Design and carry out an experiment to determine the specific identity of the substance in each of si — Qualitative Analysis Chemistry Question
Problem Context
Design and carry out an experiment to determine the specific identity of the substance in each of six numbered vials. Each vial contains one of these substances.
- baking soda (sodium bicarbonate)
- borax (sodium tetraborate)
- cornstarch (polysaccharide of glucose)
- Epsom salts (magnesium sulfate)
- powdered sugar (sucrose and corn starch)
- washing soda (sodium carbonate)
Using the materials provided (including distilled water, 10% HCl, 10% NaOH, 2% iodine solution, bromothymol blue, and phenolphthalein), devise and carry out an experiment to correctly determine the contents of each vial.
Give a brief description of your experimental plan.
Model Answer
• use equal amounts of substances (i.e. end of a spatula, small amount in the bottom of test tube or well)
• add a set volume of reagent (i.e. 0.25 mL) or add the reagents dropwise
• stir samples and observe color changes, formation of gas, formation of precipitate
Record your data and other observations.
Model Answer
• Clear data table or flow chart indicating substances combined and the results of the tests performed
• observations (color changes, production of gas, solubility, formation of precipitate)
Identify the substance in each unknown vial, giving a brief justification for that choice.
Model Answer
1: Cornstarch (Insoluble in water, interacts with starch and turn blue-black)
2: Baking Soda (Produces a gas in the presence of an acid (HCl or vinegar))
3: Epsom salts (Forms a white precipitate in the presence of hydroxide)
4: Powdered sugar (Soluble in water, interacts with starch and turns blue-black (smaller color change than cornstarch – indicating less starch present in the sample))
5: Washing soda (Produces a gas in the presence of an acid, basic in the presence of phenolphalein (darker pink than sodium bicarbonate))
6: Borax (Soluble in water and aqueous acids but no gas produced in the presence of acid, basic in the presence of phenolphalein)