Students are expected to:
Equipment: pH-sensor or universal indicator paper the self-made product (from the previous activity)
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After making the product, students are expected to establish their criteria in order to assess the “goodness” of the product. Students will carry out the experiments where self-made product is tested against the pre-determined criteria while presenting gathered data in a tabular format. Some tests, actually quite important, cannot be conducted within limited classroom time (e.g. determing shelf life of a product). Still, some tests may be applicable like measuring pH, estimating qualitatively the feeling on the skin left by the product, the odour, appearance and viscosity. Based on the findings, students draw conclusions about the product. This learning activity may be organised in a way using more open or more structured formats of student inquiry. Based on Bianchi and Bell (2008) students’ inquiry may be conducted as:
It would be probably reasonable to implement an open inquiry with students who already have sufficient experience with other formats of inquiry. More or less, it depends on a teacher’s decision which format to use taking into consideration his/her students and their previous experiences. Compared to more structured formats, more open formats certainly take more time to carry out this activity.
Therefore, the two different ways suggested to carry out this activity are: (1) using a structured inquiry, and (2) using an open inquiry.
Activity 7a. Structured inquiry
Using this format, students are provided with the research question and criteria to assess the self-made product plus, the description of procedures to follow.
They are asked to find answers to the following question:
Does the self-made product meet the predetermined criteria regarding its: (a) pH, (b) viscosity, (c) outlook, (d) smell, (e) feeling on skin? Is the self-made product comparable with analogous commercial products?
The usual pH of a cosmetic cream may vary between 5 – 8. Normal skin has a pH level of about 5.5. Skin or at least the outermost layer of it is slightly acidic. The acidic layer helps your skin retain moisture and keeps germs out. To help maintain the skin's fatty protective layer cosmetics should be a pH level similar to that of the skin itself. For example, soap that is too alkaline pH≥ 8, will break up the acid in skin, causing dryness.
However, the cosmetics that contain alpha hydroxy acids (AHA’s) may be more acidic than other. Alpha hydroxy acids found in skin-care products work best in a concentration of 5% to 8% and at a pH of 3 to 4. Beta hydroxy acid, salicylic acid, is a topical exfoliant that can reverse some of the effects of photoaging such as fine wrinkles and discolored skin. Currently, salicylic acid is the only beta hydroxy acid used as an exfoliant.
2) Feeling on skin
The product can be tested on skin, too. Volunteers should be warned to apply cream only on a very small area on skin. They should also be aware of the ingredients in order to minimise possible allergic reactions. Does it leave the skin soft, smoothe, and not very greasy? The last may be even OK when the product is meant for rough hands and/or frosty whether.
3) Odor and appearance
Odor and appearance may also be very important factors when buying a product. At the same time, the sense of smell may be quite subjective, therefore, it is suggested to use a number of „expert noses“ to estimate the attractiveness of the smell. The same issue is related to the appearance of the product.
Viscosity of different products may broadly vary depending on its purpose, still it cannot be too runny and therefore inconveniet to use, and the other way around – if it is too thick, it will be difficult to spread on skin. Students probably are not expected to measure quantitatively the viscosity, rather, they give their qualitative estimation on viscosity.
5) Shelf life
This may be a case for individual inquiry as it takes time to become rancid even for a product made without preservatives.
How long may be the shelf-life of homemade cosmetics? Products can be preserved for a couple of weeks or for three to four years depending on three factors: 1) how clean one works (disinfection is very important); 2) what kind of container one uses (dispensers are preferable because contamination is minimised); 3) the concentration of anti-microbial agents (essential oils, vitamins, and preservatives).
How do you know that the product is spoiled?
Besides a gray-green layer of mold on the surface of a product, there are several other factors indicating that a cosmetic product is severly contaminated with microbes:
Activity 7b. Open inquiry
This is an alternative to Activity 7a following the open inquiry format where students are expected to develop their own research question(s) and criteria for deciding whether the product is applicable in place of analogous commercial products. Although it is expected that students make up tests on their own, they may need the teacher’s help to decide whether the criterion is met (e.g. what is actually the normal pH of cosmetic creams).
 Bianchi, H. & Bell, R. (2008). The many levels of inquiry. Science and Children, 46 (2), 26-29.
 Shelf life is the length of time that foods, beverages, pharmaceutical drugs, chemicals, and many other perishable items are given before they are considered unsuitable for sale, use, or consumption. In some regions, a best before, use by or freshness date is required on packaged perishable foods.