The sodium in water chemistry demonstration is a dramatic chemical reaction that raises interest in science. It illustrates several properties of sodium and other alkali metals, including flame test color, reactivity in air and water, and physical properties. The reaction also demonstrates exothermic reactions, pH changes, and hydrogen production. Here is how to perform the sodium in water demonstration safety, with tips for maximizing learning.
The Reaction Between Sodium Metal and Water
Sodium and other alkali metals readily oxidize in air and react (sometimes vigorously) with water. The reactivity increases moving down the element group, so sodium is more reactive than lithium, but less reactive than potassium and much less reactive than cesium or francium. The reaction between sodium (Na) and water (H2O) forms sodium hydroxide (NaOH) and hydrogen gas (H2). Here is the balanced equation for the reaction, written two ways:
2 Na(s) + 2 H2O → 2 NaOH(aq) + H2(g)
2 Na(s) + 2 H2O(l) → 2 Na+(aq) + 2 OH–(aq) + H2(g)
Sodium hydroxide is a strong base that dissociates completely into its ions in water, raising the pH of the liquid. The reaction between the metal and water is exciting, but adding a pH indicator to the water improves interest because the water changes color as the metal reacts. You can use a pH indicator like phenolphthalein that changes from colorless to colorful as pH changes from neutral to alkaline or you can use an indicator that changes color as pH rises. Using phenolphthalein, a pink trail follows the sodium metal as it skitters across the water surface.
Setting Up the Sodium in Water Demonstration
You need sodium metal, water, a suitable reaction container, and a pH indicator (optional).
- pH indicator
A borosilicate beaker is a good choice because it withstands temperature changes, is tall enough to contain any stray sparks, and offers excellent viewing. But, any glass container works. Warm or hot water results in a more spectacular reaction than cold water.
Performing the Sodium in Water Demonstration
- Fill the beaker about half full of water.
- Add a few drops of phenolphthalein or other pH indicator to the water (optional).
- Using tongs or tweezers, drop a small piece of sodium metal onto the water.
- Stand back. Sodium immediately reacts with water, forming hydrogen gas bubbles. The exothermic reaction heats the water, which may boil. The heat often ignites the hydrogen gas. The flame displays the red color of burning hydrogen and the yellow flame-test color for sodium. Sodium hydroxide formation raises the water pH and changes the indicator color.
Concepts to Point Out
While the demonstration is simple, it illustrates several chemistry concepts:
- Sodium metal is less dense than water (0.97 g/cm3), so it floats on water.
- Sodium and other alkaline metals are soft enough to cut easily. If possible, show students how sodium metal is stored and how easily it cuts. Point out the shiny metal surface of freshly-cut sodium and how quickly the surface dulls as it oxidizes in air. Potassium oxidizes more quickly than sodium, while rubidium oxidizes instantly. Similarly, these metals react more vigorously with water.
- The reaction between an alkali metal and water is exothermic.
- The reaction is one way to make hydrogen gas.
- Hydrogen burns with a red flame. Sodium adds a bright yellow color to a flame.
- Sodium reacts with water, forming sodium hydroxide. The hydroxide dissociates in water and raises its pH.
- The chemical reaction, like most others, proceeds more rapidly at higher temperature.
- Store sodium under kerosene or mineral oil until use.
- Wear gloves while cutting the sodium metal to prevent direct skin contact. Skin contains water, after all.
- Only use a pea-sized piece of sodium metal. YouTube has many videos demonstrating the spectacular reaction of large sodium chunks in water, which also show why less-is-more in a classroom setting.
- Wear proper lab attire, including safety goggles and gloves.
- Either perform the demonstration at a safe distance from viewers or else separate viewers from the container by a safety shield. Placing the beaker on an overhead projection screen provides clear viewing as well as safety.
- Usually, a small piece of sodium behaves itself (making sodium safer than potassium and much safer than rubidium). It skitters around the water, with a small flame. However, it’s possible that the metal chunk breaks apart or sparks. So, make certain the container walls are high enough to contain the reaction.
- Similarly, the reaction generates heat. Using too much sodium, too little water, or a fragile container could lead to container breakage. It’s a good idea to set the beaker inside a large tub to contain a break or spill.
- Dispose of the liquid the same as sodium hydroxide solution: rinse it down the drain with water. Sodium hydroxide is the key ingredient in drain cleaner, but, if you want to neutralize the pH before disposal, just mix the solution with a bit of weak acid (e.g., vinegar, acetic acid). Wearing goggles during disposal is good practice, in case some unreacted sodium fragments remain.
- Atkins, Peter W.; de Paula, Julio (2002). Physical Chemistry (7th ed.). W. H. Freeman. ISBN 978-0-7167-3539-7.
- Averill, Bruce A.; Eldredge, Patricia (2007). “21.3: The Alkali Metals”. Chemistry: Principles, Patterns, and Applications with Student Access Kit for Mastering General Chemistry (1st ed.). Prentice Hall. ISBN 978-0-8053-3799-0.
- Ladwig, Thomas H. (1991). Industrial Fire Prevention and Protection. Van Nostrand Reinhold. ISBN 978-0-442-23678-6.
- National Research Council (U.S.) Committee on Prudent Practices for Handling, Storage, and Disposal of Chemicals in Laboratories (1995). Prudent Practices in the Laboratory: Handling and Disposal of Chemicals. National Academies.