Quiz — Chapter 5: Acids, Bases, and pH

17 questions. 13 multiple choice, 4 short answer. Mix of recall, application, and "explain why" questions. Answer key with explanations at the bottom.

Multiple Choice

1. Pure water at room temperature has a pH of: - a) 0 - b) 4.6 - c) 7.0 - d) 14

2. The pH scale is logarithmic. A solution at pH 3 has how many times more hydrogen ions than a solution at pH 5? - a) 2 times - b) 10 times - c) 100 times - d) 1,000 times

3. Which of the following kitchen liquids is most acidic? - a) Tap water (pH 7) - b) Whole milk (pH 6.5) - c) Apple cider vinegar (pH 2.8) - d) Baking soda solution (pH 8.5)

4. The most accurate description of what an acidic marinade does to a piece of meat is: - a) It tenderizes the meat all the way through. - b) It breaks down all of the meat's proteins into amino acids. - c) It denatures the surface proteins (a few millimeters deep) and adds flavor. - d) It has no effect on the meat unless heat is also applied.

5. Baking soda needs an acid to leaven a batter. Which of the following provides the acid in a typical American buttermilk pancake recipe? - a) Salt - b) The buttermilk itself - c) Flour - d) Sugar

6. Baking powder differs from baking soda in that: - a) Baking powder contains its own dry acid; baking soda does not. - b) Baking powder is more alkaline than baking soda. - c) Baking powder cannot be used in cake recipes. - d) Baking powder reacts only when heated; baking soda reacts at any temperature.

7. Most spoilage bacteria stop growing when the pH of the food drops below: - a) 4.6 - b) 7.0 - c) 8.5 - d) 11.0

8. A "buffer" is a chemical mixture that: - a) Speeds up acid–base reactions. - b) Resists changes in pH. - c) Concentrates hydrogen ions. - d) Boils at a higher temperature than water.

9. Why does lemon juice prevent a cut apple from browning? - a) Lemon juice contains an enzyme that cancels out browning. - b) Lemon juice's low pH inactivates the enzyme polyphenol oxidase, which is responsible for browning. - c) Lemon juice removes the oxygen from the air around the apple. - d) Lemon juice contains sugars that mask the brown color.

10. Cream of tartar is most commonly used to: - a) Tenderize meat in a marinade. - b) Dissolve fats in a sauce. - c) Stabilize egg-white foams during whipping. - d) Preserve fresh fruit.

11. Acid sharpens the perception of: - a) Bitterness only. - b) Salt and umami; suppresses bitterness. - c) Bitterness; suppresses sweetness. - d) None of the above; acid only contributes its own sour taste.

12. A traditional alkaline cooking technique developed by indigenous Mesoamerican cooks more than 3,000 years ago that softens corn, releases bound niacin, and produces masa is called: - a) Sourdough fermentation - b) Nixtamalization - c) Maillard reaction - d) Steeping

13. Which of the following is a "weak acid" (gives up only a fraction of its hydrogen ions in solution)? - a) Hydrochloric acid - b) Sulfuric acid - c) Acetic acid (vinegar) - d) All of the above are weak acids.

Short Answer

14. A friend says she made a chicken curry that "came out flat." She has already added more salt and the dish still doesn't taste right. What is one specific recommendation you could give her, and why does it work?

15. Explain in 2–3 sentences why a pinch of baking soda can rescue an over-cooked, too-sharp tomato sauce. Why might too much baking soda make the sauce taste worse?

16. Walk through the chemistry: why does adding an acid (like vinegar) to a solution containing baking soda cause bubbling? Be specific about which chemical species are reacting and what gas is produced.

17. Maya has been marinating a chicken breast in a strong lemon-juice marinade for 12 hours and the surface is now chalky and dry. Using what you learned in this chapter, explain what went wrong and what she should do differently next time.

Answer Key

1. (c) 7.0. Pure water has a balance of hydrogen and hydroxide ions, giving a pH of exactly 7.0 at 25°C (77°F). Note that water at higher temperatures has a slightly lower pH because more water molecules dissociate at higher temperatures — a detail that matters in some food-science applications but not in everyday cooking.

2. (c) 100 times. Each pH unit is a factor of 10 in hydrogen-ion concentration. From pH 5 to pH 4 is 10×; from pH 5 to pH 3 is 100×. This logarithmic scale is why small pH changes can produce large effects in food.

3. (c) Apple cider vinegar (pH 2.8). The lower the pH, the more acidic. Apple cider vinegar at 2.8 has roughly 100,000 times more hydrogen ions than baking soda solution at 8.5.

4. (c) Surface denaturation plus flavor. This is the central marinade myth — popular cookbooks claim acid tenderizes meat deeply, but in reality the acid only penetrates 1–2 millimeters and leaves the interior unchanged. Salt, by contrast, does penetrate deeply over time.

5. (b) The buttermilk. Buttermilk is acidic (pH ~4.5) because of lactic acid produced by the bacterial cultures used to make it. The acid + base reaction (acid in buttermilk + base in soda) produces CO₂ that leavens the pancake. Note: this is why "milk plus baking soda" pancakes don't work — without acidic buttermilk, the soda has nothing to react with.

6. (a) Baking powder contains its own dry acid. Baking powder is essentially baking soda mixed with a dry acid (cream of tartar, monocalcium phosphate, sodium aluminum sulfate) plus a starch buffer. It is self-contained: add water and it activates. Baking soda requires a separately added acid.

7. (a) 4.6. This is the magic number for food safety. Below pH 4.6, Clostridium botulinum (the bacterium that produces botulism toxin) cannot grow, and most other spoilage bacteria are also slowed or stopped. This is the threshold for "high acid" foods that can be safely water-bath canned.

8. (b) Resists changes in pH. A buffer is a weak acid plus its conjugate base (or a weak base plus its conjugate acid) that absorbs added hydrogen or hydroxide ions, holding the pH stable. Tomato sauce, blood, and seawater are all buffered systems.

9. (b) The low pH inactivates polyphenol oxidase. Polyphenol oxidase (PPO), the enzyme that produces brown pigments by reacting with oxygen and phenolic compounds, is most active around pH 6.5 and loses most of its activity below pH 4. Lemon juice (pH 2.2) drops the apple surface well below the active range. (Vitamin C in lemon juice also helps by reducing oxygen.)

10. (c) Stabilize egg-white foams. Cream of tartar (potassium bitartrate) is a dry weak acid that lowers the pH of whipped egg whites, stabilizing the foam structure. It is also used in homemade baking powder and in candy-making to prevent sugar crystallization.

11. (b) Salt and umami; suppresses bitterness. Acid is a versatile flavor amplifier. It sharpens the perception of salt and umami (which is why a squeeze of lemon makes a soup taste saltier even without adding salt), suppresses the perception of bitterness, and can also affect the volatility of aromatic compounds.

12. (b) Nixtamalization. This indigenous Mesoamerican technique uses calcium hydroxide (lime water; cal) to soak and cook corn. The alkaline treatment breaks down the corn's outer hull, makes the kernel easier to grind into masa, and releases bound niacin so the body can use it. Cultures that ate corn without nixtamalizing it (like 18th–19th century European populations) suffered widespread pellagra; cultures that nixtamalized did not.

13. (c) Acetic acid (vinegar). Hydrochloric and sulfuric acids are strong acids — they fully dissociate in water and are not present in food. Acetic acid (vinegar), citric acid (lemon), lactic acid (yogurt), and most of the other acids you cook with are weak acids: they release only a small fraction of their hydrogen ions, which is why they're safe to handle and consume.

14. Sample answer: Try adding a small amount of acid before adding more salt. Three drops of lime juice, a quick splash of yogurt, or a small spoon of vinegar can transform a flat curry into a balanced one. The acid amplifies the existing salt's perception (so the dish tastes saltier without more salt), suppresses bitterness from spices, and increases the volatility of aromatic compounds, making the curry smell more vivid. The most likely problem with the curry is under-acidification, not under-salting.

15. Sample answer: Baking soda is a base. When added to an over-acidic tomato sauce, it neutralizes some of the citric and other organic acids in the sauce, raising the pH from too-sharp (around 3.7) toward a more pleasant (around 4.1). Too much baking soda overshoots the neutralization, leaves residual unreacted soda in the sauce, and produces a soapy, metallic taste. The fix is precision: a quarter teaspoon of baking soda for two quarts (1.9 L) of sauce, stirred in and tasted before adding more.

16. Sample answer: Vinegar contains acetic acid (CH₃COOH), which releases hydrogen ions (H⁺) in solution. Baking soda is sodium bicarbonate (NaHCO₃), and its bicarbonate ion (HCO₃⁻) reacts with the hydrogen ions: HCO₃⁻ + H⁺ → H₂O + CO₂. The carbon dioxide is the gas that bubbles. The reaction also produces water and leaves sodium and acetate ions in solution (which together form sodium acetate, a harmless salt). This is exactly the reaction that leavens a buttermilk pancake — same chemistry, different ingredients.

17. Sample answer: Maya has marinated her chicken breast for too long in too strong an acid. Acid penetrates only 1–2 millimeters into meat, but at the surface it denatures proteins and at high concentrations or long times causes the surface to turn chalky, dry, and tough — the texture often described as "cardboard." For next time: shorter marinade time (1–2 hours maximum for chicken in a strong acid), or a milder acid base like yogurt or buttermilk (pH 4.5 instead of 2.2) which gently denatures the surface without damaging it. For deep flavor, salt the chicken ahead of time (salt diffuses where acid does not). The cooking method also matters; if she's going to grill or pan-sear after marinating, a brief acid contact gives the surface useful Maillard support without destroying the texture.