Recently, i received several enquiries and questions from students on the color of copper hydroxides and copper carbonates.

I understand why the confusion. This is because in the local Secondary Schools, many of the Science (Chemistry) teachers are only showing the students of the Blue Color PPT when they test for the Cu2+ cation using alkaline solution such as dilute NaOH or aqueous Ammonia Solution. The Blue Color PPT are due to the formation of insoluble Copper Hydroxide which is very distinct.

Many of students are confused and thought all Copper Salts have a Blue Color PPT. This is certainly NOT TRUE.

Summary on Color & Form of Common Copper Salts:
* Copper Hydroxide –> Insoluble Blue Color Precipitates (PPT)
* Copper Carbonate –> Insoluble Green Color Powders
* Other soluble copper salts –> Blue Color Solutions

Below is the appearance of Copper Carbonate powder:

Below is a Table that can help us to identify cations in an unknown solution. This is very important in Qualitative Analysis Tests.

Besides SPA and Practical Examinations where the following table will be provided, you will need to memorize the table in order to answer QA questions that come out for Multiple-Choice Questions as well as Structured Questions.

But no fret, it is not that difficult to memorize it. Start by looking at Cu2+, Fe2+, Fe3+ and NH4+ because they have very distinct colours changes:

• Cu2+ –> Blue PPT
• Fe2+ –> Green PPT
• Fe3+ –> Reddish Brown PPT
• NH4+ –> Gives Pungent Ammonia Gas upon heating with NaOH
   (Click Image to have a better view)

PS: Do print a copy and paste it at the end of your notebook if necessary.

Try out this question on Qualitative Analysis / Precipitation and share with us your answers & reasoning at the “Comments” section right below this post.

Question:
X forms a white ppt with dilute sulphuric acid and also with silver nitrate. What could solution X be?
A barium chloride
B magnesium chloride
C barium nitrate
D magnesium sulphate

My Suggested Answers:
If you are not very familiar with the solubility of the different types of salts, the best way to approach such question is to draw a simple table and write down the possible reaction products. (Click on Image to have a clearer view)

PS: Check out “Comments” below for suggested answers.

Below is a question contributed by a local chemistry student (Student X) and an avid reader of this Chemistry Blogsite.

This will be a question that combines Mole Calculations & Qualitative Analysis topics. Give it a try!

Question:
In an experiment, 4.0cm^3 of 1.0mol/dm^3 aqueous copper(II) sulphate was mixed with 8cm^3 of 1.0mol/dm^3 aqueous sodium carbonate:

CuSO4 + Na2CO3 –> Na2SO4 + CuCO3

What did the reaction vessel contain when the reaction was completed?
A. A colourless solution only
B. A green precipitate and a blue solution
C. A green precipitate and a colourless solution
D. A blue precipitate and a blue solution

Student X’s suggested answer:
What I think: copper sulphate is the limiting reactant, hence sodium carbonate is in excess. Hence, when the reaction is completed, only excess sodium carbonate remains. So, the solution remains colourless and my answer is C.

My Suggested Answer:
I agree that the Answer is C. At the end of the reaction, we will see a Green Precipitate of Copper Carbonate(s) which is insoluble in water. There will also be a colourless solution that is contributed by both the product Sodium Sulphate(aq) and the excess reactant Sodium Carbonate (aq) which are soluble in water. (Click the image below to have a clearer understanding)

PS: Check out “Comments” below for Suggested Answers.

Another of Student X question to sean@simplechemconcepts.com to be shared with the world!

Question:
A certain solid has the following properties.
• It is insoluble in water.
• It gives off a gas when heated.
• It gives off a gas when added to dilute sulphuric acid.

Which of the following is the solid?
A. Sodium carbonate
B. Copper(II) carbonate
C. Iron (II) oxide
D. Zinc hydroxide

Student X’s suggested answer:
“What I think: Sodium carbonate is soluble in water. The two oxides, Iron (II) oxide and Zinc hydroxide reacts with dilute sulphuric acid to give only salt and water. Thus, my answer is B Copper(II) carbonate.”

DETERMINATION:
“Determine that the thing can and shall be done, and then we shall find the way”
- Abraham Lincoln

Check out “Comments” below for suggested answer.

Another question sent to sean@simplechemconcepts by ‘Student X’ to be shared with all “chemists”. Go work it out!

“If you don’t practice it, You Loose It”
- Sean Chua

Question:
When a colourless solution S was added to aqueous sodium hydroxide, no visible change was observed. When aluminum powder was added, a pungent gas which gave a brown precipitate when bubbled into aqueous iron (III) chloride was obtained. What ions could be present in S?

• A. Na+ , NH4+
• B. K+ , NO3-
• C. Na+ , Br-
• D. Ca2+ , NO3-

Student X’s suggested answers:
What I think: B and D is out as a white ppt is formed with aq NaOH. Hence, either A and C. But I don’t know which one to choose between the two.

PS: Check out “Comments” below for suggested answers.

Halo students and avid readers,

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?

“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 (more…)