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Shaking for Suds: Which Type of Water is the Hardest?

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Do you want your hair to be shiny after you wash it? Do you want your clothes to stay bright and soft after laundering, and last a long time? You might think that a special shampoo or detergent can make this happen, but in this chemistry science fair project, you'll discover that how well people and things get clean has a lot more to do with the type of water used for washing than any special shampoo or soap. Did you know that water can be classified as either soft or hard? Soft water lathers and cleans better than hard water. Using this clue, you'll test different common water types to figure out which type is the softest and which is the hardest.


Areas of Science
Time Required
Short (2-5 days)
Material Availability
Readily available
Very Low (under $20)
Be sure to wear safety goggles if you are using glass jars for testing.

Kristin Strong, Science Buddies


To determine the relative hardness of different common water types.


Do you love to take a bubble bath filled with thick, hissing suds that you can sink down into? Do you like to take those bubbles and coat yourself from head to toe with white foam until you look like old man winter? Or do you sculpt the bubbles into towers and mountains, or hide your bath toys in them, so that your toys can't get eaten by an evil shark?

Three rubber ducks float in soapy water

Figure 1. This photo shows rubber duckies enjoying a bubbly bath. (Wikimedia, 2006.)

However you enjoy your bath bubbles, you might be surprised to learn that when you pour a small amount of liquid bubble soap into the water coming out of your faucet, the amount of bubbles you can make depends upon the hardness of your water. You might be thinking "What? Water isn't hard! You can easily put your hand down into it, right?" That's true, but the term hardness, when used to talk about water has a different kind of meaning. Hardness is a word that geologists and other water scientists, like hydrologists, use to describe how many tiny particles of minerals are in the water.

Minerals are the building blocks of rocks. Minerals are not made by people, but by nature. If you've ever seen a diamond, emerald, or a ruby, you've seen a special and expensive kind of mineral, called a gem. In water, the common minerals that you will find are calcium and magnesium cations (pronounced CAT-eye-un). Cations are atoms that are missing some of their electrons (the negatively charged parts of an atom).

An assortment of minerals

Figure 2. This photo shows examples of beautiful minerals. (Mila Zinkova, 2007).

How do these minerals get into water? To understand this, you first need to know a little bit about acids and bases. Pure water is neutral, which means it is not acidic or basic; however, rainwater is slightly acidic, like orange juice. As this slightly acidic rainwater passes through mineral deposits, like the calcium-containing limestone that was used to build the Great Pyramids of Egypt, it reacts with the minerals and changes them into a form that can be dissolved in water, just as you can dissolve a spoonful of sugar in a glass of water. In this way, calcium and magnesium cations get into the water supply.

Is the hardness of water the same everywhere? No, take a look at the map below of the United States from the United States Geological Survey. The areas in deep blue are where the water is the "softest," with low levels of dissolved minerals in the water. The areas in medium blue and white have greater levels of dissolved minerals, and the levels in red are areas where the water is the hardest, with high levels of calcium and magnesium cations in the water. So, if you live in states like Maine, Vermont, the Carolinas, Alabama, or Georgia, you might have never even heard of hard water, but if you live in states like Utah, Wyoming, Colorado, Nebraska, or west Texas, hard water is something you deal with every day.

Map of the United States color coded to show groundwater hardness from collection stations across the entire country

Map showing water hardness from 344 collection stations across the United States in 2005. The groundwater hardness levels are elevated to above 151 mg/l in the mid-west region of the continental United States reaching from the northern most areas of North Dakota to the southern most tip of Texas and extending west until the southern tip of California. Both the west and east coast are below 120 and 50 mg/l for water hardness with the bulk of the states with soft water stretching from northern Maine to southern Louisiana and to the east through northern Florida.

Figure 3. This map of the United States shows by color where water levels are hardest and softest. The red areas have the most dissolved calcium cations in the water and the hardest water, while the dark blue areas have the least dissolved calcium cations in the water, and therefore the softest water. (USGS, 2005.)

What happens when you have hard water? Well, the harder the water (the more calcium and magnesium hardness minerals you have in the water), the more soap or detergent you need to get things clean. In other words, harder water makes less soapy lather. So, if you live in an area with hard water and you want to wash your clothes, dishes, or yourself, it will be harder to get these things clean than if you live in an area with soft water—water without a lot of dissolved minerals. Hard water interferes or reduces lathering (the formation of suds or bubbles). Because of hard water, a film can build up on shower doors and walls, or bath tubs, sinks, and faucets. Hair washed with hard water might look less shiny than hair washed with soft water. Clothing fabrics might be dull, gray, and scratchy, and wear out more quickly. Finally, water pipes in your house, and appliances that use water might not work very well over time because of build-up of hard water deposits. Is it bad to have hard water? No. In fact, the minerals in hard water are thought to contribute to human health. Hard water is mainly just a bother when it comes to cleaning, or in keeping pipes and water-using appliances working well.

So, how do you measure the hardness of water? There are chemistry kits that will tell you approximately how many tiny parts of calcium and magnesium you have in your water, but in this chemistry science fair project, you are going to use liquid soap to compare the hardness of different kinds of water. You will see how high of a bubbly lather you can create with each type of water.

Terms and Concepts



For information about hard water, see these sources:

This source describes what minerals are:

  • Oxford University Museum of Natural History. (n.d.). What is a Mineral? The Learning Zone. Retrieved August 20, 2014.

For help creating graphs, try this website:

  • National Center for Education Statistics (n.d.). Create a Graph. Retrieved January 8, 2010.

Materials and Equipment

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Experimental Procedure

Preparing to Test

  1. Label the jars with the permanent marker.
    1. You can write down 1, 2, or 3, or write the name of each water type on a different jar, so that you can keep track of which type of water is inside each jar.
    2. The three types of water you will be testing are distilled water, bottled mineral water, and tap water.
  2. Decide how much water you want to put in each jar.
    1. Filling the jar 1/5 full with water works well. For example, this would mean putting 100 milliliters (mL) of water into a 500 mL water bottle.
    2. In your lab notebook, write down the amount of water you will add to each jar. (You will be adding the same amount of water to each jar.)
  3. Using the metric measuring cup, measure out and add the amount of water you decided on (in step 2) to each appropriate jar.
    1. For example, you may put 100 mL of distilled water in jar 1, 100 mL of bottled mineral water in jar 2, and 100 mL of tap water in jar 3.
    2. Get down so that you are eye-level with the mark as you are measuring the water in the measuring cup.
  4. While each jar is on a level surface, draw a straight line to mark the height of the water.
  5. In your lab notebook, create a data table like Table 1, below.

  Height of Soap Bubbles (in cm)
Trial Distilled Water Bottled Mineral Water Tap Water
Average of trials   

Table 1. Make a data table like this one in your lab notebook. You will record the height of the soap bubbles (in centimeters [cm]) for each different type of water.

Testing Your Waters

Important Notes Before You Begin Testing:

  • If you are using glass jars, safety goggles are recommended in case you drop a jar while shaking it and it breaks.
  • To test your waters, you will add soap and shake your jars. Then you will look at how high the soap lather is. The most important thing is to shake the jar the same way every time you test. This means you should not shake it harder or faster sometimes.
  • Before you start testing, put the lid on a jar, and, without adding any liquid soap, practice shaking it up and down a few times.
  1. Figure out how long you want to shake the jars for.
    1. It can be 15 seconds, 30 seconds, or whatever you think is best. Be careful not to shake them for too long, though, because not only will you be tired by the end, but you may also create too many bubbles
    2. In your lab notebook, write down the amount of time you chose for testing.
  2. Decide how many drops of soap you want to use. For a water bottle-sized jar, between 2 to 4 drops works well.
    1. In your lab notebook, write down the number of drops you will use.
  3. Fill your eye dropper with soap.
  4. If using glass jars, put on your safety goggles.
  5. Add the number of drops of liquid soap you decided on (in step 2) to the jar containing distilled water.
  6. Put the lid on the jar.
  7. Ask your helper to time you with the stopwatch (for the amount of time you decided on in step 1) while you shake the jar up and down.
  8. When the time is up, set the jar on a flat surface and measure the distance from the initial water line you made to the top of the bubbles. Record your results in the data table in your lab notebook.
    1. Important: If the bubbles reach the top of the jar when you are testing any water type, you must restart your experiment from the beginning, reducing one or more of the following factors because your results may not be accurate (when you restart make sure to write the new values down in your lab notebook):
      1. The amount of water used in each jar
      2. The number of soap drops used
      3. The amount of time the jars are shaken
    2. To measure where the top of the bubbles is, take off the lid and measure from the center of the jar. Because bubbles will stick to the edges of the jar, looking from the outside will not give an accurate measurement.
  9. Repeat steps 5–8 for the jar containing bottled mineral water. Be sure to record your results in the data table in your lab notebook.
  10. Repeat steps 5–8 for the jar containing tap water. Be sure to record your results in your data table.
  11. Rinse your jars out thoroughly with tap water and dry them. Repeat step 3 in the "Preparing to Test" section, above. The water level should match the mark you drew earlier. If they do not match, re-check your measurements.
  12. Repeat steps 3–11 two more times, so that you have a total of three trials for each jar. Be sure to record your results in your data table. Repeating the trials ensures your results are accurate and repeatable.

Analyzing Your Data Table

  1. Calculate the average height of the soap bubbles for each type of water. To get the average of the three trials, add up the results for trial 1, trial 2, and trial 3, and then divide by 3. Ask an adult if you need help with this step.
  2. Make a bar graph showing the average height of the soap bubbles for each type of water. You can make the chart by hand or use a website like Create a Graph to make the graph on a computer and print it out.
    1. On the x-axis (the horizontal axis), put the different types of water you tested. Make a bar for each different water type.
    2. On the y-axis (the vertical axis), put the height of the soap bubbles (in centimeters).
  3. Looking at your graph, which type of water made the most soap bubbles? Which type of water made the least? Which type of water do you think is the softest? Which type do you think is the hardest?
icon scientific method

Ask an Expert

Do you have specific questions about your science project? Our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.


  • Test rainwater, river water, ocean water, or water samples sent by your friends or family from different parts of the country.


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General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Science Buddies Staff. "Shaking for Suds: Which Type of Water is the Hardest?" Science Buddies, 23 June 2020, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p094/chemistry/water-harness. Accessed 30 Sep. 2023.

APA Style

Science Buddies Staff. (2020, June 23). Shaking for Suds: Which Type of Water is the Hardest? Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p094/chemistry/water-harness

Last edit date: 2020-06-23
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