Module 2.2: Solute, Solvent, Solutions, Acids, Bases, Salts, pH, and Buffers

Solutions

In biological systems, many chemical reactions occur in solutions. Understanding the nature of solutions is fundamental to Anatomy & Physiology.

• Solution: A homogeneous mixture of two or more substances. The solvent is the substance present in the greatest amount, and the solute is the substance dissolved in the solvent.

• Example: In a glass of water with dissolved sugar, water is the solvent and sugar is the solute. The sugar disperses evenly, forming a uniform solution.

• Water as a Solvent: Water is called the "solvent of life" due to its polarity and ability to remain liquid at a wide range of temperatures, making it ideal for dissolving many substances.

Hydrophilic vs Hydrophobic Substances

Substances interact with water differently based on their chemical properties.

• Hydrophilic: "Water-loving" substances that dissolve easily in water. These are typically ionic or polar covalent compounds, which have full or partial charges. Water molecules form a hydration shell around these solutes, stabilizing them in solution.

• Hydrophobic: "Water-fearing" substances that do not dissolve in water. These are usually nonpolar covalent molecules, such as fats and oils, which lack charged regions for water to interact with.

• Example: Table salt (NaCl) is hydrophilic and dissolves in water, while oil is hydrophobic and does not mix with water.

Colloid, Suspension, and Emulsion

Mixtures in biological systems can be classified based on the size and distribution of their particles.

• Colloid: A mixture with a dispersed phase (small particles) and a continuous phase (the medium). Colloids do not settle quickly. Example: Milk, thyroid gland colloid.

• Suspension: A heterogeneous mixture with large, visible particles that settle over time. Example: Blood cells suspended in plasma.

• Emulsion: A mixture of two or more immiscible liquids, where one liquid (dispersed phase) is distributed in another (continuous phase). Example: Emulsions formed during fat digestion.

Acids, Bases, and Salts

Acids, bases, and salts are important inorganic compounds in physiology, affecting cellular function and homeostasis.

• Acid: A substance that releases hydrogen ions (H+) when dissolved in water. Strong acids dissociate completely (e.g., HCl), while weak acids only partially dissociate (e.g., acetic acid).

• Base: A substance that releases hydroxide ions (OH-) or accepts H+ ions. Strong bases dissociate completely (e.g., NaOH), while weak bases (e.g., ammonia) only partially accept H+.

• Salt: An ionic compound formed when an acid reacts with a base, producing water and a salt. Example: HCl + NaOH → NaCl + H2O

• Electrolytes: Substances that dissociate into ions in solution, conducting electricity. Important physiological electrolytes include Na+, K+, Ca2+, Cl-, and HCO3-.

Dissociation

Dissociation is the process by which molecules (such as acids, bases, or salts) split into smaller particles, often ions, in a reversible manner.

• Example: When an acid dissolves in water, it may release H+ ions and a negative ion.

• Water Dissociation: Water molecules can dissociate very slowly into hydrogen ions (H+) and hydroxide ions (OH-):

Hydrogen Ions (H+) and pH

The concentration of hydrogen ions in a solution determines its acidity and is measured as pH.

• Hydrogen Ion (H+): A hydrogen atom that has lost its electron, essentially a single proton.

• pH: The negative logarithm of the hydrogen ion concentration:

• pH Scale: Ranges from 0 to 14. A pH of 7 is neutral, below 7 is acidic (higher [H+]), and above 7 is basic (lower [H+]).

• Relationship: Each unit change in pH represents a tenfold change in [H+]. For example, a solution with pH 5 has 10 times more H+ than a solution with pH 6.

Buffers and pH Regulation

Buffers are crucial in maintaining stable pH in biological systems, allowing cells and tissues to function properly.

• Buffer: A substance that minimizes changes in pH by absorbing or releasing H+ ions as needed.

• Mechanism: Buffers typically consist of a weak acid and its conjugate base, which can reversibly combine with H+ or OH- ions.

• Example: The bicarbonate buffer system in blood:

• If blood pH rises (becomes more basic), the buffer releases H+ to lower pH.

• If blood pH drops (becomes more acidic), the buffer absorbs H+ to raise pH.

Summary Table: Types of Mixtures

Key Equations

• pH Calculation:

• Water Dissociation:

• Acid-Base Neutralization:

Additional info:

• Stable electrolyte concentrations are vital for nerve and muscle function, including heart and brain activity.

• Negative feedback mechanisms help regulate electrolyte and pH balance in the body.