Chemistry of Water PDF

# CHEMISTRY of WATER ## Diffusion - Mixing due to molecular movement is called **diffusion**. - All particles of matter are constantly moving. - gas molecules move in straight lines until colliding with other molecules - liquid molecules slide in one direction, but are "pulled...

# CHEMISTRY of WATER ## Diffusion - Mixing due to molecular movement is called **diffusion**. - All particles of matter are constantly moving. - gas molecules move in straight lines until colliding with other molecules - liquid molecules slide in one direction, but are "pulled" by the bonds they have with the molecules around them - even in solids, atoms move by vibrating in place - those movements allow other molecules to enter the space around them ## Diffusion Rates - The rate of diffusion measures how fast molecules disperse. - A slow process, because it depends on the random movement of molecules. - Rate of diffusion is affected by: - **concentration gradient**: the difference in concentration of a substance across space - molecules always move from an area of higher concentration to an area of lower concentration - **temperature & pressure** - higher temperature = faster moving molecules - higher pressure = more frequent collisions - The size of molecules also affects diffusion rates - bigger particles bump into more obstacles & don't fit between particles as easily ## Osmosis - **Osmosis is the diffusion of water across a semi-permeable membrane.** - A membrane is like a net. - will not let bigger molecules through - some small molecules will collide and bounce back ## Interactions of Water - **Nonpolar substances are hydrophobic** - literally means "water-fearing" - will repel water - disperse into fats (oily or greasy substances) - i.e. oil & water do not mix ## Hydrogen Bonds in Water - The polar nature of water results in **hydrogen bonding** - weak attractive forces between the negative Oxygen end of the molecule and the positive Hydrogen end - gives water many unique physical and chemical properties ## Water as a Solvent - **Water is an excellent solvent because of its strongly polar molecular arrangement.** - **Hydrophilic (water-loving, polar) substances readily dissolve in water** - because the polar ends of the molecule are attracted to (and pulled apart by) the charged ends of the H₂O molecule - i.e. salt (NaCl) dissolves easily in water to become Na+ and Cl- ## Surface Tension - **Surface tension is caused by cohesion** - an attraction between H₂O molecules due to hydrogen bonding ## H-Bonding and Changing States - Water is unlike most substances, which get denser when going from liquid to solid. - **Hydrogen bonds cause water to become less dense as it freezes.** - Water in its solid state (ice) floats. - Important for aquatic species in cold climates. ## Insulating Properties of Water - Water resists temperature change. - It absorbs a huge amount of heat as it changes into vapor. - H₂O releases a lot of heat when freezing into ice. ## Review - The basic building blocks of matter are **atoms**. - The **Atomic Number** of an atom equals the number of protons in the nucleus (which = the number of electrons). - The **Atomic Mass** of an atom equals the number of Protons + Neutrons in the nucleus (average of all isotopes). - **Elements** are different than **compounds** because compounds have 2 or more different elements bonded together. - Electrons are found where in an atom? - circling the nucleus in shells (also called orbitals or energy levels). - Each energy shell can only hold a set number of electrons "(octet rule)" - The small (subscript) 2 in H₂O tells the number of Hydrogen atoms in each molecule of water. - What do we get if one neutron is added to the nucleus of a carbon atom? - an isotope - Carbon-13 (13C) - What do we get if one electron is added to the outer orbital of chlorine? - an ion is formed, creating a more stable atom due to the filled energy level - Electrons are shared by two or more atoms in covalent bonds. - **A polar molecule is one in which electrons are not equally shared, so there is a slight + and - charge at certain locations.** - What do we get if one proton is added to the nucleus of a boron atom? - a carbon ion - (C+) - The polarity of water allows it to dissolve other polar molecules (especially ionic compounds). - Non-polar compounds are hydrophobic and thus not easily dissolved in water. - Many properties of water are due to hydrogen bonding. - The cohesion of water molecules to each other is exploited by plants and animals. - Water resists temperature changes by absorbing lots of heat. - Lower density of ice causes it to float & insulate the water below. - In diffusion, molecules always move from an area of higher concentration to an area of lower concentration. - Osmosis refers to the diffusion of water across a membrane. - Adding or removing hydrogen ions changes the pH of a solution. ## The pH Scale - pH describes how acidic or basic a solution is - a solution is any mixture with one or more solutes distributed evenly in a solvent - Substances with pH less than 7 are **acids** - i.e. battery acid, hydrochloric acid (HCI) - A pH of 7 is **neutral** - i.e. water, blood - Substances with pH higher than 7 are **bases** - i.e. bleach, drain cleaner ## pH - pH stands for **potential of Hydrogen ions** - when a substance dissolves in a solution, the molecules will break apart - in acids and bases, the broken-apart molecules form either OH- or H+ - the more H+, the lower the number on the pH scale and the more acidic it is - Log scale means 10X change per unit! - Water has a pH of 7 (it's neutral) - equal # of hydrogen & hydroxide ions - Substances with pH less than 7 are **acids** - molecules separate to form hydrogen ions (H+) - more H+ than OH- - the farther from 7, the stronger the acid - Substances with pH higher than 7 are **bases** - molecules separate to form hydroxide ions (OH-) - more OH- than H+ - the closer to 14, the stronger the base ## Acids - A pH below 7 is an acid - acids form hydrogen ions (H+) in water - In strong acids, the H+ will dissociate (separate) from most or all molecules - i.e.: lemon juice, hydrochloric acid (HCI), battery acid - With weak acids, only a few of the molecules will dissociate into hydrogen ions - i.e.: tomatoes, milk, vinegar (CH3COOH or acetic acid) ## Bases - A pH above 7 is a base - also described as alkaline - bases form hydroxide ions (OH') in water - In bases, hydroxide ions (OH-) will form in two ways - by the dissociation of the base - by stripping an H+ from water - Examples of strong bases - bleach, oven cleaner, drain opener - sodium hydroxide (NaOH) - Examples of weak bases - baking soda, eggs, ammonia, dish soap