A glass of composition 85% SiO2 - 15% K2O is to be surface strengthened by ion exchange. To this end, two identical specimens approximately the size and shape of credit cards are soaked for the same amount of time at 850 ∘C. Sample A is soaked in NaCl and sample B is soaked in KCl. From the list below, select the statements that are true.

The yield strength of Sample A will decrease while the yield strength of Sample B will increase.
The yield strength of Sample A will increase while the yield strength of Sample B will decrease.
The yield strength of both Samples A and B will increase.
The yield strength of both Samples A and B will decrease.
The yield strength of Sample A will decrease. There will be no change in the yield strength of Sample B.

It's e. The last one.

To determine the effect of ion exchange on the yield strength of the glass samples, we need to consider the compositional changes that occur during the process.

During ion exchange, Na+ ions from NaCl or K+ ions from KCl will replace the K2O ions in the glass. Since Sample A is soaked in NaCl and Sample B is soaked in KCl, the Na+ ions will substitute the K2O ions in Sample A, and the K+ ions will replace the K2O ions in Sample B.

Based on this information, we can assess the impact on the yield strength of the glass samples:

1. The yield strength of Sample A will decrease while the yield strength of Sample B will increase.
This statement is true. The substitution of Na+ ions in Sample A reduces the strength of the glass structure, leading to a decrease in yield strength. On the other hand, the substitution of K+ ions in Sample B strengthens the structure, resulting in an increase in yield strength.

2. The yield strength of Sample A will increase while the yield strength of Sample B will decrease.
This statement is false. The substitution of Na+ ions in Sample A weakens the glass structure, causing a decrease in yield strength. The substitution of K+ ions in Sample B, however, reinforces the structure, leading to an increase in yield strength.

3. The yield strength of both Samples A and B will increase.
This statement is false. As explained earlier, the composition of Sample A (85% SiO2 - 15% K2O) is weakened by the Na+ ion substitution, resulting in a decrease in yield strength. Although Sample B experiences an increase in yield strength due to K+ ion substitution, Sample A does not exhibit a similar effect.

4. The yield strength of both Samples A and B will decrease.
This statement is false. The yield strength of Sample A decreases due to the Na+ ion substitution, but the yield strength of Sample B increases as a result of the K+ ion substitution. Thus, the statement is only partially correct.

5. The yield strength of Sample A will decrease. There will be no change in the yield strength of Sample B.
This statement is false. The yield strength of Sample A does decrease due to the Na+ ion substituting the K2O ions, weakening the glass structure. Sample B, however, experiences an increase in yield strength caused by the substitution of K+ ions for K2O.

Therefore, the correct statement to select from the list is:
- The yield strength of Sample A will decrease while the yield strength of Sample B will increase.