Capstone

Nine chapters of ideas, and this one puts them to work on things you already own: a tub of baking soda, a bar of soap, a rusty bike chain, a bottle of bleach, the label on a packet. The aim is not new theory but a habit: for any everyday chemical claim, name which earlier chapter explains it, and check what a source actually says. Each fact below is attributed to the Wikipedia page that states it. My own arithmetic or general knowledge is marked "(check)", and the numbers in the exercises are invented teaching values.

Example 1: Baking Soda and an Acid

The Sodium bicarbonate article gives the reaction as "NaHCO3 + H+ → Na+ + CO2 + H2O", and says "When it reacts with acid or is heated, carbon dioxide is released." In baking, it says, baking soda "is primarily used in baking as a leavening agent": the gas makes batters expand. Now tie it back:

• Chapter 8: the H⁺ on the left is the acid. Baking soda is a base reacting with an acid, so a sour ingredient in a recipe (vinegar, lemon juice, buttermilk) supplies it (my link to Chapter 8 (check)).
• Chapter 6: the equation must balance. Count the charge and atoms: one sodium, one hydrogen from the soda plus one H⁺ gives H₂O and CO₂, with matching totals each side (my check of the page's equation (check)).
• Chapter 7: using relative atomic masses (Python), the molar mass of NaHCO₃ is 84.01 g/mol and of CO₂ is 44.01 g/mol. One mole of soda gives one mole of CO₂, so 10 g of baking soda (0.119 mol) can give at most 5.24 g of CO₂ if there is enough acid (Python).

The same page also says baking soda can be used as "a gentle scouring powder" when made into a paste with minimal water, and that for odour control in refrigerators "there is little evidence that it is effective in this application". That second sentence is the course's checking habit in miniature: a popular tip, and a source saying the evidence is weak.

Example 2: Cooking and Browning

The Maillard reaction article defines the reaction as "a chemical reaction between amino acids and reducing sugars to create melanoidins", and says it is "responsible for many colors and flavors in foods", for example seared meat, bread crusts, fried onions and roasted coffee. It says the reaction "typically proceeds rapidly from around 140 to 165 °C (280 to 330 °F)", and that "Caramelization is an entirely different process from Maillard browning": caramelization involves sugars alone, while the Maillard reaction needs amino acids too.

Links: this is a chemical change, not a physical one (Chapter 1), because new substances with new colours and smells form. The temperature range is the Chapter 9 idea that heating speeds reactions; many reactions need enough energy to get over a barrier (Chapter 9). Frying versus boiling then makes sense: boiling water stays near 100 °C (my general knowledge (check)), well below that range, which is why boiled food does not brown the same way (my inference, not stated by the page (check)).

Example 3: Cleaning with Soap

The Soap article says a soap molecule has a hydrophilic (water-attracting) end and a fat-attracting end, and that "The insoluble oil/fat 'dirt' becomes associated inside micelles", so grease can be carried away with water. It also says soap "emulsifies oils, enabling them to be carried away by running water." On making it: "An alkaline solution (often lye) induces saponification", in which "the triglyceride fats first hydrolyze into salts of fatty acids", with glycerol released. On hard water: "When used in hard water, soap does not lather well, but forms soap scum."

Example 4: Rust and Sea Water

The Rust article describes rust as "an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen". It says "When iron is in contact with water and oxygen, it rusts", and calls it "an oxidation reaction specifically occurring with iron" in which iron gives up electrons to oxygen. The summary I read said salt speeds the process, so a bicycle left by the sea rusts faster than one kept inside (my everyday reading of that point (check)). For prevention it names galvanizing, "a layer of metallic zinc", and coatings "such as paint, lacquer, varnish", which keep air and water off the iron.

A simplified balanced equation is 4 Fe + 3 O₂ → 2 Fe₂O₃: four iron and six oxygen atoms on each side (Python atom count). Real rust is a mix of hydrated oxides and oxide-hydroxides according to the page, so this equation is a teaching simplification (check). Links: it is a combination reaction type of Chapter 6; iron is a metal giving up electrons as in Chapter 4's ions; and rust's speed is a Chapter 9 question, since a paint coat or zinc layer lowers how often iron meets water and oxygen (my link (check)). Sea water is a mixture of water with dissolved salts (Chapter 1), and its sodium chloride is the ionic compound of Chapter 4 (formula mass 58.44 g/mol using 22.990 + 35.45, Python).

Reading an Ingredient List or a Hazard Label

I could not verify a general rule for how ingredient lists are ordered from the pages I read, so treat this as general knowledge (check): in many countries ingredients must be listed in descending order of weight, so the first item is the largest part. Use the list like this:

1. Find the substance names. Many are plain names of compounds from Chapters 4 and 5 (sodium chloride is table salt).
2. Remember a name is not a warning. A long chemical name is just a name; water is also "dihydrogen monoxide" and harmless (my example (check)).
3. Look for percentages. The Bleach article says household bleach is typically "Sodium hypochlorite, usually as a 3–6% solution in water". So in 500 g of a 5 per cent solution there is 500 × 0.05 = 25 g of the active compound (Python).

For hazard labels, the GHS hazard pictograms article describes "a standard set of hazard warning images" in the Globally Harmonized System, with nine pictograms, GHS01 to GHS09. I summarise it in the next table; the page said it is based on older editions, so the latest rules may differ (the page flags itself as outdated).

The page also says labels carry the signal word "Danger" or "Warning" where necessary; it did not say which pictograms take which word. The corrosive pictogram links to Chapter 8's strong acids and bases (my link (check)). Also read the product's own instructions and safety sheet, which this course cannot replace.

Judging Claims: "Chemical-Free" and Others

Chapter 1 defined chemistry as the study of matter, and all matter is made of substances. The Chemophobia article says "chemical free" is "impossible if taken literally, since all consumer products consist of chemical substances", and that such labels appeal to a fear by suggesting a safer alternative. Chemists quoted there say the divide between natural and artificial is "incorrect since all things in the universe are chemical by default", and a 1992 Science paper cited on the page states that "every natural chemical is also toxic at some dose". The page also describes IUPAC's definition of chemophobia as "irrational fear of chemicals".

That does not mean every claim is nonsense, nor that every synthetic product is safe. It means the word "chemical" alone tells you nothing. A better set of questions, in the spirit of the course's "check" habit:

Where to Read More

• Encyclopaedias. The Wikipedia articles above are the sources used here.
• A school or open-university chemistry course. Look for organic chemistry (the chemistry of fats and soaps), electrochemistry (rusting) and food science (browning). These are general directions, not courses I checked.
• Chapters of this course. Return to Chapter 6 (equations) and Chapter 7 (the mole) for the calculation habits, and Chapter 8 for acids and bases.
• Practical. Only with supervision and safe materials: compare a wet and a dry nail over some days (my suggestion (check)).

Exercises