Common experiments

The following are common scientific experiments and the results thereof.

Baking soda/vinegar

A classic involves pouring/dumping/mixing vinegar and baking soda. Water is also a common ingredient.

Results

The mixture foams up as a result of the carbon dioxide that is produced. This effect is often used to make a model of an erupting volcano. Also see chemical volcano.

Chemical explanation

Acetic acid (HC₂H₃O₂ (or C₂H₄O₂ in solution), from the vinegar) reacts with aqueous sodium bicarbonate (NaHCO₃), forming sodium acetate (NaC₂H₃O₂) and carbonic acid (H₂CO₃). As also happens in carbonated water, the carbonic acid then dissociates into water and carbon dioxide (H₂O, and CO₂).

Chemical Balancing

The reaction is already balanced, no coefficients are needed.

Hydrogen generator

Pieces of metal, i.e. steel wool or magnesium ribbon, are placed in a diluted strong acid; the resulting hydrogen can be captured in one of the following ways, based on the equipment available:

*An inverted siphon leading to a bath of soapy water

*An inverted, graduated cylinder to measure volume

*An inverted vessel with a tap. If the metal is held at a fixed height, and the tap is closed, hydrogen will force the acid down out of the vessel, stopping the reaction until the tap is opened.

Results

Soap bubbles will float in air and (if small) can be lit on fire.

Based on the height of water in a graduated cylinder, the pressure can be calculated. This and the measured volume can be used with the ideal gas law to find the number of moles of hydrogen, and this can be compared to the original mass of the metal to find its formula weight.

The self-regulating tap is a good example of negative feedback, and the hydrogen can be put to other uses.

Chemical explanation

Metal ions displace hydrogen from the acid. Using magnesium and hydrochloric acid as an example, the reaction is Mg + 2HCl = MgCl₂ + H₂.

Crystallization

A sample of polyethylene terephthalate (identified by a resin identification code of 1) can be crystallized by heat. Translucent PET is not suitable for this experiment.

A flame is held several inches below a soft drink bottle and slowly brought closer, with care taken not to burn the material.

Results

Once the temperature of the bottle is high enough, the polymer will shrink and become opaque.

Chemical explanation

PET in its natural state is a crystalline resin. Manufacturers are able to produce clear products by rapidly cooling molten polymer to form an amorphous solid. Like glass, amorphous PET forms when its molecules are not given enough time to arrange themselves in an orderly fashion as the melt is cooled. At room temperature the molecules are frozen in place, but if enough heat energy is put back into them, they begin to move again, allowing crystals to nucleate and grow.

Because the crystalline structure is denser than the amorphous state, and due to surface tension, the plastic will seem to shrink.

Like most materials, PET tends to produce many small crystallites when crystallized from an amorphous solid, rather than forming one large single crystal. Light tends to scatter as it crosses the boundaries between crystallites and the amorphous regions between them. This scattering causes any crystalline regions in the PET to become opaque or translucent.