Question

Sammy was boiling some brine (seawater) but forgot that he left the heat on. When he returned to the pan 40 minutes later, he noticed a white residue left at the bottom of the pan. Which of the following best explains the white residue? (A) The metal pan rusted due to the presence of the water. (B) The water evaporated, leaving salt behind. (C) The water burned and left ashes behind. (D) The pan must have been dirty before Sammy put the brine in.

Short answer

The best explanation for the white residue left at the bottom of the pan is option (B) - The water evaporated, leaving salt behind.

Step-by-step solution

Understand the situation

Sammy was boiling brine, which is seawater. Seawater contains dissolved salts, mainly sodium chloride (NaCl) or common table salt. When the pan was left on the heat, the water in the brine started evaporating.

Analyze each option

(A) The metal pan rusted due to the presence of the water: This is unlikely because rust happens over time and not within 40 minutes. Also, rust typically appears as a red or brown residue, not white. (B) The water evaporated, leaving salt behind: When boiling brine, the water will evaporate, and the dissolved salts will remain. These salts will form a white residue. This is the most likely explanation. (C) The water burned and left ashes behind: Water cannot burn, so it cannot leave ashes behind. (D) The pan must have been dirty before Sammy put the brine in: While it's not impossible, this option is not the best explanation for the white residue, as the residue is more likely to consist of the dissolved salts in the brine.

Choose the best answer

Based on the analysis above, the best explanation for the white residue left at the bottom of the pan is option (B) - The water evaporated, leaving salt behind.

Key concepts

Dissolved Salts

Dissolved salts are an important part of many solutions such as seawater, also known as brine. In seawater, the most common salt is sodium chloride (NaCl), the same salt that's used on your dinner table. Sodium chloride, along with other minerals and salts, dissolve in water, making the water taste salty. These are called "dissolved" salts because they mix completely with the water, usually becoming invisible.

When you heat up a solution containing dissolved salts, like boiling seawater, the water begins to evaporate. As the water leaves the mixture in the form of gas, the salts can't evaporate and thus start to separate out of the solution. Eventually, these salts will gather at the bottom of the pot or other container, appearing as a solid residue.

Boiling Process

The boiling process is a physical change where water changes from a liquid to a gas (water vapor) when sufficient heat is applied. The temperature at which this occurs is known as the "boiling point." Sea level atmospheric pressure causes water to boil at 100°C (212°F), but this can change with altitude and pressure. When water boils, bubbles form in the liquid that rise and escape, which is the steam you see.

During boiling, the heat energy breaks the hydrogen bonds between water molecules, allowing them to escape into the gas phase. If there are other substances mixed with the water, like salts, they remain behind as the water turns to vapor because they have different boiling points and cannot simply evaporate. This fact is crucial to understanding why the residue is left behind when brine is boiled.

Chemistry Explanation

From a chemistry standpoint, substances like salt do not change chemically when water evaporates. This means they stay the same substance—sodium chloride, in the case of common salt—and that they keep their properties. The key concept here is that evaporation is a physical change, not a chemical change.

This means when water evaporates, only the state of water changes, but the nature of the salt does not. It remains solid after water is gone because it cannot transition to a gaseous state at normal pressures and temperatures within a household setting. Understanding this concept helps explain why, after 40 minutes of boiling, only the water leaves as vapor, leaving solid salt residue behind on the bottom of the pot.