Introduction

Whilst aluminium is mostly a metal, and displays many of the properties you would expect of a metal, it also has some non-metallic character. In this experiment you will look more closely at these properties by converting some scrap aluminium from a drinks can into aluminium sulphate (a.k.a. alum) in a process similar to that used industrially to recover aluminium from scrap metal; this is a two step reaction involving first converting the aluminium to aluminium hydroxide. You will then complete an analysis of both the alum and of aluminium nitrate to show off the amphoteric properties or aluminium and its salts.

Materials and Equipment

• 5×5 cm square of aluminium from drinks can
• 6 M potassium hydroxide solution (CORROSIVE)
• 6M ammonia solution (CORROSIVE)
• 9 M sulphuric acid solution (CORROSIVE)
• 1M aluminium nitrate solution
• 0.5 M barium chloride solution
• Sand-paper
• Heating apparatus
• Thermometer
• Buchner filtration apparatus
• Beakers of various sizes
• Ice
• Indicator paper
• Nichrome wire and tongs

Part 1: Preparation of Alum/Quantative Analysis (Calculation of % Yield)

1. Cut a 5cm x 5cm square piece of aluminium from a scrap aluminium can and sand the paint off using steel wool.  
2. Cut this piece into smaller pieces (about 0.5 cm long) and weigh out approximately 1g.  Record the weight to 3 decimal places. 
3. Place the aluminium in a 400 cm³ beaker and add 15 cm³ or water followed by 35 cm³ of 6M KOH_(aq).
   • 6M KOH is highly corrosive! Wash affected area immediately in event of skin contact. 
4. Place beaker on a gauze on a tripod and heat gently, using a thermometer to keep the temperature between 80-90^OC.
   • Hydrogen gas is evolved in this step so ensure the flame does not go near the top of the beaker
5. When the bubbles have stopped, remove from the heat, i.e., hydrogen is no longer evolved.
6. Vacuum filter the solution (using a Buchner funnel) and save the filtrate, observe the solid, keep to one side for a while and observe again at the end of the lesson, before disposing of it.
7. Carefully rinse your graduated cylinder, pour out 25-30 cm³ of 9M H₂SO_4(aq) and then add slowly to the filtrate, taking careful note of any changes.
   • 9M sulphuric acid is highly corrosive. Wash affected areas immediately in event of skin contact.
8. Heat gently while stirring until the solution becomes clear. Boil the solution down to a volume of about 45 cm³.
9.  While the solution is boiling, prepare an ice bath by filling a bowl half-way with ice and then adding water until the bowl is three-quarters full.
10. After the “boiling off”, let the beaker cool a little and then place it in the ice bath. Crystals of alum should form. Allow to cool for 15 minutes or as long as time allows.
11. After 15 minutes, vacuum filter the product and wash with 10 cm³ of ethanol. 
12. Allow to dry for a few minutes. Dry completely in the microwave at less than 50% power in 10 sec intervals until the weight no longer changes. Dry overnight in drying oven if this is not possible.
13. Record the final mass of crystals obtained and describe them.

Part 2: Qualitative Analysis of Alum

Test the solubility, acidity and reaction with barium chloride:

• Use a spatula to transfer a few of the alum crystals (about 5 mg) to a watch glass. Add 3 drops of water to the crystals. Stir gently until the crystals dissolve.
• Use a small piece of indicator paper to see whether the solution is acidic, basic, or neutral.
• Now add 1 drop of 0.5 M BaCl₂   (barium chloride) to the solution. Record your observations.

Conduct a flame-test on the alum

A reliable method for detecting the presence of potassium is the flame test. Begin by twisting a piece of nichrome wire into a small loop at one end. Grip the wire securely with a pair of test-tube tongs.

• Clean the Wire Loop Hold the nichrome loop in the hottest part of a Bunsen burner flame to remove any residual impurities. Continue heating until the wire glows red hot.
• Collect the Sample Once the loop is red hot, remove it from the flame and immediately touch it to a small cluster of potassium-containing crystals. A few crystals should adhere to the loop.
• Perform the Flame Test Slowly bring the loop with the crystals back into the flame. Observe the color and intensity of the flame carefully. Hold the loop in the flame for at least 5 seconds, or until the sample glows.
• Record Observations Note the flame color and any other visual changes. Potassium typically produces a pale lilac or light purple flame.
• Cool the Wire Remove the wire from the flame and place it on a heat-proof mat to cool.

Part 3: Acid-Base Properties of Aluminium Compounds

1. Clean 3 test-tubes and label them 1 to 3. 
2. Tube 1: Place 10 drops of 1M aluminium nitrate solution and 2 drops of 6.0 M sodium hydroxide solution, mix well and record your observations on your lab report Then add more sodium hydroxide solution to tube 1 until a reaction is observed (around 7 drops). Mix well. Record your observations.
3. Tube 2: Place 10 drops of 1M aluminium nitrate solution and 2 drops of 6.0 M potassium hydroxide solution, mix well and record your observations on your lab report Then add 4 cm³of 6M HCl and record your observations.
4. Tube 3: Add 3 cm³ of 6M ammonium hydroxide, i.e aqueous ammonia to some of your alum and record your observations.

Analysis

• Part 1: Write balanced equations to describe the two steps in the reaction.
• Calculate the % yield you obtained.
• Explain how this experiment shows off the amphoteric properties of aluminium.
• Part 2:
• Explain all observations using equations where relevant.
• Part 3:
• Explain all of your observations, explaining how they demonstrate aluminium’s amphoteric behaviour.

• Practical Science