Science Buddies: "Ask an Expert"

Hello.
For my high school science fair, I am doing the "Does Chemical Lightening Affect the Structure of Human Hair?" project. Unfortunately, I am unable to understand the procedure. From my understanding, this project is about finding out if chemical lightening treatments affect the natural elasticity of human hair? But I am not sure because it started talking about building a hygrometer which is used for measuring humidity. I am extremely confused. So basically I have to take samples of hair, dye them, and test if they're still elastic after dying them and the disulfide bonds have been destroyed? But what does humidity have to do with that? Is there any way that I can perform this experiment excluding the hygrometer? If there is anyone who can help me understand this project, I would greatly appreciate it.
Thank you,
Sydney

lab link: https://www.sciencebuddies.org/science- ... #procedure

Hello and welcome to Science Buddies!

Towards the end of the Background section for the project, you will see this:

One feature of the hair structure that you can readily investigate is how the hair strand changes with varying humidity. A strand of hair under slight, steady tension will change in length when the humidity changes. The hair will contract when the air is drier, and stretch out when the air is wetter.

Essentially, hair elasticity is readily measured and compared by subjecting the hair to different humidity conditions. In this experiment, you are constructing a hygrometer with the hair so that you can test its elasticity. So it may be confusing, but you're not exactly measuring humidity with hair; it's more like you're measuring hair with humidity. You don't need an actual, quantitative hygrometer in this experiment (though I suppose it would help quantify your data and draw nicer figures).

This is done because there's no real good, easy alternative to measuring elasticity. An engineering lab might have access to a machine that could test tensile strength, but even then, they're probably not designed for something as fine as hair; generally they test something more load-bearing, like wood or metal.

Hope that was helpful!