Chapter 3 Water Practice Sheets

Water and Life

Introduction

This section covers essential concepts related to the properties of water, solution chemistry, acids and bases, and the role of water in biological systems. Understanding these topics is fundamental for grasping how life depends on water and how chemical reactions occur in aqueous environments.

Solution Chemistry and Molarity

• Solution: A homogeneous mixture of two or more substances. The solvent is the dissolving agent (often water), and the solute is the substance dissolved.

• Molarity (M): The number of moles of solute per liter of solution. It is calculated as: $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Molecular Weight: The sum of the atomic masses of all atoms in a molecule, usually expressed in grams per mole (g/mol).

• Preparation of Solutions: To prepare a solution of a given molarity, calculate the required moles of solute, convert to grams using molecular weight, and dissolve in the appropriate volume of solvent.

Example: To make 1 L of 1.5 M NaNO3 solution, calculate moles needed (1.5 mol), then multiply by the molecular weight of NaNO3 to get grams required.

Acids, Bases, and pH

• Acid: A substance that increases the hydrogen ion (H+) concentration in a solution. Example: HCl (hydrochloric acid).

• Base: A substance that reduces the hydrogen ion concentration, often by accepting H+ or releasing OH-. Example: NaOH (sodium hydroxide).

• pH Scale: Measures the concentration of H+ ions. Defined as: $\text{pH} = -\log_{10}[H^+]$

• Buffer: A substance that minimizes changes in pH by accepting or donating H+ ions.

Example: Blood contains buffers to maintain a stable pH despite metabolic changes.

Properties of Water

• Cohesion: Water molecules stick to each other due to hydrogen bonding, contributing to surface tension.

• Adhesion: Water molecules stick to other polar or charged surfaces.

• High Specific Heat: Water can absorb or release large amounts of heat with little temperature change, helping to stabilize temperatures in organisms and environments.

• Evaporative Cooling: As water evaporates, it removes heat from surfaces, cooling organisms (e.g., sweating in humans).

• Density of Water: Water is less dense as a solid (ice) than as a liquid, so ice floats. This is due to the hydrogen-bonded lattice structure in ice.

Example: Lakes and ponds freeze from the top down, allowing aquatic life to survive beneath the ice.

Water as a Solvent

• Universal Solvent: Water dissolves many substances due to its polarity, facilitating chemical reactions in cells.

• Hydration Shells: Water molecules surround and isolate ions or polar molecules, keeping them in solution.

Example: Table salt (NaCl) dissolves in water as Na+ and Cl- ions become surrounded by water molecules.

Summary Table: Key Properties of Water

Property	Description	Biological Importance
Cohesion/Adhesion	Hydrogen bonding between water molecules and with other substances	Transport of water in plants
High Specific Heat	Absorbs/release heat with little temperature change	Stabilizes climate and body temperature
Evaporative Cooling	Heat loss as water evaporates	Prevents overheating in organisms
Lower Density of Ice	Ice floats on liquid water	Insulates aquatic environments
Solvent Properties	Dissolves polar and ionic substances	Facilitates biochemical reactions

Additional info:

• When preparing solutions, always use the molecular weight (g/mol) to convert between grams and moles.

• Evaporative cooling is critical for thermoregulation in many animals.

• The unique density of water is essential for aquatic ecosystems, especially in cold climates.