Thread: Chem lab problem~

Four bottles, labeled A through D, each containing about 5 grams of finely powdered white substance are found in a lab. Near the bottles are four labels specifying high purity and indicating that substances are glucose (C6H12O6), sodium chloride (NaCl), aluminum oxide (Al2O3), and zinc sulfate (ZnSO4). These labels belong to the bottles and each bottle has only one substance. Describe tests you would use to help find which label goes to which bottle. Then figure out what substance actually goes to which of the four bottles and explain.


I am REALLY confused on how to figure out what substance goes to which and what procedure i would take. PLEASE HELP! THANK YOUU

sodium chloride can be be dissolved in water and will then freeze at -5 C and for the glucose: To test for a polysaccharide (complex sugar), Iodine is used. Iodine turns from brown to black in the presence of polysaccharides.

sorry, can't help with the rest

U can identify aluminium oxide by it's insolubility in water.

Glucose is a monosaccharide, not a polysaccharide. Starch is a polysaccharide and can be identified with the iodine test. To test for glucose you would use Benedict's solution.

for the ZnSO₄, make a solution, then add NaHCO₃, and a precipitate of Zn(HCO₃)₂ will form. None of the other things will form a precipitate with the HCO₃^-

Originally Posted by Chemboy

for the ZnSO₄, make a solution, then add NaHCO₃, and a precipitate of Zn(HCO₃)₂ will form. None of the other things will form a precipitate with the HCO₃^-

Zn(HCO₃)₂ is water soluble. Maybe you intended ZnCO₃ (precipitated with Na₂CO₃).

I would taste them. The salt will taste like salt, the glucose will taste like sugar, and the zinc sulphate is toxic. So you will know what they all are.

Originally Posted by Dantak

I would taste them. The salt will taste like salt, the glucose will taste like sugar, and the zinc sulphate is toxic. So you will know what they all are.

Ok, would you please taste these white crystals first? (You don't know but I gave you ZnSO₄*7H₂O! )
Ok, actually, you can taste small amounts of it, it won't harm you at all. It has a slightly different taste than NaCl, but you have to know this difference!

Glucose has a dull appearance. Salt has a shiny appearance. Salt and Glucose are both souble in water. Once dissolved in water, the salt solution would have a much greater conductivity (more ionic).

The ZnSO4 would be slightly soluble. It too, when placed in water, would conduct electricity. The Al2O3 being very insoluble would be less conductive in water.

A couple more easy tests. Look at the crystal structure under a microscope. Use a refractometer to determine the refractive index. Do a hardness test. Al2O3 is very hard and used as a polishing agent.

considering this post is nearly 6 months old now and it was a homework qu. I think we can pretty much drop it