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Chemistry science fair project:
How sterile a doctor's office is? Which dilution of bleach will kill the bacteria growing in the office?




Science Fair Project Information
Title: Find out how sterile a doctor's office really is, and which dilution of bleach will kill the bacteria growing in the office.
Subject: Chemistry
Grade level: Elementary School - Grades 4-6
Academic Level: Ordinary
Project Type: Experimental
Cost: Medium
Awards: None
Affiliation: Canada Wide Virtual Science Fair
Year: 2005
Description: Collect bacteria from different locations in the doctor's office using swabs. Place the swabs in sterile water. Pour water into agar plates. Place paper disks, soaked in different dilutions of bleach, into the agar plates. Measure in mm how big the haloes are around the disks.
Link: http://www.virtualsciencefair.org/2005/powa5s0/public_html/
Short Background

Sterilization (or sterilisation) refers to any process that effectively kills or eliminates transmissible agents (such as fungi, bacteria, viruses, spore forms, etc.) from a surface, equipment, article of food or medication, or biological culture medium. Sterilization does not, however, remove prions. Sterilization can be achieved through application of heat, chemicals, irradiation, high pressure or filtration.

Chlorine bleach is an accepted liquid sterilizing agent. Household bleach consists of 5.25% sodium hypochlorite. It is usually diluted to 1/10 immediately before use; however to kill Mycobacterium tuberculosis it should be diluted only 1/5, and 1/2.5 (1 part bleach and 1.5 parts water) to inactivate prions. The dilution factor must take into account the volume of any liquid waste that it is being used to sterilize. Bleach will kill many organisms immediately, but for full sterilization it should be allowed to react for 20 minutes. Bleach will kill many, but not all spores. It is highly corrosive and may corrode even stainless steel surgical instruments.

Bleach decomposes over time when exposed to air, so fresh solutions should be made daily.

Surgical and dental instruments are also sterilized to prevent contamination and infection by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection. Most bacteria in food are killed by cooking to temperatures above 60 C (140 F).

By far the most cost-effective home disinfectant is the commonly used chlorine bleach (a 5% solution of Sodium hypochlorite) which is effective against most common pathogens, including such difficult organisms tuberculosis (mycobacterium tuberculosis), hepatitis B and C, fungi, and antibiotic-resistant strains of staphylococcus and enterococcus. It even has some disinfectant action against parasitic organisms. Positives are that it kills the widest range of pathogens of any inexpensive disinfectant; it is extremely powerful against viruses and bacteria at room temperature; it is commonly available and inexpensive; and it breaks down quickly into harmless components (primarily table salt and oxygen). Negatives are that it is caustic to the skin and eyes, especially at higher concentrations; like many common disinfectants, it degrades in the presence of organic substances; it has a strong odor; it is not effective against giardia lamblia and cryptosporidium; and extreme caution must be taken not to combine it with ammonia or any acid (such as vinegar as this may cause noxious gases to be formed). The best practice is not to add anything to household bleach except water. Dilute bleach can be tolerated on the skin for a period of time by most persons, as witnessed by the long exposure to extremely dilute "chlorine" (actually sodium or calcium hypochlorite) many children get in swimming pools.

To use chlorine bleach effectively, the surface or item to be disinfected must be clean. In the bathroom or when cleaning after pets, special caution must be taken to wipe up urine first, before applying chlorine, to avoid toxic gas by-products. A 1 to 20 solution in water is effective simply by being wiped on and left to dry. The user should wear rubber gloves and, in tight airless spaces, goggles. If parasitic organisms are suspected, it should be applied at 1 to 1 concentration, or even undiluted; extreme caution must be taken to avoid contact with eyes and mucous membranes. Protective goggles and good ventilation are mandatory when applying concentrated bleach.

Commercial bleach tends to lose strength over time, whenever the container is opened. Old containers of partially used bleach may no longer have the labeled concentration.

Where one does not want to risk the corrosive effects of bleach, alcohol-based disinfectants are reasonably inexpensive and quite safe. The great drawback to them is their rapid evaporation; sometimes effective disinfection can be obtained only by immersing an object in the alcohol.

The use of some antimicrobials such as triclosan, particularly in the uncontrolled home environment, is controversial because it may lead to the germs becoming resistant. Chlorine bleach and alcohol do not cause resistance because they are so completely lethal, in a very direct physical way.

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

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