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You may often hear the terms "soap" and "laundry detergent" used interchangeably, but you need to be aware that these two things are definitely not the same. One is better than the other for laundry's purposes and you need to know that in order to determine how best to do your laundry.
Detergent is a compound made of large molecules, the ends of which perform very different, but very crucial roles when cleaning. Using water to detergent's greatest advantage, detergent pulls stains and dirt off of the clothes with one end and attracts water with the other. This results in the maximizing of washing potential. Detergent also affects water's surface tension by reducing it, resulting in a greater effectiveness for water. With a reduced surface tension, the water can better clean clothes and the detergent is a key component in that action.
Soap, on the other hand, is very different and doesn't have the same effect on cleaning clothes as detergent does. Soap works by attracting all the kinds of substances and molecules and pulls them out of clothes or off of hands or dishes. Soap is used almost everywhere and can be very effective when dealing with specific surfaces and materials, but in the laundry soap is not your ideal choice. Soap can work, but detergent is much more effective and has a specific purpose: to clean clothes. Soap's purpose is simply to clean, which—especially when it comes to your laundry and your clothes—isn't enough. You need detergent's targeted power and utilization of water surface tension to make the most of your laundry load.
Soap and detergent are both excellent substances but one—detergent—is definitely the better choice for doing your laundry. It's focused on cleaning clothes and getting out stains, so it's just one step above soap—it's a specialized soap. While soap isn't bad, avoid it when doing your laundry and save it for your dishes instead!