand of soap kills the most microorganisms.

Another study could be testing different materials of silverware like metal or plastic. From this experiment you would be able to conclude what material is best for you to eat off because it would clean the best manually. It would also encourage people to buy whichever one stays the cleanest.

A future study could be using one temperature of water but using different methods of cleaning dishes manually. This would help find out if there is a way that would kill more microorganisms just by simply changing the method used to wash dishes.

"Are Your Dishes Clean?"Arrow Scientific - Home. Web. 21 Mar. 2011. <>.

"Bacteria." Net Industries. Web. 21 Mar. 2011.

"Blood Agar."Austin Community College - Start Here. Get There.Web. 21 Mar. 2011. <>.

Liu, Shijun. "All About Agar."Science Fairs, Answers, & Tools. Science Buddies. Web. 21 Mar. 2011. <>.

Mosher, Dave. "Manual Dishwashing."OSU Research News Index Page. Web. 21 Mar. 2011. <>.

"Petri Dishes."Laboratory Equipment, Laboratory Apparatus, Laboratory Equipments Manufacturers, Lab Equipments Online. Web. 21 Mar. 2011. <>.