Chemistry

Half-Life of Bleach

Question:

I have read on line that household chlorine bleach has half life two hours. I cannot find info on half life of acidified chlorine bleach at ph 5 or ph 6. Would it also be around two hours as well? Thank you for your time.

Response:

I believe bleach having a two hour half-life is incorrect under normal household operating conditions.

The dominant sodium hypochlorite (NaOCl,bleach) decomposition path way is shown below:

3 NaOCl à 2 NaCl + NaClO3

The rate of decomposition of bleach is determined by suspended solids, temperature, concentration, pH, heat, light, and presents of heavy metal contaminates. To account for temperature, a general rule for a 10 oC increase of temperature will be to increase the decomposition rate constant by a factor of 3.5. If the pH of the solution is from 11.86 to 13 and no solids, heat, light, metals, suspended solids are present, then sodium hypochlorite decomposes by a second order reaction (Rate = k[OCl-]2). In other words a 10 % wt of NaOCl will decompose 4 times faster than a 5% wt of NaOCl solution.

For example, If we start with a 5% wt NaOCl solution and store it at a temperature of 25 oC, after 7 days the concentration should drop to 4.964%. The half-life at 25 oC, if calculated correctly, should be 1743 days (almost 5 yrs). However, if the temperature increased to 40 oC for the same 5% wt NaOCl solution, the half-life would be only 127 days (almost 4 months).

If you want 2 hours for a half-life you are going to half to increase temperature very high, decrease pH to a very low value, or add heavy metal contaminates. You can never assume the half-life of bleach is the same at different pH.

For more technical information:
Canadian Journal of Chemistry 1956, 34(4), 465-478. (NaOCl)
Inorganic Chemistry 1992, 31(17), 3534-3541. (pH range 5-8)

- Chadron Friesen

 


 

Perchlorethylene

Question:

What is Canada’s policy on using perchlorethylene in dry cleaning company’s? Will this be banned given its toxic nature? Please let me know.

Response:

Environment Canada has listed perchloroethylene as a toxic substance under the Canadian Environmental Protection Act since 1999. It is considered a 2A carcinogen (cancer risk to humans).The regulations on perchloroethylene is given in the following website: http://ec.gc.ca/lcpe-cepa/eng/regulations/detailReg.cfm?intReg=61. I know the US Environmental Protection Agency wants to ban this material by 2020 in residential buildings. Mostly likely, Canada will follow suit.

- Chadron Friesen

 


 

Periodic Table

Question:

Where can I find a good chart of densities and a periodic table?

Answer:

If you are looking for densities of the elements, then “The Periodic Table of the Elements” by the Sargent-Welch Scientific Company is a great document.

- Chadron Friesen

 


 

Mixing Baking Soda and Vinegar

Question:

When I was 5 I saw Bill Nye the Science Guy putting baking soda and vinegar together to make a volcano. I just wanted to try the baking soda test. Once I was done I realized that after emptying the water there was a gooey blob at the end. Can you please tell me if it is a vitamin, mineral, chemical, element, etc? Thank you.

Response:

Baking Soda is known by chemists and Bill Nye as sodium bicarbonate (NaHCO3 ). Vinegar is also known as acetic acid (CH3C(O)OH). When combined, it is a simple acid base reaction that produces sodium acetate, water, and carbon dioxide (CH3C(O)OH + NaHCO3 –> CH3C(O)ONa + H2O + CO2). Mostly likely, the “gooey blob” that remains is excess sodium acetate that when undissolved.  Sodium acetate is an inorganic compound.

- Chadron Friesen

 


 

Salt Water Pool

Question:

I have a salt water pool (sodium chloride). These pools are equipped with an electric cell that turns the salt in the pool into chlorine. Since the chlorine evaporates every day, why do I not have to regularly add salt to the water? Is the chlorine produced in this pool the same as the chlorine in a traditional pool?

Answer:

As mentioned in your question, the salt added to the pool contains sodium chloride.  The chloride (Cl-), from sodium chloride, is necessary in order for the electric cell to convert or oxidize the chloride to chlorine (Cl2).  The chlorine then reacts with the water so at the appropriate pH it forms a  low concentration of disinfectants called hypochlorous acid (HClO) and hypochlorite (ClO-).   Once these disinfectants are used up with organics, splashed out of the pool, or some of the chlorine dissipates, then more chloride has to be added to the water as the new “chlorine source”.  In regular swimming pools that use tablets, the tablets generate the same disinfectants, hypochlorous acid (HClO) and hypochlorite (ClO-).  By the way, hypochlorite is also the active ingredient in bleach.

- Chadron Friesen

 


 

Supersaturated solutions

Question:

I have two questions about supersaturated solutions: 1) What happens at the molecular level that certain amount of water dissolves more solute than its capability? 2) When a salt dissolves in water, salt ions are surrounded by certain number of water molecules to keep the ions separated. If a solution is unsaturated or saturated there is plenty of water molecules, and the ions are surrounded by sufficient number of water molecules. How does this happen for supersaturated solutions? In other words, if a solution is a supersaturated solution, what kind of arrangement/alignment occurs between the ions of solute and water molecules that plenty of solute stay dissolved in certain amount of water?

Answer:

1)

At the molecular level, we would expect supersaturation too look similar to undersaturation, except that on average there are more dissolved ions. There's a continuous change from one state to the other, so there's nothing really exciting if you look at a molecular picture. However, if you look at the big picture, it is more stable for two phases to co-exist (water and a solid precipitate) than for the ions to be in solution. At the level of the whole system, it would prefer to be separate; however, at a micro scale, it requires too much energy to create nuclei of solid. An alternative way to consider this might be that at the molecular level, it is too unfavorable to create interfaces of water and a solid, so the ions remain dissolved in water. (Even though by definition, being supersaturated means the system as a whole would prefer to have some solid, undissolved material.)

2)

In many cases of supersaturation, there are many times more water molecules than there are salt ions, so one would expect the ions are still surrounded by a sufficient number of water molecules. For example, table salt (NaCl) has a solubility of 360 g per 100 g of water. This means there are roughly 10 water molecules for every pair of salt cation (positive) and anion (negative). In this case, the ions may begin to interact, depending on how the water surrounds the ions (often water forms "shells" that surround the ions). However, for a more extreme case of barium sulfate, there are ~5 000 000 water molecules for every pair of ions. For all intents and purposes, the molecular environment of this supersaturated solution is nearly identical to the undersaturated solution, though on average there will be slightly more ions. However, at the level of the whole system, the small energy difference averaged over many molecules adds up, so the system will seek to become more stable by creating precipitates (undissolved solid).

At the other extreme, such as the case of sodium acetate in water, you can dissolve a surprisingly large amount into water. In fact, at 100 C, you can dissolve 170 g into 100 g of water. (There is 1 molecule of acetate for every 2.5 water molecules!) This is partly because of the unique structure of acetate, which allows it to interact with water through hydrogen bonding in the same way that water interacts with itself. (Because it can have similar interactions, a lot can be dissolved into water.) In this case, the local structure (that is, the molecular environment) will be very different in the case of undersaturation and supersaturation, and salt-salt interactions will play an important role when the solution is supersaturated.

-Michael Gaultois

 


 

The source of oxygen on Earth

Question:

If water is formed from hydrogen and oxygen, how could it have formed on earth if the only source of oxygen is from plants, and plants could have only evolved with water present. Perhaps there was another source of oxygen even though I have not heard of any except possibly in the Earth's crust.

Response:

Plants are not the only source of oxygen on Earth. Oxygen is in fact the most abundant element in the Earth’s crust. So, as the solar system was forming, oxygen was present to form the planets and, in turn, the first rocks. Then, water and the atmosphere were formed (pooled) and their compositions evolved to what they are today.

Time line: First crust about 4.5 billion years ago. Atmosphere without oxygen about 3 billion years ago. Increasing oxygen content in atmosphere, continental drift, mountain formation. Evidence of first life via fossils dating back to 1.5 billion years. Abundant life in the sea at about 1 billion years ago. Atmosphere as today and first land plants about 400 million years ago.

-Ann Therriault

 


 

Mutations caused by radiation

Question:

If ionizing radiation can damage DNA and cause mutations, would it be possible to produce consistent mutations on a given genome, provided that the radiation damages the same points to the same degree every time?

Answer:

Indeed, ionizing radiation is a powerful mutagen and a driver of evolutionary change in organisms on our planet, although that was more so when our planet was much younger and didn't have the protective atmosphere it does presently.  Ionizing radiation works by breaking the bonds that hold DNA together, as well as by reacting with the DNA and surrounding water to give chemical reaction products that have different structures than the originals.  These changes in structures and breaks in the DNA may be repaired by some of the repair enzymes and proteins in the cell without any loss in integrity of the DNA blueprint to life, but more likely there will be a change in either the encoded RNA or the next generation of DNA.  Unfortunately, the bonds that break and the site of the structural changes are random.  With 3 billion bases, sugars and phosphates to choose from, all with similar susceptibility to damage, the task to make evolution directed in this way is insurmountable.

- Glen Loppnow


Does temperature have an effect on pH?

Question:

Does temperature have an effect on the pH of orange juice? If so, why?

Answer:

Orange juice is a rather complex mixture, so I will answer the question for water. After all, orange juice is mostly water. pH is a measure of the concentration of a specific ion in a solution: the hydronium ion. The higher the concentration of hydronium ions in solution, the lower the pH will be. Leaving cumbersome details out, liquid water partially dissociates into hydronium ions and hydroxide ions. Although this dissociation is minimal (i.e. most water stays as H2O), the extent to which this dissociation occurs is influenced by temperature. Higher temperatures tend to increase the concentration of hydronium ions and thus decrease pH. CAUTION: this does not mean that water becomes acidic at higher temperatures!

-Juan Beltran

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