# Homework Help: Question about light absorbance with liquid solutions

Tags:
1. Apr 4, 2016

### RoboNerd

1. The problem statement, all variables and given/known data

A solution of Co+2 ions appears red when viewed under white light. Which of the following statements is true?

A) a spectrophotometer set to the wavelength of red light would read a high absorbance
b) if the solution is diluted, the amount of light reflected by the solution will decrease
c) all light with a frequency that is lower than that of red light will be absorbed by it
d) electronic transmissions within the solution match the wavelength of red light.

2. Relevant equations
none

3. The attempt at a solution

I originally put down B as if I have a diluted solution, then my light absorptivity will change by beer's law. However, the text says that the amount of light reflected is the same, regardless of concentration.

I do not understand why the text says so and why D is the right answer (and why the other ones are wrong). Could you please help?

2. Apr 5, 2016

### ehild

If a solution absorbs a range of wavelength from white light you will see the complementary color. A pink solution of Co2+ complex absorbs in the blue/green range of wavelength and you see the solution pink.

The absorption is caused by electronic transitions from lover levels to higher ones, so the color corresponds to the wavelengths of those photons which cause these transitions.

Last edited: Apr 5, 2016
3. Apr 5, 2016

### RoboNerd

OK, I see that absorption is caused by electronic transitions.

4. Apr 6, 2016

### ehild

Look at the spectrum and color card I attached before. Compare the absorbance between 650-700 nm with that at 450-550nm. What is the color of light at these wavelength ranges?
So are the statements true:
A) a spectrophotometer set to the wavelength of red light would read a high absorbance
What is the absorbance for red light, with wavelength of 650 nm? Is it high, compared to the absorbance at 550 nm?

B) if the solution is diluted, the amount of light reflected by the solution will decrease
The low absorbance of a dilute solution does not influence the reflection appreciably.

C) all light with a frequency that is lower than that of red light will be absorbed by it
Is the wavelength lower or higher than that of the red light if the frequency is lower?

5. Apr 10, 2016

### RoboNerd

For A, the absorbance of red light is significantly lower than that of the 500-550 nm range.

B) Why is this the case that "low absorbance of a dilute solution does not influence the reflection appreciably"?

C) Wavelength will be higher, so there will be lower absorbance as we are going towards the right on the absorbance graph where there is a higher wavelength.

6. Apr 10, 2016

### ehild

The reflectance depends on the complex refractive index n-ik, where n is the real refractive index and k is the imaginary part of it. The absorbance is proportional with k. In the problem, the refractive index of the solution was a mixture of those the cobalt compound and of the solvent, perhaps water. The refractive index in such cases is mainly determined by that of the solution, and is not changed appreciably by further dilution.

7. Apr 10, 2016

### RoboNerd

Ahh. That explains a lot. So if I have a unique type of solution with substances A and B, then I will have a unique refractive index, and the amount of light reflected will be approximately the same!

Now, what do you think about the rest of my "arguments" for why the other answer choices are right/wrong?

Thanks a lot!

8. Apr 10, 2016

### ehild

The other statement were true, so what about the questions A, C, D, are they true or false? Is the absorbance high for red light? Is all light with frequency lower than that of the red light absorbed? In case B, you can not know, how the reflection exactly changes by the dilution, but it does not change much. As for D, I do not know what "match" means. The electronic transitions cause the absorption, but the transition energy corresponds to the blue-green right. The material does not absorb red light.
I still have some doubt if the solution appears red in transmitted light of in reflected light. The colors can be complementary in case of a thick layer of solution.

9. Apr 12, 2016

### RoboNerd

Aha. I see now how to approach this question. Thanks for the time so much!

10. Apr 12, 2016

### RoboNerd

I think that the electronic transmissions correspond to the solutions color, but the solutions color is red, so the solution is colored red.

11. Apr 12, 2016

### ehild

The electronic transition causes the absorption, The absorbed light is not seen, it is missing (or gets weaker) in the spectrum. If red line is absorbed, the solution looks green. If the solution is red, green line is absorbed.