Water Lecture PDF

Water Water is obviously fundamental to life. Cells are approximately 80% water. Water is often described as the “solvent of life”; -a solvent is a substance that dissolves a solute; the result is a solution -many different molecules a...

Water Water is obviously fundamental to life. Cells are approximately 80% water. Water is often described as the “solvent of life”; -a solvent is a substance that dissolves a solute; the result is a solution -many different molecules are dissolved in the water of a cell -there are thousands of different chemical reactions happening in the water of a cell (“aqueous-based reactions”) polar covalent bonds But water is not a universal solvent. There is no universal solvent. The fact that water is not a universal solvent is very important in understanding how cells work. Modified from OpenStax Biology 2e.  Water Polar covalent Many of the Properties of Water are δ- δ- Explained by the Polar Covalent Bonds of the O bond Water Molecule δ+ δ+ Polar covalent bonds containing H lead to the possibility for H-bonding. H bond H H H bonds are weak, transient bonds that result from polar covalent bonds containing δ- δ- δ- δ- H. The extensive H-bonding in water causes it O O to be a liquid at room temperature. δ+ δ+ δ+ δ+ Other molecules of similar low molecular H H H H mass all are gases room temperature. Water molecules participate in multiple hydrogen-bonding interactions with nearby water molecules. Modified from: OpenStax Chemistry Atoms First H bonds Polar covalent bonds  Water Water Exhibits Cohesion Due to H-bonding, water molecules are attracted to each other; they stick to each other. H bonds This explains the liquid nature of water at room temperature. Water Exhibits Adhesion Water is attracted to large polar/charged molecules. Water will stick to those molecules, even though By User Qwerter at Czech wikipedia: Qwerter. Vectorized by Magasjukur2 - File:3D model hydrogen the molecules are too large to be dissolved. bonds in water.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid =14929959 Interactions between water and glass tubes, and between mercury and glass tubes. Glass has charges, and the charges attract the partial charges of water. In contrast, liquid mercury (Hg) does not have charges and does not adhere to glass tubes. By MesserWoland - own work created in Inkscape, based on the graphics by Daniel Stiefelmaier, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1353236  Water PRHaney Creative Commons Attribution-Share Alike 3.0 In devices used for measuring the volume of water-based fluids (burettes, graduated cylinders etc), there will a “meniscus” due to adhesion between water and the device. When measuring the volume of water-based fluids, keep your eye parallel to the bottom of the meniscus for an accurate reading. A meniscus as seen in a burette https://chem.libretexts.org/Bookshelves/General_Chemistry/Chem1_%28Lo full of colored water. wer%29/02%3A_Essential_Background/2.04%3A_The_Meaning_of_Measure  Water Two Very Important Terms: Hydrophilic and Hydrophobic Hydrophilic – Literally means “water-liking”; hydrophilic molecules are polar and/or charged, and exhibit mutual attraction with water molecules. Hydrophilic molecules may be soluble in water, if they are not too large. Hydrophobic – Literally means “water-fearing”; hydrophobic molecules are composed largely of non-polar covalent bonds (e.g. as in fats/oils); hydrophobic molecules exhibit mutual repulsion with water. Hydrophobic molecules are not soluble in water. Oil is hydrophobic, and oil and water do not mix. As this macro image of oil and water shows, oil does not dissolve in water but forms droplets instead. This is due to it being a nonpolar compound. Nonpolar compounds are “hydrophobic” (= “water-fearing”), while polar compounds (e.g. glucose) are “hydrophilic” (= “water-liking”). OpenStax Biology.  Water – Hydration Shells/Spheres Small and Smallish Hydrophilic Molecules are Water Soluble (= will dissolve in water) Water is very good at dissolving salts (which dissociate in water). H2O NaCl Na+ + Cl- Dissolved ions are surrounded by three-dimensional hydration shells. Note the orientation of the water molecules (attraction of opposite charges): OpenStax Biology. When table salt (NaCl) is mixed in water, spheres of hydration (= hydration shells, hydration spheres, solvation shells) are formed around the ions. These diagrams are greatly simplified; keep in mind that the actual shells/spheres are three-dimensional. Water – Hydration Shells/Spheres Another view of a hydration shell around a Na+ ion. The δ- of the water molecule is oriented towards the Na+ (i.e. opposite charges attract). Keep in mind that the shell is a sphere (which is very hard to represent in two dimensions). By Taxman - http://bio.winona.edu/berg/ILLUST/Na+H2O.gif, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3648176  Water Water will also dissolve small polar molecules (molecules that have partial charges), e.g. sugars: Glucose Sucrose (= glucose-fructose) C is implied By NEUROtiker - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2951918 By Ben; Yikrazuul - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?cu You do not need to memorize the detailed structures of these rid=1019158 sugars. There is more information about “implied” C atoms in the next section of the course (Carbon Chemistry). Consider the many O-H and O-C bonds in glucose and sucrose: these are polar covalent bonds (have partial charges, δ- and δ+), and sugars are polar molecules; → H2O is attracted to sugars and “solvates” them.  Water What About Large Polar Molecules? Solubility of polar molecules decreases with size of the molecule. Very large polar molecules are not water-soluble, despite having partial charges (and being hydrophilic). But, water is nonetheless attracted to these molecules (but does not dissolve them). Starch is composed of many linked glucose molecules. A small portion of a starch molecule. Starch has many polar covalent bonds. C is implied By Picasa author kalaya - http://picasaweb.google.com/Teaychula/MiGropRatNa#5423611 By NEUROtiker - Own work, Public Domain, 872763475698, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3962569 https://commons.wikimedia.org/w/index.php?curid=9939132 You do not need to memorize the detailed structure of starch.  Water Why are Hydrophobic Molecules Hydrophobic? Hydrophobic molecules are composed largely of non-polar covalent bonds. Non-polar covalent bonds do not have partial charges or have relatively few partial charges. Important biological non-polar covalent bonds include: C–C C–H Fats are hydrophobic; -have many C-C and C-H bonds (with a few polar covalent C-O bonds) Structure of a fat (= triglyceride, triacylglycerol) OpenStax Biology 2e You do not need to memorize the detailed structure of a fat.  Water – Dissociation and pH Dissociation of Water Water can reversibly dissociate (split into smaller units): H2O ⇌ H+ + OH- (proton + hydroxyl ion) “⇌” means reversible, or equilibrium The dissociation can also be thought of as: 2 H2O ⇌ H3O+ + OH- (H3O+ = the hydronium ion) Strong acid – e.g. hydrochloric acid, shows almost complete dissociation, unlike the equilibrium for the dissociation of water: HCl → H+ + Cl- Acidity – there are various ways to think about acidity; For us, acidity is H+s (protons); high acidity = high [H+]. A strong acid would have a high [H+]. pH – measure of acidity pH = -log[H+]; pH 7 is considered to be “neutral”, pH < 7 is “acidic”, and pH > 7 is “alkaline” or “basic” [H+] of pure water = 10-7 M (molar, moles/L) negative log 10-7 = 7 = the pH of pure water  Water – Dissociation and pH pH – measure of acidity pH = -log[H+] André Karwath aka Aka Creative Commons High [H+] = high acidity = low pH. Attribution-Share Alike 2.5 Generic E.g. lemon juice has pH ~2; → [H+] = 10-2 M Low [H+] = low acidity (or is basic/alkaline) = high pH. E.g. household bleach has pH ~13; → [H+] = 10-13 M. Adina Firestone Creative Commons Attribution-Share Alike 2.0 pH 7 = neutral. E.g. pure water (no dissolved gases) is pH 7. → [H+] = 10-7 M.  Water – Dissociation and pH Effect of CO2 on the pH of Water Unlike other gases, CO2 reacts with water. CO2 + water produces carbonic acid (a weak acid), which causes water to acidify (increase the [H+]: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- ⇌ H+ + CO32- carbonic acid bicarbonate ion carbonate ion The CO2 and HCO3- equilibrium is very important in the human body, affecting things such as blood pH. Also has implications for photosynthesis, and the pH of distilled (pure) water. CO32- becomes an important part of the equilibrium as pH increases and affects water chemistry in alkaline water bodies (e.g. in southern SK).  Water – Dissociation and pH pH of pure water (no dissolved gases) = 7 pH of lemon juice ~ 2 (lemon juice is acidic and has a high [H+]) pH of household bleach ~13 (bleach is basic/alkaline and has a low [H+]) The pH scale measures the concentration of hydrogen ions (H+) in a solution. pH = -log[H+] Note: the pH of distilled water (= pure water) is 7 only if it is completely pure water. Contact with air, which contains CO2, will cause the water to acidify due to carbonic acid formation (the pH of distilled water in equilibrium with the atmosphere is ~5.7): CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- OpenStax Biology.  Water as Solvent - Summary Water as a Solvent – the polar covalent bonds of water, which lead to a δ- on the O and a δ+ on the H, cause water to be a polar molecule; as such, water is a good solvent for small polar and charged molecules (which are hydrophilic) Effect of Molecule Size on Water Solubility - the solubility of polar molecules decreases as the molecules get larger Molecules with Mainly C-H and C-C Bonds are not Water Soluble – these molecules do not have partial or full charges, and do not interact with water, i.e. they are hydrophobic and are not water soluble; a common example is vegetable oil, which does not mix with water; water is not a universal solvent