Decomposing Water Lab

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Students will use a 9v battery to decompose water into hydrogen and oxygen.

Big Idea

A safe lab that allows students to see the decomposition of water into its component elements.

NGSS Background

This lesson is based on California's Middle School Integrated Model of NGSS.

NGSS Performance Expectation (PE): (MS-PS1-2) Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occured.

Science and Engineering Practice (SP) 4: Analyzing and Interpreting Data

Disciplinary Core Ideas (DCI): PS1.A: Structure and Properties of Matter - Substances are made from different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms.

Crosscutting Concepts (CCC) 1: Patterns - Macroscopic patterns are related to the nature of microscopic and atomic-level structures.

This lab is designed to provide the students with a simple, safe, and effective method of decomposing water with electrolysis. A 9v battery is used as the source of electrolysis. Students analyze the data collected (observations) to determine if a chemical reaction took place and what type of reaction it may be (SP4). Once the 9v battery is submersed in water, bubbles of hydrogen and chlorine will be observable. Recognizing the pattern that the formation of new bubbles is evidence of a chemical reaction helps in determining a cause (electrolysis) and effect (bubbles) relationship (CCC). This lesson allows my students to pose testable questions (can water be split?) and seek patterns in their results.

I run this lab as a series with two other labs, Surface Area Lab and Catalyst Lab (Elephants Toothpaste).

Set-up (Prelab)

15 minutes

Equipment Needed

  1. 9 volt battery (I ask for donations from parents when they switch out their smoke alarm batteries.)
  2. 150mL beaker (any glass jar would work)
  3. Sodium chloride (NaCL) - table salt
  4. Distilled water (H2O)
  5. Stop watch
  6. Stir rod


  1. Fill the beaker with about 140mL of distilled water (pure H2O)
  2. Add 1 pinch of sodium chloride (NaCl – table salt) to the beaker of distilled water (pure H2O). Stir until dissolved.
  3. Carefully submerge the 9 volt battery into the beaker of water.
  4. Time the reaction for 1 minute.
  5. Carefully remove and dry the battery.

TIP: You'll need a great number of 9v batteries for this lab. How long they last depends on how long you allow your students to keep the 9v battery in the water. They really discharge quickly. Over the course of one day (45 student labs), I'll go through about 15 batteries. I ask for donations from parents. At the end of October most people have been trained to switch out their 9v batteries in their smoke alarms. I send home a note and typically get 10 batteries returned. Don't buy new batteries for this lab as it would get rather expensive.

Student Activity (Lab)

15 minutes

Pass out a copy of the lab Decomposing Water lab to each student. I provide all the necessary supplies and safety goggles in a plastic box at the center of each table. It is imperative that students wear safety goggles for this activity.

This is a fairly simple lab for the kids to perform. They add a pinch of salt to distilled water in order to encourage electrical conductivity, then carefully place a 9v battery in water.

The chemical formula is as follows:

Electrolysis splits both the salt and the water into chlorine gas, hydrogen gas, and sodium hydroxide (lye). The only gases that are bubbling up from the 9v battery terminals are chlorine gas and hydrogen gas. The hydrogen bubbles are from the positive terminal (seen on the left in the video) and chlorine bubbles are from the negative end (seen on the right in the video). If this reaction continued the sodium hydroxide (lye) would turn the water a yellow color.

Decomposing Water into Hydrogen Gas and Oxygen Gas through Electrolysis


Student Publishing (Postlab)

15 minutes

 As part of this activity the students have to answer seven questions.

  1. What did you observe?
  2. Draw a diagram of your observations.
  3. What conclusions can you infer from your observations? (Students often ask what 'infer' means.)
  4. Why did one end of the battery produce more bubbles?
  5. Why were the bubbles produced?
  6. Where were the bubbles from?
  7. What is the evidence that a chemical reaction occurred? 
  8. How does your evidence support the claim that a chemical reaction did or did not occur?

An easy method of checking for understanding and spotting misconceptions is to look at one student drawing per group. I allow my students to work together to answer these questions and looking at one drawing is the same as looking at all their drawing (within the group). I can easily spot misconceptions within a few minutes of checking 8-10 drawings out of 40 students (see - Student Activity Reflection).

Please note that the student work example did not include the final question, "How does your evidence support the claim that a chemical reaction did or did not occur?" This was added after I realized that I was not the NGSS expectation of expressly supporting claims with evidence. NGSS wants students to make claims, provide evidence, and include a reasoning of how the specific claims are related to the documented evidence.





45 minutes

Here is a Powerpoint Types of Chemical Reactions that explains the four different types of chemical reactions that I teach. There are more categories, but this is what I teach eighth graders.

I created a mnemonic sentence to help them to remember the four types of chemical reactions.

Squids don't carry roses.

  1. Synthesis
  2. Decomposition
  3. Combustion
  4. Replacement 

For review I have also included: Evidence of a Chemical Reaction. This covers the evidence needed in order to prove a chemical reaction has occurred.

I created another mnemonic sentence to help students to remember the evidence of a chemical reaction.

Cats tell stories of ghosts.

  1. Change in color/light
  2. Change in temperature
  3. Formation of a solid (precipitate)
  4. Formation of an odor
  5. Formation of a gas