In this experiment, students determine what is meant by an alloy, produce their own (solder) and identify ow the alloy’s properties differ from those of its constituent elements.

Student sheet

In this practical I will be:

  • Using my scientific knowledge and understanding to explain what is meant by an alloy.
  • Producing an alloy (solder)
  • Analysing how the properties of my alloy differ from its constituent elements.
  • (Extra) Calculating the density of my alloy, and comparing this with the densities of the constituent elements.

Introduction:

You are working on a piece of jewellery for a client which requires several delicate pieces of silver to be joined together. You know that plumbers often use join metal pipes in peoples’ houses using a mixture of tin and lead, called solder.

Unfortunately, you don’t know what the ratio of metals is best, so like all good science-artists, you decide to investigate further…

Equipment per group:

  • Eye protection
  • Thermal protection gloves
  • Lead turnings (TOXIC) 2 g (In countries where lead is prohibited for use in school then the tin–silver–copper combination can be used as reliable and easy to work with as a replacement for the lead. If this is chosen as the route, then the formulation is 95.5% tin, 3.9% silver, 0.6% copper. It is known as SAC solder from the chemical symbols of each of the elements (Sn, Ag, Cu). In this case the quantities of metals per group should be 7.95 g tin, 0.325 g silver and 0.5 g copper. Ignore the tin quantity given below.)
  • Tin turnings 2 g
  • Carbon powder 2 g

Each working group requires:

  • Crucible
  • Pipe clay triangle
  • Bunsen burner
  • Tripod
  • Heat resistant mat
  • Spatula
  • Tongs
  • Casting sand- enough to fill a sand tray.
  • Metal sand trays or sturdy metal lids- 2, or white tile
  • Balance
  • Stirring rod

Method:

Wear eye protection.

Tie long hair back.

Open windows to provide adequate ventilation.

Wash hands after concluding the activity

Making the alloy:

  1. If using casting sand, fill one of the sand trays with casting sand and push your finger into it to make an indent. This is your cast.
  2. Use a balance to weigh out 1g of the tin turnings and keep to one side

If following the non-lead route then put 7.95 g of tin into the crucible first. Ignore 3 below.

  1. Weigh 1g of lead turnings into the crucible.
  2. Put the crucible onto a pipe clay triangle. Rest this on a tripod on a heat resistant mat and make sure that it is stable.
  3. Heat the crucible strongly with a Bunsen burner until the lead (tin) is molten- this should not take longer than 5 minutes.
  4. Add a spatula of carbon powder to the top of it to prevent a skin forming.
  5. Add the tin to the molten lead and stir with a stirring rod until both metals are molten and thoroughly mixed.

If following the non-lead route than you add 0.5 g copper and 7.95 g silver to the molten tin and carbon. Mix the molten metals thoroughly then follow 8 below.

  1. Move the Bunsen away from the tripod and put it onto a yellow flame.
  2. Wearing thermal protection gloves and eye protection, pick up the crucible using the tongs, and pour the molten metal into the cast or onto a ceramic tile.
  3. Take great care as you do this to avoid splashing or dripping of the molten metal.
  4. Let it cool down completely before you touch it.

Testing the alloy:

Hardness testing

  • Try to scratch the alloy with the metals that made the alloy and with the alloy itself.
  • Also try to scratch each metal with the other metals.
  • The one which does not scratch the other is the hardest.
  • Which metal is the hardest?
  • Is the alloy harder or softer than either of the metals?

Density testing

  • Hold the metals in one hand and the alloy in the other.
  • Which seems to be the heaviest/most dense?

Melting-point testing

  • Put the alloy, a piece of each metal, both about the same size as the alloy, 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 the metals have melted, stop heating.
  • Which metal melts the first?
  • Which metal melts second?

Comparing all the answers from the tests make some deductions about the way the properties of the alloy are the same or different from the starting metals.

Going further:

  • Find the mass of a piece of the alloy.
  • Work out the volume of the same piece by the displacement method.
  • Then calculate the density using the formula:

Density\; =\; \frac{Mass}{Volume}

Compare the density of the solder alloy with the density of the starting metals.

Try repeating the activity but varying the proportions of metals to tin and see if it makes a difference to the properties of the solder.

Theory:

In everyday language, we often use the word metal to describe what an alloy is. In its scientific usage, the term metal means a metallic element.

A mixture of metallic elements is called an alloy. So, iron (an element) is a metal; but steel (a mixture of iron with other elements such as carbon, nickel, chromium, etc.) is an alloy.

An alloy is a mixture of either pure or relatively pure chemical elements, forming an impure substance retaining the characteristics of a metal.

An alloy is distinct from an impure metal, such as wrought iron, in that, with an alloy, the added impurities are usually desirable and will typically have some useful benefit.

Alloys are made by mixing two or more elements; at least one of which being a metal. This is usually called the primary metal or the base metal, and the name of this metal may be the name of the alloy. The other constituents may or may not be metals but, when mixed with the molten base, they will be soluble, dissolving into the mixture.

When the alloy cools and solidifies (crystallizes), its properties will often be quite different from those of its individual constituents.

The alloy solder can be made by heating together the metals lead and tin. Tin/lead solders, are also called soft solders, contain tin concentrations between 5% and 70% by weight.

Alloys commonly used for electrical soldering are 60:40 tin/lead (Sn/Pb). This type of solder melts at 188°C

In plumbing, metal pipes are joined together with an alloy called solder. In the past, a high proportion of lead was used in the alloy, typically 50:50 tin: lead. This made the alloy solidify more slowly, so that it could be wiped over the joint to ensure it was watertight, before soldering.

When the significance of lead poisoning was fully appreciated lead water pipes were no longer used, and copper pipes were used instead. But lead solder was still used until the 1980s. More recently, research has shown that even small amounts of lead can be detrimental to health, so lead in plumbing solder was replaced by silver (food grade applications) or antimony, with copper often added, and the proportion of tin was increased.

Teacher and Technician sheet

In this practical students will:

  • Use their scientific knowledge and understanding to explain what is meant by an alloy.
  • Produce an alloy (solder)
  • Analyse how the properties of solder differs from its constituent elements.
  • (Extra) Calculate the density of their alloys, and compare this with the densities of the constituent elements.

Introduction for Teachers:

This activity could open with a discussion about alloys.

  • Do they know what an alloy is?
  • What they are used for?
  • Where we might see alloys in the world around us?

This is designed to gain some perception of the ideas students have about glass as a material. For reference the following notes might help. The key words for this activity are:

  • Mixture;
  • Metal;
  • Element;
  • Crystal.

In its scientific usage, the term metal means a metallic element. An alloy is a mixture or solid solution composed of a metal and another element. But in everyday language, we often use the word metal to describe what an alloy is.

That is because a mixture of metallic elements is called an alloy. Iron is a metal element. Steel is a mixture of iron with other elements such as carbon, nickel, chromium, etc. and so is an alloy.

An alloy is a mixture of either pure or relatively pure chemical elements, forming an impure substance retaining the characteristics of a metal. Alloy constituents are usually measured by mass.

An alloy is distinct from an impure metal, such as wrought iron, in that, with an alloy, the added impurities are usually desirable and will typically have some useful benefit.

Alloys are made by mixing two or more elements; at least one of which being a metal. This is usually called the primary metal or the base metal, and the name of this metal may be the name of the alloy. The other constituents may or may not be metals but, when mixed with the molten base, they will be soluble, dissolving into the mixture.

Unlike pure metals, most alloys do not have a single melting point; rather, they have a melting range in which the substance is a mixture of solid and liquid. When the alloy cools and solidifies (crystallizes), its mechanical properties will often be quite different from those of its individual constituents.

A metal that is normally very soft and malleable, such as aluminium, can be altered by alloying it with another soft metal, like copper. Although both metals are very soft and ductile, the resulting aluminium alloy will be much harder and stronger.

Adding a small amount of non-metallic carbon to iron produces an alloy called steel. Due to its very-high strength and toughness (which is much higher than pure iron), and its ability to be greatly altered by heat treatment, steel is one of the most common alloys in modern use. By adding chromium to steel, its resistance to corrosion can be enhanced, creating stainless steel, while adding silicon will alter its electrical characteristics, producing silicon steel.

Hence alloys may be a homogeneous solid solution, a heterogeneous mixture of tiny crystals, a true chemical compound, or a mixture of these. Alloys are used more extensively than pure metals because they can be engineered to have specific properties. This leads to a number of definitions of different types of alloys:

  • amalgam an alloy containing mercury;
  • eutectic mixture a mixture of substances having a melting point lower than that of any of its components;
  • microstructure the fine structure of a pure metal or alloy, as revealed by magnifications of 25x or greater.

The alloy solder can be made by heating together the metals lead and tin.

In countries where lead is prohibited for use in school then the tin–silver–copper combination can be used as reliable and easy to work with as a replacement for the lead. If this is chosen as the route, then the formulation is 95.5% tin, 3.9% silver, 0.6% copper. It is known as SAC solder from the chemical symbols of each of the elements (Sn, Ag, Cu). In this case the quantities of metals per group should be 7.95 g tin, 0.325 g silver and 0.5 g copper.

Tin: lead solders, also called soft solders, with tin concentrations between 5% and 70% by weight. Alloys commonly used for electrical soldering are 60:40 Tin: lead (Sn: Pb) which melts at 188°C.

In plumbing in the past, a high proportion of lead was used, commonly 50:50. This made the alloy solidify more slowly, so that it could be wiped over the joint to ensure it was watertight, before soldering. Lead water pipes were displaced by copper when the significance of lead poisoning was fully appreciated but lead solder was still used until the 1980s. Since even small amounts of lead have been found detrimental to health lead in plumbing solder was replaced by silver (food grade applications) or antimony, with copper often added, and the proportion of tin was increased.

Curriculum Range:

This practical is really designed for secondary students and the aim is to gain some understanding of the way the materials are used to create artefacts. It links with:

  • Setting up simple practical enquiries, comparative and fair tests;
  • Reporting on findings from enquiries and observations, including oral and written explanations, displays or presentations of results and conclusions;
  • Using straightforward scientific evidence to answer questions or to support their findings;
  • Build a more systematic understanding of the chemistry of metals and alloys by exploring the way metals can be used to make a different substance with useful properties;
  • Ask questions and develop a line of enquiry based on observations of the real world, alongside prior knowledge and experience;
  • Use appropriate techniques, apparatus, and materials during laboratory work, paying attention to health and safety;
  • Make and record observations using a range of methods for different investigations; and evaluate the reliability of methods and suggest possible improvements;
  • Present observations using appropriate methods;
  • Interpret observations and identify patterns using those observations to draw conclusions;
  • Present reasoned explanations, including explaining data in relation to predictions and hypotheses;
  • Learn about the concepts of mixture, metal, element and crystal.

Hazard warnings:

Wear eye protection.

Tie long hair back.

Wash hands after concluding the activity

  • Lead, Pb(s), (harmful if swallowed or inhaled, reproductive toxin, Specifit Target Organ Toxin on Repeated Exposure)- see CLEAPSS Hazcard/SSERC hazardous chemicals database
  • Tin, Sn(s) - see CLEAPSS Hazcard/ SSERC hazardous chemicals database. (Low hazard)
  • Carbon, C(s) - see CLEAPSS Hazcard/ SSERC hazardous chemicals database.  (probably an eye / respiratory irritant)

It is a useful precaution to check the tongs since some tongs in schools do not grip well and the hinges often stick. Ask your technicians to check them before starting the experiment.

Casting sand may be available from the school Design and Technology or art departments. If unavailable from these departments it is possible to use a ceramic tile (old bathroom tile) for small quantities. If a tile is used, a sand tray will not be required.

Materials (Equipment):

  • Eye protection
  • Thermal protection gloves
  • Lead (harmful if swallowed or inhaled, reproductive toxin, Specifit Target Organ Toxin on Repeated Exposure) 2 g
  • Tin 2 g
  • Carbon powder (eye/respiratory irritant) 2 g

Each working group requires:

  • Crucible
  • Pipe clay triangle
  • Bunsen burner
  • Tripod
  • Heat resistant mat
  • Spatula
  • Tongs
  • Casting sand
  • 2  Metal sand trays or sturdy metal lids or white tile
  • Balance
  • Stirring rod

Technical Notes:

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.

In countries where lead is prohibited for use in school then the tin–silver–copper combination can be used as reliable and easy to work with as a replacement for the lead. If this is chosen as the route then the formulation is 95.5% tin, 3.9% silver, 0.6% copper. It is known as SAC solder from the chemical symbols of each of the elements (Sn, Ag, Cu).

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.

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 subjective.

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

Results:

The metals melt (within 5 minutes) and, when the other metals are added to it, they fuse in the carbon powder.

The alloy is easy to produce in a crucible on a pipe clay triangle.

If the alloy has fused properly it can be separated from the carbon when cooled and then weighed. There may be some loss due to smaller parts not fusing properly or being left in the carbon powder.

The alloy can be left to cool in the sand/crucible or placed into a beaker of water. Drain the excess water and dry using filter paper.

The alloy is harder than the constituent metals and takes longer to melt than lead or tin.

Taking the work further:

To more accurately determine the density of the solder.

To add in some mathematics you could get the students to weigh the alloy to find the mass. Then to work out its volume by displacement of water in a Measuring cylinder or a Eureka can.

They could then calculate the density from the formula:

Density\; =\; \frac{Mass}{Volume}

They can then compare the density of the solder alloy with the lead and tin they added.

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