Page 33 - Curriculum Visions Dynamic Book

Sodium sulphate
Also known as Glauber’s salt, sodium sulphate is commonly found dissolved in drinking water. Sodium sulphate occurs as thick evaporite beds and can be mined by pumping hot water down a well and pumping the dissolved sulphate back up.
Sodium sulphate is used instead of sulphuric acid for some processes, for example in making dyes, paper and glass, and to help molten metals flow (when used as a flux).
Sodium sulphate crystals melt at 32°C, meaning that they will melt at high room temperatures. On melting they absorb very large amounts of heat (the salt has a large latent heat). This means that sodium sulphate can be used in some forms of solar storage heating. The Sun’s rays can be used
to melt the sulphate, and later, as it cools and solidifies during the night, it releases its heat.
flux: a material used to make it easier for a liquid to flow. A flux dissolves metal oxides and so prevents a metal from oxidising while being heated.
latent heat: the amount of heat that is absorbed or released during the process of changing state between gas, liquid or solid. For example, heat is absorbed when a substance melts and it is released again when the substance solidifies.
saturated solution: a solution that holds the maximum possible amount of dissolved material. The amount of
material in solution varies with the temperature; cold solutions can hold less dissolved solid material than hot solutions. Gases are more soluble in cold liquids than hot liquids.
Also...Photographer’s hypo
Every time you have a photograph printed, the printer will have used a chemical called hypo. This is short for the chemical sodium thiosulphate. It is used to “fix” the image after it has been developed.
The photographic film contains a surface coating, or emulsion, of chemicals. This emulsion, a kind of gelatin, contains minute crystals or grains of silver compounds spread evenly over it The smaller the size of the crystals,
the finer the grain and the better the eventual picture quality. (This is why we talk about “graininess” when referring to photograph quality.)
When the camera shutter is worked, the iris opens
on the camera and light reaches the emulsion of the film. The light causes a chemical reaction to occur that results in small groups of silver atoms clumping together. Wherever silver atoms clump together within a crystal or grain they will provide the image on the film.
Developing a negative requires several chemical stages. The first chemical stage happens in a darkroom, where the developer solution converts the clumps of silver atoms into tiny particles of pure silver.
However, the emulsion still contains a mix of chemicals that would react to light and fog the film; this mix has to be removed before the film can be exposed to the light. This is the job of the hypo. The hypo dissolves away the remaining silver-containing compounds. After fixing, the film is stable, and all other processing can be done in normal light.
The hypo reacts with the silver compounds of the emulsion that have not been changed to pure silver, but leaves the pure silver alone. When the film is washed in water, everything except the silver is removed and the film surface contains just silver. The film is now a “negative” in which all light areas are shown dark and all dark areas are shown transparent or light. The positive print (the actual photograph) is produced by shining a light through the negative onto a piece of light-sensitive printing paper.
EQUATION: Making photographer’s hypo
Sodium sulphite + sulphur ➪ sodium thiosulphate Na2SO3(aq) + S(s) ➪ Na2S2O3(aq)
 Sodium thiosulphate crystals
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