Investigate the Properties of Liquids

Background Information for Teachers

Different materials are characterized by different properties. This is not only true for solids but also for liquids. Liquids and solids share many properties. Among them are, for example, the transmittance, which measures how much light is able to pass through a material, and the density. The density is different for each individual compound and defined as the mass of a compound divided by its volume. In other words, the more matter there is in a certain amount of volume, the denser a substance is. All types of matter—solids, as well as liquids—are made up of many different atoms. Depending on the mass of these atoms, their size, and the way they are arranged, different substances will have different densities. One cubic centimeter (1 cm³) of rock, for example, is much heavier than 1 cm³ of wood. This is because there is much more matter in the same volume of rock compared to the wood. Liquids can also have different densities. Freshwater, for example, has a density of about 1 gram per cubic centimeter (g/cm³) at room temperature. The density of an object will determine whether it sinks or floats in water. Any substance—either liquid or solid—that has a higher density than water will sink, whereas substances with a lower density than that will float (Figure 1).

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 1. Objects with a density higher than water will sink (left) and objects with a lower density than water will float (right).

There are also some properties that are used to specifically characterize a liquid. Surface tension or viscosity are two examples. The viscosity of a liquid is a measure of its resistance to deformation or flow. It is also often referred to as the "thickness" of a liquid. The thicker a liquid is, the higher is its velocity. Corn syrup, for example, has a higher viscosity than water. An easy way to measure viscosity is to measure how long it takes for a liquid to pour out of a cup into another container, and how long it takes until it takes the shape of the receiving container. Water will do that almost instantly, whereas corn syrup will take some time to pour out and settle.

Liquids also experience surface tension. The molecules a liquid is made of pull on each other or exert a force. As shown for water in Figure 2, a water molecule in the middle of a droplet gets pulled equally in all directions by the neighboring molecules. However, a molecule at the surface of the droplet gets pulled mostly inward by the molecules below it. This means that all the molecules at the surface help "hold together" the droplet of water, which is called surface tension. The surface tension of different liquids can be determined by measuring how much volume of each liquid fits on a penny before it spills over the edge of the penny.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 2. A diagram showing the forces on water molecules that create surface tension. Note that the diagram is not to-scale—there are quintillions of water molecules in a single drop!

In this lesson plan, students will investigate the properties of different liquids. By pouring, smelling, or stacking them on top of each other, they will realize that not all liquids have the same properties. They can vary in color (transmittance), smell, viscosity, or density (mass per volume).

Additional Background Links

• What is viscosity?, from RheoSense
• Liquid, from Wikipedia
• Density, from The Physics Hypertextbook
• States of Matter, from cK-1

Prep Work (15 minutes)

1. Fill the 16-oz. plastic cup with enough water for all groups and add a couple of drops of green food coloring.
2. For each student group, pour 1 Tbsp. of each liquid (dark corn syrup, dish soap, colored water, vegetable oil) into small 2-oz mini cups and close them with the lids.
3. Prepare one extra set of mini cups with the four liquids.
4. Fill one separate mini cup with 1 Tbsp. apple cider vinegar and close it with a lid.

Engage (20 minutes)

1. Start the lesson by introducing the concept of liquids and some of their properties. Ask students about their favorite drinks.

   What is your favorite drink for breakfast?

   What makes this drink your favorite breakfast drink?

   How is your favorite drink for breakfast different from other breakfast drinks?

   Collect students' answers. Focus on responses that describe tastes, colors, smells, or other properties, such as stickiness or thickness. To wrap up the discussion, highlight the ways some of the breakfast drinks are different and/or similar in taste, color, smell, stickiness or thickness.

2. Continue the discussion. Ask:

   How are all of the breakfast drinks the same?

   How can we describe all of the breakfast drinks?

   How are the breakfast drinks different from breakfast foods?

   Listen to students' replies. During the discussion, point out that all breakfast drinks are liquids and breakfast foods are mostly solids.

3. Have students name examples of solids versus liquids. Write them in two columns on the board.
4. Then show students the corn syrup, the dish soap, the colored water, and the oil. Haves students identify each liquid and add them to the "liquid column" if not listed yet. Ask students about the four liquids.

   How do you think these four liquids are similar or different from each other?

   Use students' responses to point out that liquids, just like solids, can have different properties. A material property is a physical property that does not depend on the amount of the material.

5. Ask students if they can name some liquid properties.

   What are some properties of liquids that you know?

   Collect students' replies. They might mention the color of a liquid, or its taste and sweetness. If students struggle, give them some examples of liquid properties. You can find an extensive list in the Additional Background section.

6. Tell students that they will plan and conduct an investigation to find out about each of the liquids' properties. In their investigation, they will observe the ways the four liquids are different or similar to each other.

Explore (45 minutes)

1. Ask students what properties they could investigate and make a list together. The list should include properties such as color, smell, taste, density (mass per volume), and viscosity ("thickness," or how fast or slow a liquid flows). Also ask them how they would test the property that they suggest. Note: For 2nd-grade students, consider simplifying the concept of density (mass per volume) to just mass. You can also refer to viscosity as "thickness."

   What liquid properties could we investigate?

   How could we test each of the suggested properties?

   Use students' responses to develop an investigation plan together and describe what information or evidence they will collect for each of their selected tests that allows them to differentiate the different liquids. Together with students, decide that you are going to investigate the color (transparency), the smell, the viscosity (thickness), and the density (mass) of each liquid. If students want to test something else, it is up to you to decide whether they can continue with their own tests.

2. After brainstorming with your students how to test each of the selected properties, do a brief demonstration of each test to guide groups through the individual steps of their investigation.
3. Split the students into groups of 2–3 and provide each student with a worksheet. Then give students 15–20 minutes to conduct their tests in their groups. Tell them they should discuss their observations within their groups after each test and record their test results on their student worksheet.
   1. Color: Ask each group to investigate the color of each of the liquids.

      How do the colors compare?

      Can you see through the different liquids?

   2. Smell: Have each group investigate the smell of each liquid.

      Do all of the liquids have a smell?

      How does each liquid smell?

      How does their smell differ from each other?

   3. Viscosity: Instruct students to carefully pour each liquid from its cup into another empty cup. Have them observe how each of the liquids behaves while pouring, and how long it takes to empty the cup.

      Does the liquid flow quickly or slowly?

      Which liquid flows the fastest or the slowest?

      How long does it take to empty the cup with the liquid? Does it take more than 5 seconds?

   4. Density: Instruct students to carefully pour all four liquids into the same empty mini cup, one after the other. Let them wait and observe the liquids for about one minute until they have settled. Note: The order in which students pour the liquids into the cup does not matter. The denser liquids will always settle to the bottom, whereas the less-dense liquids will float on top of the denser layers.

      What do you notice?

      Do the liquids mix?

      Which liquid settles on top, which one at the bottom?

      What does this teach you about how heavy (dense) each of the liquids is?

   The video below shows how students can investigate the density of different liquids.

   

   https://www.youtube.com/watch?v=lYITnkFxa5A

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Reflect (20 minutes)

1. Once each group has completed their investigation, bring all students together and discuss each tested liquid property with the whole class.

   What did you observe with each of the liquids?

   What patterns did you observe in the properties of the different liquids?

   How were the properties of all of the liquids similar or different?

   Did you find a property that is unique for each liquid?

   What do your observations teach you about liquids in general?

   Have students share their observations. Guide them to conclude that each liquid has different properties. These properties can be used to classify and identify them. Mention that classifying means sorting them based on similar properties.

2. Together with students, classify the different liquids based on their observed properties.

   Can we use the patterns we observed in the properties of the four liquids to classify (sort) them?

   Can you sort the liquids from heaviest to lightest, thickest to thinnest, or lightest to darkest?

   Show the students the fresh set of four mini cups that each contain one of the liquids. Then let them guide you in sorting the cups based on their properties. Have them explain the reasoning for their sorting.

3. Challenge students to identify an unknown liquid based on its properties. Show them the prepared mini cup filled with apple cider vinegar. Allow students to come closer to smell the liquid and then pass the mini cup around (with closed lid!) so they can have a closer look, or shake the cup to gauge the liquid's viscosity. Then ask them to tell you what liquid this is. Have them be specific and tell the type of vinegar as well. Discuss:

   How could you tell which liquid this is?

   What liquid properties helped you identify the liquid in the mini cup?

   Students probably can tell by the smell that the liquid is vinegar. The viscosity should tell them that the liquid is similar to water. They can infer from the color that it is apple cider vinegar. If they have difficulties, prompt them by asking what the color of the liquid reminds them of. The color looks similar to apple juice.

4. End the lesson by reiterating that materials, including liquids, have different observable properties that can be used to describe and classify them.

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Materials Scientist and Engineer

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Career Profile

Material scientists and engineers study all aspects of materials to develop new products or to improve existing ones, such as airplane wings, computer chips, cars, and bridges. They work with ceramics, plastics, metals, and even create new materials that can be used to make products stronger and better. To find out if their new materials have the properties they need, material scientists and engineers spend a lot of time in laboratories doing material testing. Read more

Lesson Plan Variations

• Have students investigate an additional liquid, such as rubbing alcohol. Keep all liquid amounts the same. Have students observe and discuss the ways the liquids flowed or what layers they formed in the density column.
• Ask students to come up with other properties that they could test with their liquids. These could include taste (if safe!), stickiness, or surface tension. For information on how to test the surface tension of liquids, review Science Buddies' Measuring Surface Tension of Water with a Penny project.
• In addition to stacking the liquids on top of each other, have students measure the same amount of liquid into separate cups and weigh each liquid on a scale. This is another way of testing each liquid's density (mass per volume). The results should confirm their previous findings.