Solids are found in streams in two forms, suspended and dissolved. Suspended solids include silt, stirred-up bottom sediment, decaying plant matter, or sewage-treatment effluent. Suspended solids will not pass through a filter, whereas dissolved solids will. Total dissolved solids, or TDS, can be determined by using a Vernier Conductivity Probe to determine the ability of the dissolved salts and their resulting ions to conduct an electrical current. The conductivity is then converted to TDS.
The TDS concentration in a body of water is affected by many different factors. A high concentration of dissolved ions is not, by itself, an indication that a stream is polluted or unhealthy. It is normal for streams to dissolve and accumulate fairly high concentrations of ions from the minerals in the rocks and soils over which they flow. If these deposits contain salts (sodium chloride or potassium chloride) or limestone (calcium carbonate), then significant concentrations of ions will result.
In this experiment, you will use a Conductivity Probe to measure the total dissolved solids in a water sample.