Thursday, September 26, 2013

Chemistry Thursday: Chemical reactions

I see it all the time: Vendors being asked if they have lye in their soaps because it's a "dangerous" ingredient (which is usually followed by someone saying they have a friend who makes soap and she doesn't use lye...). If the soap maker said no, there is no lye in her soap, she'd be right, even if she used it to make the soap. Why? Because the chemical reaction that turns oils into soap uses up all the lye so there's nothing left! This specific reaction is called saponification.

What does it mean when we say there's been a chemical reaction? How does that different from a physical reaction? 

A physical reaction in a substance means that a characteristic of it has been changed, but it's still the same substance. If we look at water, this means we can heat it up to make it become steam or cool it to become ice, but it's still water. This is called a state change, or a change in the state of its being. We can put half the water in a jug and the other in a cup and, although the amount of water has changed, it's still water.

A chemical reaction (above) in a substance means the substances have been changed and aren't the thing they were before. You can see that A and B are separate, but come together and are now bonded.

Processes like food being digested - it goes from being food to waste products - or burning a piece of wood - it goes from being wood to being ashes - are chemical reactions.  When we get to the end of a chemical reactions, the reactants - the starting points of the process - are changed into something else. We start with one thing and end up with another. If we started with water and carbon, we won't have water (H2O) and carbon (C) on its own when the reaction ends. We end up with another substance entirely. We still have 2H, 1O, and 1 C in the final product, but they will be re-arranged into something new.

We can't say that the things into which things are combined are the same as the original thing. Sodium (Na) comes together with chlorine (Cl),  andwe end up with NaCl or table salt. On its own, sodium reacts adversely to water, and on its own, chlorine makes up a gas that makes breathing difficult. Together, they make a taste sensation!

Related links on chemical reactions:
Chem4Kids explanation (really great!)
Six types of chemical reactions (interesting)
Chemical reactions (far more detail)

If we add a strong acid and strong base together - something like hydrochloric acid (HCl) and lye/sodium hydroxide (NaOH) - we get water (and sometimes a salt). It's called a neutralization reaction. Combining one thing and another thing in the right proportions means we can end up with something else through a chemical reaction.

A lot of people think we should be combining alkaline and acidic things on our skin - for instance, using an alkaline shampoo because our hair is acidic - but we don't want to do that. A neutral pH isn't necessarily a good thing for our skin - we tend to like acidic things around pH 4.5 to 5.5 for our skin, up to 6.5 for our hair.

Related posts on pH and our products and skin:
Chemistry of our skin: pH and our skin care products
Chemistry Thursday: Let's take a look at pH
Chemistry Thursday: How to measure pH
Question: pH and bath products

I mentioned in the post on making gels that we were using lye to neutralize the gel, and someone remarked that we can't use lye in a leave in product. But there's no lye left in the product if we use it at the right levels because it's an acid-base chemical reaction that leaves none of the lye in the product when its done. In the post on molarity, I mentioned that it's all about the concentration. If I add the right amount of lye to the gel, it will cause a chemical reaction that leaves no lye behind!

Related posts on chemistry:
Chemistry Thursday: Molarity
Weekend Wonderings: ...w/v or molarity

Saponification is the process of making soap by adding lye to oils to hydrolyze the triglycerides (the oil) to create a sodium or potassium salt of a carboxylate. In other words, we add lye to the oils to break up those oil molecules to create soap. The glycerin is the by-product because we broke up those oils. The picture above shows the chemical reaction. If you look at the picture, you can see why there's no lye left. The NaOH (sodium hydroxide) molecule doesn't exist any more: The Na O and H are used in other molecules. You can see the Na is attached to an oxygen in the third structure and the O and H. (The R is a functional group and not relevant for this discussion.)

Related links on saponification:
About Chemistry (great pictures)
Carboxyl Acid Derivatives (very science-y, University of Calgary)

To summarize: Adding lye to a gel is a chemical reaction that shouldn't leave any lye behind when the reaction completes. 

Thanks to Corry for her beautiful soap pictures. I want to eat all of them, and I know they're soap!

Related links:
Neutralization (Elmhurst University)


canfieldfive said...

*slow clap* Thank you! I hate hearing how people won't use handmade soap because it "has a dangerous chemical". That's just misinformation!

Thanks also for showing what saponification looks like on a chemical level. I've always wondered!

Kim said...

Susan, I was wondering if you could clear up some chemistry that's confusing to me. What is the difference when mixing for a dilution vs a ratio? For example, if I was mixing a dilution of 1:1 water and lye, how would that differ from a 1:1 ratio?


Jodi said...

Hi Susan,
After washing my face I'm about to put my homemade "anti-aging" facial serum on my face. A wonderful idea occurs to me and I squirt about an equal amount of pure glycerin into the serum in the palm of my hand, mix it around, and rub it all over my face.
... Wow! After about 30 seconds my facial skin is very warm, almost tingling. What happened? Soon after, the warmth had subsided and there was no visible red spots or damage to my skin. Phew!
Do you know what chemical reaction happened with that mixture? And, is this a good finding or my skin or a bad finding for my skin?
Thanks so much for your major support!
P.S. The ingredients in my facial serum are:
Black Currant Seed Oil
Meadowfoam Seed Oil
Macadamia Seed Oil
Borage Seed Oil
Evening Primrose Oil
Rosehip Fruit Oil
Kukui Seed Oil
Pomegranate Seed Oil
Sea Buckthorn Fruit Oil
Calendula Extract
Chamomile Extract
Green Tea Extract
Lavender EO

Susan Barclay-Nichols said...

Hi canfieldfive! It drives me insane to hear how lye is dangerous in general. Yeah, we have to be careful around it, but that doesn't mean it's dangerous when mixed with something else.

Hi Jodi and Kim. Take a look at today's Weekend Wonderings to see answers to your questions!

Kim: I'm not sure what the difference might be, but I put forth a few thoughts.

Jodi: I think it's the glycerin!

Kim said...

Hi Susan -
Thanks for answering my question! Where my wonderings come from is what was meant by mixing a range between 1:1 to 10:1 dilutions of white vinegar. I wasn't sure if they meant 10 parts water to 1 part vinegar, or what, so I looked it up and found where some biology people are even confused about what to mix up. In this discussion it is explained
"Don't mix dilution and ratio...
Ratio means you have x parts of A and y parts of B.
x : y
Dilution refers to the total volume.
x : (x+y)"
What do you make of this?

Susan Barclay-Nichols said...

Hi Kim. If you told me something was 1 part vinegar to 10 parts water (by weight), I would put 10 grams of vinegar and 100 grams water for a total of 110 grams. If you told me to dilute 10 grams of vinegar with 100 grams of water, I would put 100 grams of water into the 10 grams of vinegar and get 100 grams. If you told me that we wanted 10% vinegar, I'd go with 10 grams of vinegar and 90 grams of water for a total of 100 grams. And for dilution - I need to know more information. If you said we had to dilute the 10 grams of vinegar to have 100 grams in the end, then I would add 90 grams of water. If you wanted to have 200 grams in the end, I'd add 190 grams. I would prefer to work in moles, as this is what I was taught in chemistry, but you figure out the ratio - 1:10, let's say - and add enough water to dilute it.