Experiment 6: Separating Solids

Date

8 March 2007 by Quest 1. Links in with Chapter 6 of our book.

Aim

To separate a mixture of three solid substances.

To get used to working with chemicals and lab equipment like test tubes and a meths burner.

Method

Put mixture in a test tube and use a funnel to add water to float the suspected sawdust. Plug with a bung and shake as required. Skim floating material from the top of water and drain by pouring through filter paper. Keep water for later. Place sample in microplate.

Clean remaining sample with more water and drain water. (Optional: Heat over meths burner to dry it out.) Place sample in microplate.

Put filtered water from earlier in a test tube. Use boiling chips to make the boiling smoother while evaporating the water over a meths burner. Place sample in microplate and remove boiling chips.

To determine if the white substance is sugar or salt use a metal spatula to hold a sample over a meths burner to see it it burns. Compare burnability with known samples of salt and sugar.

Equipment

Test tubes and bungs.
Cups and beakers.
Water, water dropper (known as a pipette).
Filter paper, funnel.
Meths burner, lighter, boiling chips.
Metal spatula, spoon.

Results/Observations

The mixture appears to be sawdust, small particles of stone (ie, coarse sand), and white crystals.

After adding water only a small amount of the suspected sawdust floated as the rest was trapped in the sand. The mixture needed to be shaken to get the sawdust away from the other substances to allow it to float. Most of the sawdust then floated. The sand sunk to the bottom. The white crystals dissolved.

After draining the water with the floating material off it was clear that it was sawdust.

The sand was left in the bottom of the test tube, and after washing the remaining sawdust away looked like wet sand.

The original filtered water retained from the mixture was cloudy and a pale yellow. It got yellower as it was boiled off. When it had been boiled off it left a white or off-white substance behind that looked like crushed sugar or salt. Some soot was deposited on the outside of the test tube.

When heating the substance directly over the flame it first turned brown, then black, then turned pale grey or white, but did not burn.

When a known salt sample was heated it crackled and popped but did not burn and did not change in appearance. As soon as the hot metal spatula touched the known sugar sample the sugar turned brown and melted then turned black and burned in the flame.

Conclusion

The three substances were sawdust, sand and salt. The last substance could not have been sugar as it did not burn.

Waterlogged sawdust doesn't necessarily float.

Doing experiments is fun. Boiling chips are important. More experiments are needed to get used to working with chemicals.

Discussion

All the elements in the periodic table were separated because of their different physical and chemical properties. Some elements have very similar properties so are very hard to separate.

To separate our mixture a physical property of each substance was used. Sawdust floats on water. Salt dissolves. Sand was left over because it is heavier than water and does not dissolve. If a fourth substance was added, iron filings, what is a physical property of iron filings that the others do not have that would allow us to separate the iron filings from the mixture?

Pykrete is a mix of sawdust and water ice. It melts very slowly due to low thermal conductivity and has strength closer to concrete than normal ice. Like concrete it has a high crush resistance, so a column just 25mm across could bear the weight of an ordinary car. During World War II the British had plans to make a huge "unsinkable" floating island of pykrete to use as an aircraft carrier. They worked out that even a direct torpedo hit would only make a small dent.