Clearing Water with Alum
Publication No. 12003
Where does drinking water come from and how is it purified? In this demonstration, a commercial purification procedure for removing suspended solids from drinking water will be introduced.
Alum, AlK(SO4)2•12H2O, ¼ teaspoon (~ 1.5 g)*
Ammonia, 1 teaspoon (5 mL)*
Water, tap, 1 L
Beakers, 600-mL, 2
Measuring spoon or scoopula
Sand, 1 tablespoon (23 g)*
*Materials included in kit.
Wear chemical splash goggles, chemical-resistant gloves and a chemical resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The liquid may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The sand may be placed in the trash according to Flinn Suggested Disposal Method #26a.
Before beginning the demonstration, ask the students where their drinking water comes from. The initial discussion may revolve around your area’s waterways and the condition of these waterways. Ask the students where the water goes when they flush the toilet. Many students will be surprised that it may be to the same source as their drinking water.
To lead directly into the demonstration, ask the students how dirt and other solid particles may be removed from drinking water. Many students are under the impression that some form of mechanical filtration such as a paper or metal strainer is used. Stress that these methods may be impractical for filtering large volumes of water due to the propensity of the system getting clogged and restricting water flow.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesDeveloping and using models
Disciplinary Core IdeasMS-PS1.A: Structure and Properties of Matter
MS-ESS3.C: Human Impacts on Earth Systems
HS-PS1.A: Structure and Properties of Matter
HS-PS2.B: Types of Interactions
HS-ESS3.C: Human Impacts on Earth Systems
Crosscutting ConceptsSystems and system models
Structure and function
MS-ESS3-3. Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.
Many of the suspended particles in water are so small that their removal by filtration is just not feasible. Most of these small particles are negatively charged. The negatively charged particles are naturally repelled and remain suspended in water. A process known as coagulation is used to help “break up” the suspended particles. Coagulation is a chemical technique that destabilizes the particle suspension in water. In this demonstration, coagulation is put into motion by using a chemical known as alum (aluminum potassium sulfate). The alum helps neutralize the negatively charged particles and allows for attraction of the particles.
In order for coagulation to occur using alum, a certain degree of alkalinity is required. When ammonia is added to the water the optimum pH is obtained. If the pH is correct, alum reacts with ions naturally found in water to produce insoluble precipitates such as aluminum hydroxide, Al(OH)3, and calcium sulfate, Ca(SO4). See Equation 1. The insoluble, neutrally charged particles will then fall downwards out of solution (see Figure 1). In this demonstration, the entire process should occur in less than a minute.
Coagulation is generally followed by a technique known as flocculation. Flocculation is a slow mixing technique that further promotes the aggregation of the coagulated particles. After coagulation and flocculation, a water treatment facility would then have to siphon off the top layer of clean water and treat it with chlorine (commonly in the form of bleach) to kill bacteria.
Flinn Scientific would like to thank Dale Moss, Carrick High School, Pittsburgh, PA, for the idea for this demonstration.