Acids, Bases, and pH Basics

Chemistry is the study of matter's composition, structure, properties, and interaction.
Matter: Anything occupying space and having mass, composed of elements.
Element: A substance that cannot be broken down by ordinary chemical means. Approximately $25$ elements are common to living systems, with Oxygen ( $65\%$ ), Carbon ( $18.5\%$ ), Hydrogen ( $9.5\%$ ), and Nitrogen ( $3.2\%$ ) being the most prevalent in humans.
Atom: Smallest part of an element retaining its properties.
- Subatomic Particles:
  - Protons ( $P^+$ ): Positive charge, located in the nucleus.
  - Neutrons ( $N^0$ ): No charge, located in the nucleus.
  - Electrons ( $E^-$ ): Negative charge, in orbitals around the nucleus.
- Atomic Number: Number of protons in the nucleus.
- Atomic Mass: Sum of protons and neutrons in the nucleus.
- Isotopes: Atoms of the same element with the same number of protons but a varied number of neutrons. Used in medical imaging, tracing, and dating.

Molecule: Chemical union of two or more atoms.
Compound: Molecules composed of different elements.
Chemical Bonds:
- Ionic Bonds: Formed by the attraction between oppositely charged ions, which result from the loss or gain of electrons. Ionic compounds dissociate in water.
- Covalent Bonds: Formed by the sharing of electrons; strong and common in living systems.
  - Nonpolar Covalent: Equal sharing of electrons (e.g., C-H bonds).
  - Polar Covalent: Unequal sharing of electrons (e.g., H-O bonds in water). Nonpolar and polar substances do not mix.
- Hydrogen Bonds: Weak bonds between a hydrogen atom (positively charged region) of one polar covalent molecule and an oxygen or nitrogen atom (negatively charged region) of another polar covalent molecule. Essential for molecular shape and water's properties.

Solution: A uniform mixture of two or more substances.
Solvent: The dissolving medium (e.g., water).
Solute: The dissolved substance.
Aqueous Solution: A solution where water acts as the solvent.
Electrolytes: Substances that release ions ( $Na^+$ and $Cl^-$ from $NaCl$ ) when dissolved in an aqueous solution.

Definition: A substance that donates a hydrogen ion ( $H^+$ ) in solution.
Properties:
- Contribute one or more $H^+$ to a solution.
- Sour taste.
- May be corrosive or poisonous.
- React with metals to liberate hydrogen gas ( $H_2$ ).
- Neutralize bases.
- Affect indicator color (e.g., turn phenolphthalein colorless, phenol red yellow).
Common Examples: Sulfuric acid ( $H2SO4$ ), Hydrochloric acid ( $HCl$ ), Citric acid (lemon juice), Acetic acid (vinegar).

Definition: Decrease hydrogen ion concentration or release hydroxide ions ( $OH^-$ ) in solution.
Properties:
- Bitter taste.
- Slippery feel.
- May be corrosive or poisonous.
- Neutralize acids.
- Affect indicator color (e.g., turn phenolphthalein pink, phenol red pink).
Common Examples: Sodium hydroxide ( $NaOH$ ), Calcium hydroxide ( $Ca(OH)2$ ), Magnesium hydroxide ( $Mg(OH)2$ ).

Definition: The negative logarithm of the hydrogen ion ( $H^+$ ) concentration in moles per liter ( $pH = -\log[H^+]$ ).
Logarithmic Nature: A pH difference of $1$ unit represents a $10$ -fold difference in $H^+$ concentration.
Range: $0-14$ .
- Acidic: pH < $7$ .
- Neutral: pH = $7$ .
- Basic (Alkaline): pH > $7$ .
Hydrogen Ion ( $H^+$ ) and Hydroxide Ion ( $OH^-$ ) Relationship: As $H^+$ increases, $OH^-$ decreases, and vice versa.

Phenolphthalein: An indicator that is pink in basic solutions and colorless in acidic solutions.
pH Paper: Paper treated with an indicator, changes color, and is compared to a color chart to determine approximate pH.
pH Meter: An instrument used for precise pH measurements.
Natural Indicators: Some plant materials (e.g., purple cabbage extract) change color with pH.

Carbonic Acid Formation: Carbon dioxide ( $CO2$ ) in water forms carbonic acid ( $H2CO_3$ ), lowering pH.
Antacids: Neutralize excess stomach acid (e.g., hydrochloric acid, $HCl$ ) to relieve heartburn.