Chemistry & Water, pH Scale - Flashcards

A set of practice questions (Q/A format) covering chemistry basics, bonds, water properties, pH, acids/bases, and buffers based on the lecture notes.

What is the difference between an element and a compound?

An element is a pure substance that consists of one type of atom, while a compound is a substance formed by chemical bonding of two or more elements in fixed ratios (e.g., NaCl).

What are valence electrons?

Electrons in the outermost shell that determine an atom's bonding properties.

How do covalent, ionic, and hydrogen bonds differ?

Covalent bonds share electrons between atoms; ionic bonds involve transfer of electrons creating oppositely charged ions that attract; hydrogen bonds are weak attractions between a hydrogen atom bonded to a highly electronegative atom and another electronegative atom.

What makes a molecule polar?

Unequal sharing of electrons due to differences in electronegativity, producing partial charges (δ+ on one side and δ− on the other).

Where are the partial charges located in a polar molecule like water?

Partial positive charges are near the less electronegative atoms (e.g., hydrogen), and partial negative charges are near more electronegative atoms (e.g., oxygen).

Why does water have high surface tension?

Hydrogen bonding creates cohesive forces at the air–water interface.

Why is ice less dense than liquid water?

In ice, hydrogen bonds form a lattice that spaces molecules further apart, making ice less dense than liquid water.

Why does water have a high heat capacity?

Many hydrogen bonds can be formed and broken, absorbing or releasing energy without large changes in temperature.

Define hydrophilic.

Water-loving; substances that attract water (usually polar or charged).

Define hydrophobic.

Water-fearing; substances that repel water (usually nonpolar).

What does the pH scale measure?

The negative logarithm of the hydrogen ion concentration: pH = −log[H+].

What is an acid?

A substance that increases the H+ concentration in solution (donates H+).

What is a base?

A substance that increases OH− concentration or accepts H+, often raising pH.

How do buffers work?

Buffers absorb excess H+ or release H+ to resist changes in pH.

What is sodium chloride (NaCl)?

An ionic compound formed by Na+ and Cl−; dissolves in water via hydration of ions.

What happens to Na and Cl during NaCl formation?

Sodium loses an electron to become Na+, chlorine gains an electron to become Cl−; opposite charges attract to form an ionic bond.

How does NaCl dissolve in water?

Na+ and Cl− become surrounded by water molecules (hydration).

What is a covalent bond?

A bond formed by sharing electrons between atoms, creating a molecular orbital (e.g., H2).

What is a polar covalent bond?

A covalent bond with unequal sharing of electrons, producing partial charges (e.g., in H2O, δ+ on H and δ− on O).

What is a hydrogen bond?

A weak attraction between a partially positive hydrogen and a partially negative electronegative atom (such as O or N).

How are electrons arranged in hydrogen and carbon shells?

Hydrogen: 1 electron in a single spherical first shell. Carbon: 2 electrons in the first shell and 4 electrons in the second shell (second shell has four orbitals: one spherical and three dumbbell-shaped).

What is the octet rule?

Atoms tend to fill their outer shell to eight electrons (stable valence).

How many electrons does carbon have and where are they found?

Carbon has 6 electrons: 2 in the first shell and 4 in the second shell.

Define heat, temperature, and specific heat.

Heat is energy transferred due to a temperature difference; temperature measures average kinetic energy; specific heat is the amount of heat required to raise the temperature per unit mass.

Why does water have a high heat capacity relative to sand?

Heat energy disrupts hydrogen bonds in water; new bonds form quickly, so temperature rises slowly.

What percent of the human body is water?

About 60% of the body is water.

Why do coastal areas have milder climates than inland areas?

Water’s high heat capacity buffers temperature changes, moderating coastal climates.

What is the difference between acids and bases on the pH scale?

Acids increase H+ concentration (lower pH); bases increase OH− concentration or neutralize H+ (higher pH).

What is the relationship between pH and H+ concentration?

pH = −log[H+]; lower pH means higher H+ concentration.

What is a buffer in the context of blood pH?

A substance that rapidly absorbs/releases H+ to keep blood pH within a narrow range.