Chapter 1 Water Chemistry (2)

WATER CHEMISTRY

Properties of Water

• Complexity of Water

  • Despite having the simple formula H2O, water is a complex chemical solution.

  • "Pure" water is practically nonexistent in the natural environment.

  • Natural water always contains dissolved minerals and gases from interactions with the atmosphere, rocks, organic matter, and living organisms.

Pure Water

• Definition

  • True pure water (H2O) is a compound with chemically bonded constituents in a fixed proportion.

  • The term "pure water" can be misleading; it often refers to water that has been mechanically filtered to remove impurities.

  • In the industry, it can also refer to drinking water with reduced bacterial content.

Chemical Composition of Water

• Molecular Structure

  • H2O is colorless and odorless; it comprises 2 hydrogen atoms and 1 oxygen atom connected by covalent bonds.

  • The proportion of components remains fixed at a 2:1 ratio of hydrogen to oxygen.

Molecular Geometry of Water

• Shape

  • The molecular geometry is bent with a tetrahedral electron pair geometry and an angle of 104.4 degrees.

pH of Water

• Definition of pH

  • pH measures the acidity or basicity of aqueous solutions (ranges from 0-14, 7 is neutral).

  • In neutral pure water, the hydrogen ion concentration is 10^-7 gram-equivalents per liter, corresponding to a pH of 7.

Acidity of Water

• Acidic and Basic Water

  • Acid waters have a pH less than 7, and the most acidic waters can reach a pH of 1.

  • Basic or alkaline waters have a pH greater than 7, potentially reaching 14.

Natural Acidity of Rainwater

• Formation of Acidity

  • Natural rainwater is slightly acidic due to interaction with carbon dioxide (CO2), forming carbonic acid (H2CO3).

  • Some carbonic acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3−).

Chemical Reactions in Rainwater

• Reactions

  • H2O + CO2 = H2CO3

  • H2CO3 = HCO3− + H+

  • Each reaction affects the pH of rainwater, depending on the atmospheric CO2 concentration.

Influence of Carbonic Acid Concentration

• Effect of CO2

  • The concentration of carbonic acid in rainwater depends on atmospheric CO2.

  • Higher levels of CO2 lead to increased acidity in rainwater, lowering the pH from neutrality (7).

Current Atmospheric CO2 Levels

• Statistics

  • The current average CO2 concentration in the earth's atmosphere is approximately 0.30%.

  • Using this value, hydrologists estimate that the concentration of H+ in rainwater at equilibrium is 10^-5.7 moles per liter, which is approximately a pH of 5.7.

Significance of pH Levels

• Implications

  • A pH of 5.7 is classified as slightly acidic; acid rain is typically defined as rainwater unnaturally made acidic by human activities such as emissions of nitrogen and sulfur oxides.

Interaction of Acidic Rainwater

• Chemical Reactions

  • Slightly acidic rainwater interacts with land-derived dust, resulting in dissolved salts like calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+).

Role of Carbonic Acid

• Effects on Geology

  • Carbonic acid plays a significant role in the chemical weathering of rocks, contributing to soil formation, limestone cavern creation, and sinkhole formation.

  • Lower pH levels increase mineral solubility in water.

States of Water

• States of Matter

  • Water naturally exists in three states: solid (ice), liquid (water), and gas (water vapor).

  • Below freezing (32°F), water is a solid; between freezing and boiling points, it is a liquid; above boiling point (212°F), it is a gas.

Water Distribution on Earth

• Coverage and Forms

  • Approximately 71% of the Earth's surface is covered with water, mainly in the oceans, which hold about 96.5% of all Earth's water.

  • Water exists as vapor in the atmosphere, in rivers and lakes, in ice caps, glaciers, aquifers, and within living organisms.

  • Water is in constant motion due to the water cycle, transitioning between different states.

Liquid Water

• Conditions for Liquid State

  • Water must be between 32°F and 212°F to remain in liquid form.

  • Precipitation is the primary mechanism through which liquid water returns to Earth's surface.

Condensation and Precipitation

• Formation of Clouds

  • Water vapor condenses to form liquid water, leading to cloud formation and subsequent precipitation.

Ice Formation

• Frozen Water

  • Ice is the solid state of water, expanding as it freezes and trapping air molecules.

  • This unique property causes ice to float in water.

Formation of Frost and Ice Crystals

• Frost Formation

  • Frost occurs when a surface cools below the dew point, causing vapor to condense and freeze into ice crystals.

Icicles Formation

• Cause of Icicles

  • Icicles form when warm air leaks from attics melts snow, and the resulting water drips into colder air, creating ice dams.

Snowflake Formation

• Process

  • Snowflakes form when water vapor condenses on particles in the atmosphere, resulting in growing ice crystals influenced by temperature and humidity.

Types of Precipitation

• Types

  • Snow, sleet, hail, and rain are different forms of precipitation that occur under varying atmospheric conditions.

The Phenomenon of Sleet

• Definition

  • Sleet consists of transparent ice pellets formed by the freezing of raindrops or mostly melted snowflakes.

Water Vapor

• Gas State

  • Water exists as vapor when in its gaseous state, mainly boiling at or above 212°F.

Water Vapor in Space

• Importance

  • Water is crucial for human life and plays a vital role in molecular clouds and the formation of celestial bodies.

Physical Properties of Water

• Key Properties

  • Boiling point: 100 °C (212 °F)

  • Freezing point: 0 °C (32 °F)

  • Density: 1 g/cc at 4 °C

  • Water is nearly colorless, tasteless, and odorless.

Chemical Structure of Water

• Molecular Bonds

  • Comprised of one oxygen atom and two hydrogen atoms, forming a 105° angle with a "V" shape.

  • Water molecules possess a dipole moment due to uneven charge distribution caused by hydrogen bonding.

Hydrogen Bonds in Water

• Attraction Forces

  • Water molecules experience weak attractions called hydrogen bonds due to their polar nature, contributing to unique properties of water.

Polarity of Water

• Molecular Interactions

  • Water is a polar molecule with unequal distribution of charge leading to hydrogen bonding, which plays a role in its solubility properties.

Unique Properties of Water

• Hydrogen Bonding Effects

  • Unique properties of water are attributed to hydrogen bonds including cohesion, adhesion, high surface tension, and solvent capabilities.

Cohesion and Adhesion

• Definitions

  • Cohesion: Attraction between water molecules.

  • Adhesion: Attraction of water molecules to other polar surfaces.

Capillary Action

• Functionality

  • Water’s adhesion and cohesion contribute to capillary action, enabling water to rise in thin columns, critical for plant water transport.

Density and Ice**

• Impact On Bodies of Water

  • Ice is less dense than liquid water, allowing it to float and insulate water bodies, thereby enabling aquatic life to survive in winter.

Surface Tension

• Definition

  • Surface tension is the property of water’s surface that allows it to resist external forces, critical for insect movement on water surfaces.

Universal Solvent

• Solvent Properties

  • Water is a universal solvent as it dissolves various ionic and polar substances, important in biological and chemical processes.

High Specific Heat

• Temperature Regulation

  • Water has a high specific heat, enabling it to moderate temperatures and affect climate and ecosystems significantly.

Importance of Water as a Resource

• Global Importance

  • Water is essential for life, environmental stability, and industry, affecting billions of people worldwide.

Water Scarcity Issues

• Challenges

  • Global water scarcity affects 2.2 billion individuals; inadequate supply leads to significant health issues and environmental challenges.