Overview
This lecture introduces the basic chemistry of water, including its atomic structure, bonding, properties, solubility, pH, and its role in emulsification, particularly for cosmetics.
Atomic Structure and Valence Electrons
- Atoms are the smallest units of matter and are made of protons (positive), neutrons (neutral), and electrons (negative).
- Protons and neutrons form the nucleus; electrons orbit outside at varying distances.
- The first electron shell holds up to 2 electrons; the second shell holds up to 8 (the octet rule).
- Valence electrons are those in the outermost shell and determine chemical bonding.
- Atoms are stable when their outer shells are full (octet rule).
Chemical Bonding in Water
- Atoms not satisfying the octet rule form bonds by sharing valence electrons.
- Covalent bonds involve sharing valence electrons between atoms (e.g., H2, H2O).
- Water (H2O) has two hydrogen atoms (each with 1 valence electron) bonded to oxygen (with 6 valence electrons).
- Oxygen requires two more electrons to complete its octet, achieved by sharing with two hydrogens.
- Unshared electron pairs on oxygen are called lone pairs.
Water Structure and Polarity
- Oxygen's high electronegativity creates a partial negative charge on oxygen and partial positive on hydrogens, making water polar.
- Water molecules form hydrogen bonds (a strong type of intermolecular force).
- In ice, hydrogen bonds form a stable, structured network; in liquid water, bonds are less orderly.
Unique Properties of Water
- Water has a high boiling point (100°C) due to hydrogen bonding.
- Water can self-ionize to form hydronium (H3O+) and hydroxide (OH-) ions.
- Pure water has equal H3O+ and OH– concentrations; pH is 7 (neutral).
- pH below 7 is acidic (more H3O+), above 7 is basic (more OH–).
Solubility and "Like Dissolves Like"
- Water's polarity makes it an excellent solvent for other polar substances (like dissolves like).
- Ionic compounds dissolve easily in water via hydration (water molecules surround ions).
- Non-polar substances (like oils) do not dissolve in water and form separate layers.
Emulsification and Skincare Applications
- To mix oil and water, an emulsifier is needed (e.g., soap, egg yolk).
- Emulsifiers are amphiphilic: hydrophilic (water-loving) part and hydrophobic (oil-loving) part.
- Amphiphilic molecules form micelles, trapping oil inside and allowing stable mixtures.
- This principle is key in cosmetics and food, like salad dressings.
Key Terms & Definitions
- Atom — The smallest unit of matter, made of protons, neutrons, and electrons.
- Valence electron — An electron in the outermost shell, key to chemical bonding.
- Octet rule — Atoms are stable when their outermost shell holds 8 electrons (except hydrogen/helium).
- Covalent bond — A bond formed by sharing valence electrons.
- Polarity — Distribution of charges leading to positive and negative regions in a molecule.
- Hydrogen bond — Attraction between a hydrogen atom and a highly electronegative atom (e.g., oxygen).
- Hydrophilic — Water-loving; dissolves easily in water.
- Hydrophobic — Water-fearing; does not dissolve in water.
- Amphiphilic — Contains both hydrophilic and hydrophobic parts.
- Micelle — A spherical structure formed by amphiphilic molecules in water.
Action Items / Next Steps
- Review atomic structure, electron shells, and the octet rule.
- Study examples of covalent bonding, especially in water.
- Practice identifying acids, bases, and neutral substances using pH.
- Read further on emulsifiers and their application in skincare products.