Bleach is a commonly used household chemical that contains sodium hypochlorite as its active ingredient. When bleach is exposed to air, the sodium hypochlorite slowly breaks down through evaporation. The rate of evaporation depends on several factors including temperature, air circulation, the amount of bleach exposed, and the concentration of sodium hypochlorite in the bleach.
Factors Affecting Bleach Evaporation
Here are some of the key factors that determine how quickly bleach will evaporate:
Temperature
Higher temperatures cause bleach to evaporate more quickly. At room temperature (68-77°F), sodium hypochlorite evaporates slowly from bleach. But at temperatures above 100°F, the evaporation rate increases significantly. This is because higher kinetic energy at higher temperatures allows sodium hypochlorite molecules to more easily break free from the liquid surface.
Air Circulation
Good air circulation or airflow over the surface of the bleach will carry away evaporated sodium hypochlorite molecules faster. This maintains a concentration gradient that causes more rapid evaporation. Stagnant air above bleach allows concentrations to build up and slows the evaporation rate.
Exposed Surface Area
The more surface area of bleach exposed to air, the faster it will evaporate. For example, pouring bleach into a shallow tray will expose much more surface area and cause faster evaporation compared to keeping the same amount of bleach in a bottle.
Concentration
Bleach comes in different concentrations, with some products containing up to 8.25% sodium hypochlorite. Products with higher sodium hypochlorite concentrations will evaporate faster because there are more hypochlorite molecules available at the surface to escape into the air.
Testing Bleach Evaporation Rates
To get a better understanding of how quickly bleach evaporates under different conditions, some simple experiments can be performed. Here are a few examples:
Experiment 1
Take two small, open containers and pour equal amounts of bleach into each, such as 50 mL. Place one container in a warm spot around 100°F and the other in a colder spot around 60°F. Allow both to sit for 2 hours, then measure how much liquid remains. You should notice significantly more evaporation from the warmer sample.
Experiment 2
Set out two open containers with equal volumes of bleach, placed side-by-side. Use a fan to blow air across only one of the containers. After leaving them for an hour, the sample with the fan should show more evaporation than the other.
Experiment 3
Take two small containers and fill one shallow tray with 50 mL of bleach, and keep the other 50 mL in a deeper bottle with a smaller surface area. Check them after 3-4 hours. The shallow tray should show faster evaporation than the bottle due to more surface area exposed.
Typical Bleach Evaporation Rates
Under normal indoor conditions, here are some estimates for how quickly different amounts of bleach typically evaporate:
| Starting Amount | Amount Evaporated After 24 Hours | Amount Evaporated After 1 Week |
|---|---|---|
| 250 mL | 5-10 mL | 25-50 mL |
| 500 mL | 10-15 mL | 50-75 mL |
| 1 L | 15-25 mL | 75-125 mL |
As shown, the evaporation rate is generally proportional to the starting amount of bleach. Under standard conditions around 70°F, an open container of bleach can lose 2-5% of its volume per day through evaporation of sodium hypochlorite. Over one week, exposed bleach may lose 10-15% of its original volume.
Maximizing Shelf Life by Minimizing Evaporation
To extend the shelf life of bleach and make it last longer, follow these tips to minimize evaporation:
- Store bleach in cool, dry places away from direct sunlight.
- Keep bleach in an opaque, tightly sealed bottle whenever possible.
- Only open bleach when needed, and close the bottle afterward.
- Buy bleach in smaller containers to limit air exposure.
- Avoid pouring bleach into shallow pans or trays.
Bleach also degrades more quickly when diluted, so only mix bleach solutions as needed. Following these steps will keep evaporation in check and extend the usable life of your household bleach.
Conclusion
In summary, bleach containing sodium hypochlorite evaporates slowly at normal temperatures when left exposed to air. Factors like heat, air circulation, concentration, and exposed surface area can all increase the evaporation rate significantly. Simple experiments can demonstrate the effects of these variables on bleach evaporation over time. Being mindful of proper bleach storage and handling allows maximizing shelf life by minimizing unnecessary evaporation.