Basic Chemistry Review and Properties of Water

The Foundation: Electronegativity

Chemical Foundations

Biological systems are aqueous solutions where life's chemistry happens. It all starts with electronegativity—the 'greediness' of an atom for electrons.

Oxygen is highly electronegative. It pulls electrons toward itself, often creating partial charges in molecules like water. Carbon and Hydrogen have similar electronegativities. Because they share electrons equally, they form nonpolar bonds, which are the backbone of fats and cell membranes. Welcome to Biology 150. To understand how cells function, we must first look at the atoms that build them. The most critical concept here is electronegativity—an atom's affinity for electrons. Notice how electronegativity increases as we move toward Oxygen and Nitrogen. These atoms are the primary drivers of molecular structure in biology.

Predicting Bond Polarity

Drag the atoms into the bonding zone to see how electronegativity differences create polar or nonpolar covalent bonds.

Carbon and Hydrogen share electrons almost perfectly. This is a nonpolar covalent bond, meaning the molecule won't have charged poles. Because Oxygen is much more electronegative than Hydrogen, the electrons spend more time near the Oxygen. This creates a polar covalent bond with partial charges. Let's put this into practice. Drag two atoms into the center to see what kind of bond they form. Remember: 'Like' electronegativities share equally.

Water: The Hydrogen Bond

Hydrogen Bonding

Water's unique properties emerge from hydrogen bonds. These are weak attractions between different molecules.

Crucial Distinction: Covalent bonds hold 1 molecule together; Hydrogen bonds connect 2+ molecules.

Water is a polar molecule. The polar covalent bonds inside the molecule create partial charges. These charges allow a hydrogen atom on one water molecule to be attracted to the oxygen of another. This is a hydrogen bond. Don't fall into the common trap! These solid lines are covalent bonds—they are strong and stay within the molecule. These dashed lines are hydrogen bonds—they are the 'social' bonds between neighbors.

Life-Sustaining Properties of Water

Hydrogen bonding gives water five emergent properties essential for life. Explore each one below.

Because of those hydrogen bonds, water behaves unlike any other substance. Select a property to see how it supports life on Earth. In most substances, the solid is denser than the liquid. But not water! As it freezes, hydrogen bonds push molecules into a spacious crystalline lattice, making ice float and insulating the life below. Cohesion is water sticking to itself. This creates surface tension and, combined with adhesion to cell walls, allows water to climb tall trees against gravity. Water has a high specific heat. It absorbs a lot of energy before the temperature actually rises because that energy is used to break hydrogen bonds first. This keeps our oceans and bodies at stable temperatures.

The Logarithmic pH Scale

Acids, Bases, and pH

The pH scale measures the concentration of hydrogen ions [H+]. It is logarithmic: pH = -log[H+].

A shift from pH 7 to pH 5 means the solution is 100 times more acidic.

Life is sensitive to the concentration of hydrogen ions. The pH scale is logarithmic, meaning small numbers mask huge changes. Moving from pH 7 to 6 increases the H-plus concentration by 10 times. Moving to pH 5 increases it by 100 times! As you move the slider, notice how the concentration of ions changes exponentially. A low pH number means a high concentration of acid.

Biological Buffers: Maintaining Homeostasis

The Bicarbonate Buffer

Buffers minimize changes in pH by absorbing or releasing H+ ions. The human blood uses the carbonic acid-bicarbonate system.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

If your blood becomes too acidic, there are too many H-plus ions. The reaction shifts to the left, combining those extra ions with bicarbonate to form CO2, which you then breathe out. If your blood becomes too basic, the reaction shifts to the right. Carbonic acid dissociates to release more H-plus ions, bringing the pH back down. Your blood must stay at a pH of about 7.4. To do this, it uses a buffer system. This chemical seesaw shifts back and forth to keep things stable.

Calculating pH and Solubility

Apply your knowledge to these three scenarios. Look at the [H+] concentration and bond types to make your predictions.

Let's see if you can apply these principles. Complete these three quick calculations and predictions.

Final Challenge: The Mystery Molecule

Analyze the provided molecule. Identify its bond types and predict how it will interact with water.

Here is a mystery molecule with several O-H groups and a large C-H tail. Based on what you've learned, tell me: will this molecule dissolve in water? Explain your reasoning using the terms 'polarity' and 'hydrogen bonding'.