Making solder as an alloy of tin and lead

In this experiment, students make their own alloy, heating lead and tin together to produce solder. They then investigate three properties of the alloy and compare these with lead, including hardness, melting point and density.

The most likely incident in this experiment is a student burning themselves, so warn them about the equipment being hot.

If students are not sure how to use tongs correctly (pouring molten metal can be hazardous and lead to burns) it is worth demonstrating how to use them safely. Some tongs in schools do not grip well. Technicians must check them before starting the experiment.

Lead is a toxic metal and if it is heated for too long or too high above its melting point it can start to give off fumes. Ensure that the laboratory is well-ventilated. Warn students not to breathe the fumes given off by their experiment and tell them to heat the metals for the shortest time possible to get them to melt. Wash hands after handling lead.

The experiment should be completed in 40 minutes. However, this depends on the experience and practical abilities of the students. Further time may be required for allowing the equipment to cool before it can be put away.

Equipment

Apparatus

• Eye protection
• Thermal protection gloves
• Each working group requires:
• Crucible
• Pipe clay triangle
• Bunsen burner
• Tripod
• Heat resistant mat
• Spatula
• Tongs (see note 6 below)
• Casting sand (see note 7)
• Metal sand trays or sturdy metal lids, x2 (see note 7)
• Balance (no decimal places needed)

Chemicals

• Lead (TOXIC, DANGEROUS FOR THE ENVIRONMENT), about 2 g
• Tin, about 2 g
• Carbon powder, about 2 g

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Lead, Pb(s), (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC056. 
• Tin, Sn(s) – see CLEAPSS Hazcard HC102A. 
• Carbon, C(s) – see CLEAPSS Hazcard HC021.  
• Some tongs in schools do not grip well and the hinges can stick. Technicians must check them before starting the experiment.
• Casting sand may be available from school Design and Technology departments. If unavailable, a ceramic tile (eg old bathroom tile) could be used instead. If a tile is used, a sand tray will not be required.

Procedure

Making the alloy

1. Weigh out 1 g each of lead and tin. Put the lead into the crucible, but keep the tin to one side.
2. If using casting sand, fill one of the sand trays with casting and push your finger into it to make an indent. This is your cast.
3. Put the crucible onto a pipe clay triangle. Make sure that it is stable on a tripod and mat.
4. Heat the crucible strongly with a Bunsen burner until the lead is molten. Add a spatula of carbon powder to the top of it to prevent a skin forming.
5. Add the tin and stir with a spatula until the metals are both molten and thoroughly mixed.
6. Move the Bunsen away from the tripod and put it onto a yellow flame. Wearing thermal protection gloves, pick up the crucible using the tongs, and pour the molten metal into the cast or onto a ceramic tile. NOTE: take great care as you do this to avoid splashing or dripping.
7. Let it cool down completely before you touch it.

Testing the alloy

1. To test hardness, try to scratch the alloy with the lead, and the lead with the alloy. The one which does not scratch is the hardest.
2. To test density, hold the lead in one hand and the alloy in the other. Which seems to be the heaviest/most dense?
3. To test melting point, put the alloy, a piece of tin and a piece of lead onto a sand tray. They should all be the same distance from the middle of the dish. Heat the dish gently in the middle. When two of the metals have melted, stop heating.

Teaching notes

The expected results of the tests are that the alloy is clearly harder and scratches the lead. The lead does not leave a mark on the alloy.

The density of the alloy should be less than that of the lead, but this test is fairly subjective. You could get the students to weigh the alloy and to work out its volume by displacement. They could then calculate the density (mass/volume) if you prefer a more accurate version.

The lead melts first, followed by the tin. The alloy has the highest melting point – demonstrating clearly very different properties from its constituent metals.

This experiment can stand alone as a demonstration of how the properties of a metal can be changed by alloying, or you could follow it up by asking students to explain the results of the hardness testing in terms of the structure of the metals.

A good answer includes reference to the layer structure of metals and how alloying can prevent the layers from sliding over each other, making it more difficult to change the shape of the metal. This results show that the alloy is harder than the pure metal as it is more difficult to change its shape and therefore to scratch it.

For those doing this as a fun experiment, it is possible to cast the alloy in a variety of shapes by changing the shape of the cast in the casting sand.