Adding Salt to Water to Cook Faster

There is something I would like to point out when cooking with salt. There is a myth out there that continues to live on and it’s getting worse. I don’t know whether to blame reality television cooking shows or maybe people don’t think through things any more.

The myth is this that if salt is added to water, the boiling point rises and you can cook your food much faster. It is true but it won’t cook that much faster. It has everything to do with how much salt you add to the saucepan of water on your stove. You don’t add enough salt to elevate the boiling point to the point where cooking would occur faster. Or at least I hope you don’t. It would be a very very salty meal.

The boiling point of a substance or an element is the temperature when the vapour pressure of the liquid equals the pressure surrounding the liquid. A liquid at atmospheric pressure will have a higher boiling point than a liquid in a lower pressure like in a vacuum. At higher altitudes, say at the top of Mount Everest, the boiling point will also be lower for that substance.

Boiling point elevation is the name to the observed raising of a boiling point of a liquid when a non-volatile solute is added. The solution that results has a higher boiling point than the pure solvent. This can happen when salt is added to water. The boiling point can be accurately measured with an Beckmann thermometer, usually when hundredths of a degree need measuring and when small changes need monitoring.

Now back to the situation at hand. Raising the boiling point of water in saucepan on a stove. There are some calculations ahead to work out how much non iodised table salt, (NaCl) needs to be added to 500mL of water to raise the boiling point by one degree celsius.

The mathematical equation used to determine boiling point elevation is:

ΔT = iK_bm

where:  ΔT = change in temperature in °C

i = van ‘t Hoff Factor

K_b = constant that relates molality of a solution to the boiling point elevation of the solvent

m = molality of solute (mol/kg)

The information that we do have is:

ΔT = 1°C, i = 2, K_b = 0.512 °C kg/mol

What we don’t know is the molality of the solution, m but we can work it out by rearranging the original equation to:

m = ΔT/iK_b

m(NaCl) = 1/(2 x 0.512)

m(NaCl) = 0.976 mol/kg

We can now begin to work out how much table salt is needed to raise the boiling point of 500mL of water 1°C. I am going to make the assumption that the starting temperature of the water being used is 25°C. The density of pure water, (I know that tap water isn’t pure water and has dissolved minerals and salts in it but I want to keep things simple), at 25°C is 0.997 g/mL.

The first thing I need to work out is how much 500mL of pure water at 25°C weighs.

m(H₂O) = 0.997 g/mL x 500 mL

m(H₂O) = 498.5 g

m(H₂O) = 0.4985 kg

Now I can work out the number of moles of table salt in 0.4985 kg of water that will raise the boiling temperature by 1°C.

n(NaCl) = m(NaCl) x m(H₂O)

n(NaCl) = 0.976 x 0.4985

n(NaCl) = 0.4865 mol

I can now convert the number of moles of NaCl to the mass in grams.

m(NaCl) = n(NaCl) x M(NaCl)

m(NaCl) = 0.4865 x 58.44

m(NaCl) = 28.43 g

I’ve calculated that 28.45g of table salt is needed to be added to 500mL of water at 25°C to raise the boiling point by 1°C. It doesn’t sound like a lot of salt. It’s roughly 30g of table salt in 500mL of water. It could even be palatable and add flavour to say vegetables or pasta being cooked.

Well that is until you compare the concentration of salt the saucepan of water to the concentration of salt in open sea water. The average concentration of salt in open water is 3.5% or 3.5 g of salt for every 100mL of sea water. What we have in the saucepan is 28.45g of salt in 500mL of water which converts to 5.70g of salt for every 100mL of water. That’s more salt than sea water. Sea water tastes really salty. The saucepan of water isn’t sounding so tasty now.

I do think that the simple experiment of adding salt to water to elevate boiling point needs to be taught to vanquish the myth that adding salt will significantly speed up cooking. It won’t. It will make some foods taste better but that’s it.

Making Water Marbles

Today I was asked by someone whether I had seen a Youtube video on making spheres from water using household ingredients. I hadn’t and I was also intrigued and surprised that as a chemist I hadn’t heard of this potentially awesome science demonstration to show everyone.

So I watched the video…

It didn’t sit well and I exclaimed, “This can’t be real!” and what I couldn’t understand was the perfect round uniform spheres created. There were no smaller ones, larger ones, or even badly shaped ones. Such perfection in chemistry is unheard of. Well, unless everything is absolutely controlled which is damn near impossible. If I have learned anything in organic synthesis labs is that creating perfect structures from liquid requires the vessel to be perfectly still, protected from any dust entering and time. Plunging a hand into a solution just isn’t going to produce perfection.

I found the claims of the chemicals’ polarity reversing in contact in air to be incredulous. I don’t know what gas in air could cause such a sudden shift in dipole movement so suddenly. Is there anything and ever in high enough concentration for this to happen and more importantly, what class of chemicals would act in this way? If you have any suggestions, leave a comment because I’m all out of ideas.

I find it most irritating when someone creates a hoax based on chemistry and sends it out especially when it’s claiming that it can be recreated from household items. People believe this and try only to be disappointed with no explanation why it didn’t work and chalk up yet another negative association with chemistry. Yay. (The “Yay.” is filled with sarcasm and a face that looks similar to this, ~_~.)

Total Bull-sheet

I know it’s the internet and people lie all the time, heck, it even happens on television, but does the world really need yet another reason to dislike or distrust chemistry? Sometimes I dream about going to family occasions or meeting new people who don’t recoil in fear at the mention of chemistry. But it’s really never going to come true is it? Chemistry is just evil and lies about chemistry just make things worse.

There is some good news. The water marbles do exist. They are usually marketed as crystal soil or flowers beads by florists as an alternative way of providing water to flowers and/or as a decorative feature for floral arrangements. They are useful for a science demonstration on refractive index.

Bacteria Spotting

Millions of people in the developing world die from drinking water contaminated by bacteria. Current methods for detecting pathogens require laboratory equipment and specialised training which can be quite taxing on remote communities to operate. A cheap and simple test is sorely needed.

A collaboration between the University of Massachusetts, University of Puerto Rico and the Georgia Institute of Technology has led to the development of a colour test strip to detect bacteria published in the Journal of the American Chemical Society. The strip changes from yellow to red in 10 minutes if bacteria are present.

On a piece of filter paper, researchers applied a mixture of gold nanoparticles, the enzyme, β-galactosidase, and a yellow dye, chlorophenol-red-β-D-galactopyranoside in a particular way. The surface of the nanoparticles are positively charged to it attracts the negatively charged enzyme making it inactive. A freely moving enzyme reacts can react with the yellow dye changing it into a red colour so unless the enzyme is freed, the test strip remains yellow.

How would an enzyme particle become freed? This is where the bacteria comes into play. Microbes are negatively charged and bind to the nanoparticles much more strongly than the enzyme particle so the enzyme particles are effectively bumped out of place and freed. The enzyme is then free to react with the yellow dye to turn it red, indicating the presence of a microbe.

At the moment the colour strip can detect 10,000 bacteria per mL of water but this is not yet sensitive enough to be of use in the field because the most virulent bacteria can cause disease in as low concentrations between 10 and 100 bacteria/mL. At the moment, the nanoparticle, enzyme and bacteria are not very specific but if selectivity can be demonstrated, it would most likely bring a revolution to testing water quality.

Related articles

• A Simple Test Spots Bacteria (pubs.acs.org)

Soft Metals

There’s a group of metals which are soft. They are so soft that you can slice through them with a knife. This group of metals have always been a bit of a puzzle for me, these crazy alkali metals. Sure, I know the chemistry but they are nothing like the metals I am accustomed with. Shiny, (well ok, these soft metals are shiny), hard and not reacting violently with water on contact. They have never made any sense to me and well, I just find them weird. Always have, always will.

All of the alkali metals, lithium, sodium, potassium, rubidium, caesium, and francium, are all highly reactive. It is this property that means that they are never found in elemental forms outside of a laboratory. They are always stored in either mineral oil or kerosene to prevent exposure with the air and water. If you have ever spent any time polishing tarnished silver, be thankful that you have never had to polish any of these metals. They tarnish very easily and can do so in front of your eyes.

Place these metals in water and what results is a violent reaction occurs producing an alkali metal hydroxide releasing hydrogen gas. In some instances, there is enough heat to cause the hydrogen gas to catch alight. The general chemical reaction between the alkali metal and water is:

2 M _(s) + 2 H₂O _(l) → 2 MOH _(aq) + H_2 (g)

where M represents an alkali metal.

This video that I include was shown to me in my final year of high school simply because it illustrates the similarity of properties of elements in a group quite well and also because the final reaction involving caesium was especially violent. Listen for low whistle at the end of the video.

Slime

I grew up in the 80′s and like many children of the 80′s, Ghostbusters I and II were part of my childhood movie going experience. The common theme from the movies was slime, and a lot of it. It covered everything and everyone in its path. In the first movie, there was a ghost made of slime named Slimer.

Toy stores were stocked with all sorts of slime and associated products. There were creatures in slime, sticky slimy hands and blobs of slime. It came in a variety of colours, red, blue, black, yellow, gold, silver though with the help of the Ghostbusters movies, green was popular. As an adult I still love playing with slime and instead of buying slime these days, I make it myself using this recipe from the Powerhouse Museum.

For an explanation of how the slime is formed, watch this video. You’ll also discover that you eat one of the ingredients but the slime made is definitely not edible!

The slime can be stored in a airtight bag or container in the fridge when it isn’t being used or played with.