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Quiz: The Molecules of Food

Test your understanding of chemical bonds, water, pH, and the four macromolecule families with these questions.

1. Which type of chemical bond holds atoms together INSIDE a molecule?

  1. Hydrogen bond
  2. Ionic bond
  3. Covalent bond
  4. Polar bond
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The correct answer is C. Covalent bonds form when atoms share electrons; they are strong bonds that hold atoms within a molecule together. Hydrogen bonds are weak attractions that act BETWEEN molecules, not within them. Ionic bonds form when one atom donates an electron to another and are medium-strength. "Polar bond" describes an unequal covalent bond but is not a separate bond category from covalent bonding.

Concept Tested: Chemical Bonds in Food

2. Water is called the "universal solvent" mainly because

  1. It has a very low boiling point that allows it to evaporate quickly
  2. Its polar nature allows it to dissolve ionic and polar substances
  3. It contains both hydrogen and oxygen atoms
  4. It forms covalent bonds with every substance it contacts
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The correct answer is B. Water's polarity allows its partially negative oxygen end to attract positive ions and its partially positive hydrogen ends to attract negative ions, pulling ionic and polar substances apart into solution. Option A is incorrect — water actually has a high boiling point for its size. Option C describes water's formula but not why it dissolves things. Option D is false — water does not form covalent bonds with dissolved substances.

Concept Tested: Water as Universal Solvent

3. A water activity (aw) value of 0.60 means that a food

  1. Contains 60% water by weight
  2. Has a pH of 0.60
  3. Has very little free water available for microbial growth
  4. Will spoil within 60 hours at room temperature
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The correct answer is C. Water activity (aw) measures how much water is "free" and available — not the total water content. An aw of 0.60 means almost all water is bound to food molecules, leaving very little for microorganisms to use. This is why honey (aw ≈ 0.60) is shelf-stable. Option A confuses aw with percent water content. Option B confuses aw with pH. Option D is an invented claim unrelated to the definition of water activity.

Concept Tested: Water Activity (aw)

4. Which of the following is a disaccharide?

  1. Glucose
  2. Starch
  3. Sucrose
  4. Cellulose
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The correct answer is C. Sucrose (table sugar) is a disaccharide made of one glucose and one fructose unit joined by a glycosidic bond. Glucose is a monosaccharide — a single sugar unit. Starch is a polysaccharide — thousands of glucose units chained together. Cellulose is also a polysaccharide, making the structural fiber in plant cell walls.

Concept Tested: Disaccharides

5. The difference between digestible starch and indigestible cellulose is

  1. The number of carbon atoms in the glucose molecule
  2. The type of linkage (α vs. β) between glucose units
  3. Whether the polysaccharide contains fructose or glucose
  4. The temperature at which the polysaccharide was formed
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The correct answer is B. Both starch and cellulose are made of glucose units, but starch uses α-glycosidic bonds (digestible by human enzymes) while cellulose uses β-glycosidic bonds (which humans cannot break). Option A is wrong — both use glucose (C₆H₁₂O₆). Option C is wrong — both are composed of glucose, not fructose. Option D is irrelevant to digestibility.

Concept Tested: Polysaccharides

6. A peptide bond forms when

  1. A hydrogen atom bridges two water molecules
  2. Two fatty acids join end-to-end through an oxygen bridge
  3. The amino group of one amino acid reacts with the carboxyl group of another, releasing water
  4. Glucose and fructose link together by sharing an electron pair
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The correct answer is C. A peptide bond forms through a condensation reaction between the –NH₂ (amino) group of one amino acid and the –COOH (carboxyl) group of another, releasing a water molecule. Option A describes a hydrogen bond between water molecules, not a peptide bond. Option B describes an ester bond between fatty acids, not a peptide bond. Option D describes a glycosidic bond between monosaccharides.

Concept Tested: Peptide Bonds

7. Why does oil float on top of water in salad dressing instead of mixing with it?

  1. Oil is less dense than water and contains nonpolar molecules that cannot interact with polar water
  2. Oil has a higher pH than water, causing chemical repulsion
  3. Oil molecules are too large to dissolve in water
  4. Oil is a protein and proteins do not dissolve in water
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The correct answer is A. Oil is nonpolar and less dense than water. The "like dissolves like" rule means nonpolar oil cannot interact with polar water molecules, so the two liquids separate, with the less-dense oil rising to the top. Option B is incorrect — the separation is about polarity, not pH. Option C is wrong — many large polar molecules dissolve in water just fine. Option D is false — oil is a lipid, not a protein.

Concept Tested: Lipids Overview

8. Which fat type is typically SOLID at room temperature?

  1. Olive oil
  2. Canola oil
  3. Butter (saturated fat)
  4. Sunflower oil
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The correct answer is C. Saturated fats have straight carbon chains that pack tightly together, making them solid at room temperature. Olive oil, canola oil, and sunflower oil are all predominantly unsaturated fats with kinked chains that cannot pack as tightly, so they remain liquid at room temperature. Butter is solid because most of its fat is saturated.

Concept Tested: Saturated vs. Unsaturated Fats

9. What makes a buffer useful in food chemistry?

  1. It raises the boiling point of water in a solution
  2. It resists changes in pH when small amounts of acid or base are added
  3. It provides energy by breaking down into glucose
  4. It prevents proteins from unfolding during heat treatment
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The correct answer is B. A buffer resists pH changes by using a weak acid and its conjugate base to absorb added acid or base, keeping the pH nearly constant. This is important in bread dough fermentation, sports drinks, and many biological systems. Option A describes a colligative property, not a buffer. Option C describes carbohydrates. Option D describes heat stability, not pH buffering.

Concept Tested: Buffers in Food Chemistry

10. Which property of hydrogen bonds best explains why water has such a high boiling point for such a small molecule?

  1. Each hydrogen bond is extremely strong — stronger than a covalent bond
  2. Water molecules form large numbers of hydrogen bonds simultaneously, requiring a lot of energy to break
  3. Hydrogen bonds prevent water from evaporating at temperatures below 0°C
  4. Hydrogen bonds cause water molecules to convert directly from solid to gas
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The correct answer is B. Individual hydrogen bonds are about 20 times weaker than covalent bonds, but water molecules form hundreds of them simultaneously. Breaking all of them to convert liquid water to steam requires a large amount of energy, which is why water's boiling point (100°C) is far higher than expected for a molecule of its size. Option A is the opposite of the truth. Option C confuses evaporation with freezing. Option D describes sublimation, not the boiling point explanation.

Concept Tested: Hydrogen Bonding in Food