Question

1. how are the items in each of the following pairs similar? how are they different?

a. copper and silver
b. distilled water and salt water
c. table sugar and table salt

Answer:

Part a: Copper and Silver

Similarities:

• Both copper (Cu) and silver (Ag) are metals belonging to the transition metals group in the periodic table.
• They exhibit characteristic metallic properties such as high electrical conductivity, high thermal conductivity, malleability (can be hammered into sheets), ductility (can be drawn into wires), and they have a shiny (lustrous) appearance.
• Both are used in applications related to electrical wiring and jewelry (due to their aesthetic appeal and workability).

Differences:

• Atomic Properties: Copper has an atomic number of 29, while silver has an atomic number of 47. Their atomic masses are different (copper: ~63.55 g/mol, silver: ~107.87 g/mol).
• Color and Luster: Silver has a brighter, whiter luster compared to copper, which has a distinctive reddish - brown color.
• Cost and Rarity: Silver is generally more expensive and less abundant in the Earth's crust compared to copper.
• Reactivity: Copper is more reactive than silver. For example, copper reacts with atmospheric oxygen and moisture to form a greenish patina (copper carbonate/hydroxide), while silver tarnishes (forms silver sulfide) more slowly and under different conditions (exposure to sulfur - containing compounds).

Part b: Distilled Water and Salt Water

Similarities:

• Both are aqueous solutions (or in the case of distilled water, a pure liquid that can form solutions) and are composed mainly of water molecules ($H_2O$).
• They are both clear, colorless liquids at room temperature and are used in various domestic, industrial, and biological processes related to hydration, cleaning, or chemical reactions.

Differences:

• Composition: Distilled water is a pure substance (composed only of $H_2O$ molecules), while salt water (usually referring to a solution of sodium chloride, $NaCl$, in water) is a mixture (it contains $H_2O$ molecules, $Na^+$ ions, and $Cl^-$ ions).
• Electrical Conductivity: Distilled water is a very poor electrical conductor because it has very few ions. Salt water, on the other hand, is a good electrical conductor due to the presence of mobile $Na^+$ and $Cl^-$ ions.
• Boiling and Freezing Points: The boiling point of salt water is higher than that of distilled water, and the freezing point of salt water is lower than that of distilled water (this is due to the colligative properties of solutions).
• Taste and Use in Biology: Distilled water is tasteless and not suitable for drinking in large amounts for biological systems (as it lacks essential minerals), while salt water has a salty taste and is not potable for humans (it can cause dehydration due to osmotic effects) but is the natural environment for marine organisms.

Part c: Table Sugar (Sucrose) and Table Salt (Sodium Chloride)

Similarities:

• Both are white, crystalline solids at room temperature and are commonly used as food additives (sugar as a sweetener, salt as a flavor enhancer and preservative).
• They are both soluble in water (to different extents and via different mechanisms).

Differences:

• Chemical Composition and Bonding:
• Table sugar (sucrose, $C_{12}H_{22}O_{11}$) is a molecular compound (a disaccharide, made up of glucose and fructose units) and is held together by covalent bonds within the molecule and intermolecular forces (like hydrogen bonding and van der Waals forces) between molecules.
• Table salt (sodium chloride, $NaCl$) is an ionic compound composed of $Na^+$ and $Cl^-$ ions held together by strong ionic bonds.
• Taste: Sugar has a sweet taste, while salt has a salty taste.
• Electrical Conductivity in Solution: When dissolved in water, sugar solution does not conduct electricity (since it does not dissociate into ions), while salt solution is a good electrical conductor (as $NaCl$ dissociates into $Na^+$ and $Cl^-$ ions).
• Melting and Boiling Points: Due to the strong ionic bonds in $NaCl$, it has a much higher melting point (801 $^\circ$C) and boiling point (1465 $^\circ$C) compared to sucrose, which has a melting point of around 186 $^\circ$C (and decomposes upon further heating rather than boiling in the traditional sense).
• Biological Role: Sugar is a source of energy (carbohydrate) in the diet, while salt is essential for maintaining electrolyte balance in the body (but excessive intake can be harmful).