Powder detergents have a water softener base, usually sodium carbonate. This water softener binds minerals in the water, allowing the surfactant in the detergent to work effectively.
Liquid detergents generally don’t contain water softeners, because soft water doesn’t contain many minerals. As a result they are better for soft water areas. Rain water and distilled water is soft.
To find out water quality, contact your water provider, check on your council website or Google it for your suburb.
For water hardness 20- 60ppm (softer water), use a liquid detergent.
For water hardness 60- 120ppm (moderately hard water), use a powder detergent.
For water hardness 120ppm and above (very hard water) add a laundry booster (eg Vanish) or sodium carbonate to the main wash cycel. Check the packaging for quantities, but generally adding 1/4 cup is sufficient. To know whether enough water softener has been added, check to see how clean the items are out of the machine and look for enough suds in the cycle.
When powder detergents are used in soft water, they can create excess suds. Excess suds are not an issue in a top loader machine, but are in a front loader machine. Excess suds create a cushioning effect, which prevent friction between items, which prevents everything from being washed properly. See the Excess suds in Front Loader page for troubleshooting techniques.
For more information on what is in detergents and the function of the raw materials see the Mainstream Detergents, Fragrance in Detergent, Enzymes, Optical Brighteners, Plant based, Sensitive & ‘Eco’ Detergents pages and The Wash Wise Detergent Ingredients Info Sheet.