L4: Understanding Acid-Base Theory

What is the reason lemon juice is acidic?

• It contains citric acid

• It contains baking soda

• It is neutral

• It is basic

Answer explanation

Explanation: Lemon juice is acidic because it contains citric acid, which is a weak organic acid. This is the primary reason for its sour taste and low pH.

Fun Fact: Vinegar contains acetic acid, a weak acid with a pKa of around 4.76. This mild acidity is just strong enough to kill bacteria but gentle enough to preserve the flavors and textures of vegetables, making it ideal for pickling.

What gas is produced when baking soda and vinegar are mixed?

• Oxygen

• Carbon dioxide

• Nitrogen

• Hydrogen

Answer explanation

Explanation: When baking soda (sodium bicarbonate) reacts with vinegar (acetic acid), it produces carbon dioxide gas, which is responsible for the fizzing and bubbling observed in the reaction.

• Fun Fact: Baking soda, or sodium bicarbonate, is a mild base that neutralizes odors by reacting with acidic odor molecules, turning them into neutral, odorless compounds. This is why an open box of baking soda in the fridge absorbs bad smells! Baking soda doesn’t just mask odors; it actually neutralizes them by reacting with acids. This same property makes it useful as a gentle cleaning agent around the house and even as a natural toothpaste alternative!

What do acids do according to Arrhenius's theory?

• They donate protons

• They accept protons

• They neutralize bases

• They create bases

Answer explanation

Explanation: Acids are defined by their ability to donate protons (H+ ions) in a chemical reaction according to Arrhenius's theory. This characteristic distinguishes them from bases, which accept protons. Therefore, the correct answer is that acids donate protons.

Fun Fact: According to Arrhenius’ theory, acids release hydrogen ions (H⁺) in water. Lemon juice, which contains citric acid, releases H⁺ ions, making it acidic. This acidity allows lemon juice to break down stains, dissolve minerals, and even kill some bacteria—making it a natural and effective cleaner!

What is the pKa of an acid indicative of?

• Its strength

• Its color

• Its smell

• Its taste

Answer explanation

Explanation: The pKa of an acid indicates its strength; a lower pKa value means a stronger acid, as it dissociates more in solution. The other options (color, smell, taste) are not related to the pKa value.

Fun Fact: pKa helps explain why vinegar (acetic acid) and lemon juice (citric acid) are perfect for marinades. Both have relatively low pKa values, meaning they are acidic enough to break down proteins in meat but not so acidic that they "cook" the meat like stronger acids would. This is why vinegar and lemon juice are great for marinating!

What happens to tooth enamel when exposed to acids with a low pKa?

• It strengthens

• It dissolves

• It remains unchanged

• It becomes whiter

Answer explanation

Explanation: Tooth enamel dissolves when exposed to acids with a low pKa because these acids have a strong ability to donate protons, leading to the demineralization of the enamel structure.

Fun Fact: Stomach acid, primarily hydrochloric acid (HCl), has a very low pKa of around -6. This extremely low pKa means it’s a strong acid that fully dissociates in water, making it powerful enough to break down food quickly and aid in digestion.

What happens when an acid loses a proton?

• It becomes a conjugate base

• It becomes a stronger acid

• It remains the same

• It becomes a neutral compound

Answer explanation

Explanation: When an acid loses a proton (H+), it transforms into its conjugate base. This is a fundamental concept in acid-base chemistry, where the acid and its conjugate base are related by the loss or gain of a proton.

Fun Fact: Baking soda, or sodium bicarbonate, acts as a base and reacts with stomach acid (HCl) to relieve heartburn. When baking soda (a base) accepts a proton from stomach acid, it forms its conjugate acid (carbonic acid), which quickly breaks down into water and carbon dioxide gas, providing instant relief.

What is the relationship between strong acids and their conjugate bases?

• Strong acids have weak conjugate bases

• Strong acids have strong conjugate bases

• They are unrelated

• Weak acids have strong conjugate bases

Answer explanation

Explanation: Strong acids completely dissociate in water, resulting in weak conjugate bases. This means that the stronger the acid, the weaker its conjugate base, making "Strong acids have weak conjugate bases" the correct choice.

Fun Fact: Hydrochloric acid (HCl) is a strong acid, which means it dissociates completely in water and has a very weak conjugate base, the chloride ion (Cl⁻). This weak conjugate base doesn’t interfere with HCl’s cleaning power, making hydrochloric acid highly effective for dissolving stubborn mineral deposits and rust in industrial cleaning.

What is baking powder made of?

• A mixture of acids and bases

• Only acids

• Only bases

• Neutral compounds

Answer explanation

Explanation: Baking powder is a leavening agent that contains both acids (like cream of tartar) and bases (like baking soda). When mixed with liquid, these components react to produce carbon dioxide, which helps baked goods rise.

Fun Fact: Baking soda (sodium bicarbonate) can act as both an acid and a base, which is why it’s so versatile in cooking and cleaning. In recipes, baking soda releases carbon dioxide gas when it reacts with acidic ingredients (like vinegar or lemon juice), causing dough or batter to rise. But it can also neutralize strong acids, making it helpful as an antacid for heartburn relief.

What is the effect of shampoo being slightly acidic?

• It keeps hair healthy

• It damages hair

• It makes hair greasy

• It has no effect

Answer explanation

Explanation: Slightly acidic shampoo helps maintain the natural pH balance of hair, which keeps it healthy by preventing damage and dryness. This is why the correct answer is that it keeps hair healthy.

Fun Fact: Shampoos are often slightly acidic, with a pH between 4.5 and 5.5, to match the natural acidity of your scalp and hair. This acidity helps to smooth down the hair cuticles, reducing frizz and making hair appear shinier.

What do curved arrows represent in organic chemistry?

• Electron movement

• Proton movement

• Molecule stability

• Chemical reactions

Answer explanation

Explanation: In organic chemistry, curved arrows are used to represent the movement of electrons during chemical reactions. They indicate how electrons are transferred or shared between atoms, making 'electron movement' the correct choice.

Fun Fact: In cellular respiration, your body uses acid-base reactions to release energy from glucose. Curved arrows in reaction mechanisms can illustrate how enzymes help transfer protons (H⁺ ions) and electrons in these reactions, driving the production of ATP, the cell’s energy currency.