Last minute question feedback [QCE chem]

[Bri MT]

Hi,

sharing a question I was asked here so that it might benefit others as well:

‘Explain how the cleaning action and solubility of soap in hard water is related to their chemical structure’ (3 marks)

My answer:
Hard water contains metal cations such as Mg2+ and Ca2+. When soap is used in hard water the metal cations neutralise the charge of the hydrophilic end of the soap molecules. This forms an insoluble compound and renders the soap molecule ineffective in its cleaning action as it is no longer able to dipole interactions with water molecules. 

First of all, nice work on identifying the characteristics of hard water, looking at the structure and properties of the molecules, and then explaining the conclusion. Good structure and sequencing here with an appropriate level of detail for 3 marks. The only critical feedback I would have here is this last part where you say "as it is no longer able to dipole interactions with water molecules" - I'm assuming you meant something like "experience dipole-dipole bonding with water molecules?"

For reference I'm putting in an image from the 2012 HSC examiners report NOTE THAT THE QUESTION WAS DIFFERENT to help you be confident in what you can discuss for this:



If you really want to get into soap practice you could also look at:

> HSC 2016 question 8 (experimental design focused)
> HSC 2018 question 31 (soap structure and action)


Good luck & I hope this helps!


Edit:

I have set up notifications on this post so I will know if any replies are posted. If you have a chem question you want answered before the exam tomorrow reply to this post. To reply you will need to be signed into your free atarnotes account, you can quickly sign up here. To embed an image upload it to imgur and get the bbcode share link.

[A.Rose]

Thanks so much!
Would you be able to check my answer for this questions as well (see attached)?
Thanks!

[Bri MT]

Thanks so much!
Would you be able to check my answer for this questions as well (see attached)?
Thanks!

No worries!

The sample response did well in outlining the forces present but imo your response was more targeted to the actual question and would also be a high scoring one. The second half of the sample answer alluded to similar boiling points but did not explain this as well as your response.