The hard calcium carbonate shell of an egg is dissolved in hydrochloric acid. If the de shelled egg is then placed in a concentrated NaCl solution, what observable change will occur in the egg and why?

Introduction / Context:
This question illustrates osmosis, a key concept in biology and chemistry. When an egg shell is removed with acid, the remaining membrane behaves like a semipermeable membrane, allowing water to move in or out depending on the surrounding solution. Placing the de shelled egg in a concentrated salt solution creates an osmotic gradient that drives water movement. Recognising whether the egg shrinks or swells tests your understanding of hypertonic and hypotonic solutions and the direction of osmotic flow.

Given Data / Assumptions:

• The hard shell, composed mainly of calcium carbonate, is dissolved by hydrochloric acid, leaving behind the soft membrane and interior of the egg.
• The egg is then placed in a concentrated NaCl solution, which is hypertonic compared with the egg interior.
• The membrane around the egg acts as a semipermeable membrane.
• We assume no mechanical damage to the membrane, so osmosis can occur normally.

Concept / Approach:
Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential). When the egg, whose interior fluid has a lower salt concentration, is placed in a concentrated NaCl solution, the outside solution has a higher solute concentration. The solution is hypertonic to the egg contents. Water will move out of the egg and into the salt solution. As water leaves, the egg loses volume and appears shrunken or shrivelled.

Step-by-Step Solution:
Step 1: Recognise that dissolving the eggshell in HCl exposes the semipermeable membrane surrounding the egg white and yolk.Step 2: Understand that concentrated NaCl solution has a higher solute concentration than the fluids inside the egg.Step 3: Identify the concentrated NaCl solution as hypertonic relative to the egg interior.Step 4: Recall that in a hypertonic environment, water moves out of the cell or structure through osmosis.Step 5: Apply this principle to the egg: water will move from inside the egg through the membrane into the surrounding NaCl solution.Step 6: As water leaves, the egg loses volume and becomes smaller and wrinkled, meaning it shrinks.

Verification / Alternative check:
You can verify your reasoning by comparing with a related experiment where a de shelled egg is placed in pure water. In that case, water moves into the egg because the external solution is hypotonic, and the egg swells. The opposite behaviour is seen in a concentrated salt or sugar solution, where the egg shrinks due to water loss. These classic demonstrations are used in schools to teach osmosis and support the conclusion that a concentrated NaCl solution makes the egg shrink rather than swell.

Why Other Options Are Wrong:
Option b, "The egg will swell", would be correct for a hypotonic external solution such as pure water, not for a concentrated salt solution. Option c, "The egg will become harder", confuses mechanical hardness with osmotic effects; the membrane remains soft, only the volume changes. Option d, "There will be hardly any visible change", is incorrect because concentrated NaCl typically produces a noticeable shrinking effect over time as water leaves the egg.

Common Pitfalls:
A common mistake is to mix up the terms hypertonic and hypotonic and their effects on cells or tissues. Students sometimes assume that the presence of any solution around the egg will cause swelling, forgetting that direction of water movement depends on relative solute concentrations. Another pitfall is to overlook the role of the membrane and think only the dissolved shell matters. To avoid these mistakes, always compare the solute concentration inside and outside and then apply the rule that water moves from lower solute concentration to higher solute concentration across a semipermeable membrane.

Final Answer:
When the de shelled egg is placed in a concentrated NaCl solution, the egg will shrink because water moves out of it into the hypertonic salt solution by osmosis.