Friday, December 28, 2007

Chemistry Practical Question

This is one of the latest question sent in on Chemistry Practical. It's a good question which I think some students would love to find out the answers as well, isn't it? ;)

Open for discussion. Feel free to contribute!

I wanted to ask what's the correct/appropriate way to wash test tubes when you're having a Chemistry practical? Because i've faced shortage of time, and it's because i wasted a lot of time to wash my test tubes. Some students maybe more "kiasu" and they washed the test tubes multiple times to ensure it's clean.

For example, i've 6 test tubes, but i'm required to do 7 or 8 experiments, so i've got to wash the test tubes. However, the test tube i'm re-using again, may have traces of the previous solution left and thus, affecting the result of the current one. Please do advise me on the correct and advisable method to wash my test tubes!

Is it okay to wash my test tube just with plain tap water or distilled water? Or both? And i do believe most schools provide a brush, which can fit into the test tube to remove solid particles or ppt which hardens and stays in the test tubes, am i required to use it, or it can be ignored?

This is approximately how i wash my test tube

1. I empty the solution into the sink
2. I rinse the test tube with tap water first
3. Then, i rinsed it again with distilled water.

I've another question on Chemistry practical. My teacher once mentioned that if you're doing titration, e.g. you're given Solution P and Solution R, to put the respectives into the burette or pipette, she told me that if you're pouring Solution P into the pipette for titration, you can just simply wash it with Solution P and need not wash it with distilled or tap water anymore. How true is that?


Gary said...

What i think is we can actually forgo the rinsing of test-tube(tap-water) and just wash with distilled water directly.Distilled water is in fact cleaner than tap-water,right?

I think it is safer to wash with distilled water then putting in your solution.

Because in titration,everything MUST BE CLEAN!What i do for prac is just spam distilled water forgoing tap-water.Its simply a waste of time washing anything with tap water.(I use a lot of distilled water since i don't use water)

Correct me if I'm wrong!Thanks in advance.


ruth said...

Just food for thought, but I don't think most of you realise that, during our normal chemistry practicals, the distilled water provided on the table is usually tap water. It's just far too impractical and not economical to have so much distilled water on hand just for practices.
Seeing that our results are pretty accurate during our practices, so how bad can it really be?
At least that's my view for QA
In my two years of studying for Os, I've only seen the absence of cleaning the testtube with distilled water affecting my experiment once.

As for VA, during titration, it's wiser to do a thorough cleaning. I had the same problem bout the time management. But I still believe that doing a thorough, slow and steady job is a MUCH better idea.
After my Os and prelims, when we compared results to the teacher (in case you don't know, your results are compared to your teacher's who did it earlier, so technically it's wiser to take heed to your teacher's method, maybe ) mine was very accurate compared to others who kinda just raced through it.

BTW be careful of experiments using methyl orange. BE EXTREMELY CAREFUL BOUT THE COLOUR CHANGE. Super tricky. This year, they made the stop point change from red to orange(I think) instead of colourless to orange. Super frustrating and lots of people messed up their VA measurements cause of this.
Jia you, O level batch of 2008!

Sean Chua said...

Halo students and ex-students,
I am Sean, the co-author of this O Level Blog (for the Chemistry part)

All of you have highlighted some important points here with regards to the correct procedure for chemistry practical.

Let me sum it up with the most comprehensive & time effective method that i have came up with several teachers:

During Volumetric Analysis and Qualitative Analysis, what we are testing on are essentially the reactions between ions, i.e. Cations and Anions. And this is a very impt clue on how we should conduct the practical.

I would suggest the following steps if you need to wash the test-tubes cos u need to re-use:
1) Choose the test-tube that contains only solution (colored or colorless).
2) Pour the content into the sink.
3) Wash once with distlled water.
4) Use brush to scrup few quick times.
5) Use distilled water to wash tubes thoroughly with two more washings.
6) Ready for next test.

PS: Tap water should not be used as they contains Ions, and those ions present might interfere with your results. Distilled water has less of such problems.

PSS: Soap should not be used cos generally they are alkaline and contains the anion OH- hydroxide ions, which might interfere with results.

PSSS: Choos a tube that contains solution instead of one that contains PPT. This will prevent waste of time in washing tubes.

Hope you will find this information useful.

Btw, i have migrated to a new website to focus on O Level Chemistry. Do visit us there to find out more effective study methods and exam tips.

For this New Year, i have prepared a present for all. A FREE Video on Tips for Balancing of Chemical Equations. Go download for FREE at

Sean =)