How does soap make dishes clean?

Informal Education | Daily Do

How does soap make dishes clean?

Chemistry Is Lesson Plan NGSS Phenomena Physical Science Science and Engineering Practices Informal Education

Teachers and families across the country are facing a new reality of providing opportunities for students to do science through distance and home learning. The Daily Do is one of the ways NSTA is supporting teachers and families with this endeavor. Each weekday, NSTA will share a sensemaking task teachers and families can use to engage their students in authentic, relevant science learning. We encourage families to make time for family science learning (science is a social process!) and are dedicated to helping students and their families find balance between learning science and the day-to-day responsibilities they have to stay healthy and safe.

Interested in learning about other ways NSTA is supporting teachers and families? Visit the NSTA homepage.

What is sensemaking?

Sensemaking is actively trying to figure out how the world works (science) or how to design solutions to problems (engineering). Students do science and engineering through the science and engineering practices. Engaging in these practices necessitates students be part of a learning community to be able to share ideas, evaluate competing ideas, give and receive critique, and reach consensus. Whether this community of learners is made up of classmates or family members, students and adults build and refine science and engineering knowledge together.

Introduction

In today's Daily Do, How does soap clean our dishes?, families participate in a Dinner Table Discussion (see below) about the phenomenon of "cutting grease" with soap. This sensemaking discussion has four parts:

Families raise the question How does soap clean our dishes? by introducing the phenomenon of "cutting grease" with soap. Students and their families can make first- hand observations and/or talk about how kitchen surfaces (counters and stovetops), dishes, pots and/or pans feel when they are greasy. Why do some things still feel greasy even after you wash them?
Families ask students to explain what they currently understand about what it means to wash things clean with soap.
Families prompt students to generate questions about what they observe when something is washed with water only verses washing with water and soap. Students can also be prompted to make observations of washing greasy dishes using different kinds of soap. For example, does hand soap from the bathroom work the same as kitchen dish soap?
Families watch a video and/or do an activity together to find some answers to their questions about how soap works to get greasy objects clean.

What are Dinner Table Discussions (DTD's)?

This activity is called a Dinner Table Discussion (DTD). Dinner Table Discussions do not have to physically happen at the dinner table. Rather, they are intended to facilitate connections for the family around a discussion about science ideas wherever you may congregate for a meal. Whether you cook dinner at home or order take-out, the Dinner Table Discussions are centered around relevant science phenomena and raise common questions children have about the world around them. The goals of DTD’s are to:

foster connection among the family through discussion of relevant science ideas.
prompt students and their families to think about what they currently know.
help students and their families ask what they want to know more about.
discover something new that moves everyone along the learning continuum of a particular science idea.

Like Daily Do’s, these types of activities are considered “micro-learning experiences”. They are not intended to replace classroom science learning, and are not intended to be used as “home school” stand-alone science lessons. They are not intended to result in being able to generate robust, complete scientific explanations of phenomena. Conversely, they are intended to move student thinking along the continuum of learning.

These are intended to be family-style discussions, with provided parent talk-moves, that stimulate thinking among family members and move everyone along the continuum of learning. Each dinner table discussion has these components to them linked below. These components provide fertile ground for the discussion to be authentic, phenomena-driven, rooted in science, and focused on sensemaking.

Dinner Table Discussion - Guidance for Families

If this is your first Dinner Table Discussion in the Daily Do series, NSTA recommends reading the guidance before trying your first family discussion.

Dinner Table Discussions have four main components. The following guidance will support you in facilitating your family discussion.

Introducing the Phenomena & Raising the Question

Our goal is to raise a puzzling question for students that does three things: (1) prompts them to think about what they currently know, (2) makes them ask what they want to know more about, and (3) helps them discover something new that moves them along the learning continuum.

Tell me what you know....

We want to foster children explaining what they think they understand to be true. These previous understandings are critical to exposing what they know and the questions they have. As they work to explain their current understandings, they will realize they don’t know as much as they think, which will spur the generation of further questions.

What questions do you have?

In developing insufficient explanations for things, students generate authentic questions they have that are the pathway to discovering the answer. In other words, these are our explanatory questions. That, if we were able to investigate, we would understand more about what we currently don’t understand. Our goal here is to generate lots of questions, but anticipate the common ones. The common questions are central to developing an explanatory idea, and we want to foster that environment by giving adult family members discussion prompts (talk moves) to facilitate the discussion for students as they work to articulate what they want to know more about.

Pursuing Common Questions

Our goal here is not to develop a robust and complete scientific understanding of a particular phenomenon. However, our goal is to help students/children understand a puzzling phenomenon more deeply than they do. Learning is a continuum, and our goal with these discussions are to move students further along the continuum; not get them to the end. The objective is to stimulate thoughtful discussion that is rooted in a scientific phenomenon and a scientific explanation.

How does soap make our dishes clean?

Have you ever gotten a dish out of the cupboard and it still felt greasy/slippery, like maybe is wasn't cleaned all the way? Have you ever wondered why some foods seem harder to get off dishes when you wash them ? Many of you have probably tried to wipe up something greasy just to find the spot you cleaned still feels slippery. In today's Daily Do, we will figure out some things about soap and why it works so well on our greasy dishes!

Introducing the Phenomenon & Raising the Question

Many students are familiar with washing dishes after a meal. Whether washing dishes by hand or in a dishwasher, soap is always part of the process. But why is soap so important - what does it do? Ask students if they have ever gotten a "clean" dish or pan out that still felt greasy. Can they think of a bowl or lid that just always seems greasy, like maybe one used to make popcorn? Have them think about what they did next - did they use the dish or not? Did they wash it again before they used it? Why?

Dishes in soap

If students are unfamiliar with what a greasy dish feels like, consider investigating by smearing a drop of oil on a plastic dish and asking students to try washing it off using water only. What they notice? Then, have students use both soap and water to wash the dish. What differences do they notice between using water only and using both water and soap?

Next, put some water is dish (a clear dish is best) and then pour some oil on top of the water, like the picture at right. Sprinkling cocoa powder or black pepper on top of the oil will create a more dramatic affect. Have students make some observations about what they see in the dish. Next, let one drop of dish soap fall into the center of the dish and ask students to make observations. Ask students to share what they noticed and ask them to think about any questions they have about what they saw.

If a hands on activity is not possible, consider using the video below of dish soap dropped into a dish of oil and water (black substance on top is cocoa powder).

Making Observations and Developing Initial Explanations

Have students make an initial model to explain what they observed in the investigation(s). Tell each student to divide their paper into three columns and label them Before, During, and After. Have students draw a model to explain what they think is going on in each of the three stages:

What did the oil and water (and cocoa powder) in the container look like before the dish soap was added?
What happened when the soap was added? (What moved? What changed?)
What did the oil and water in the container look like after things stopped moving?

Next, ask students to share their model with another student, a small group, or someone else in the home. Have student notice similarities and differences between their models. Prompt students to explain their models by asking them about specific interactions such as:

Do you think anything would happen to the oil and water if we just let is sit there?
What do you think happens to the oil when the soap is added?
What do you think happens to the water when the soap is added?
What does this tell you about how soap works to get dishes clean?

Next, have a discussion about what was common in the models. Commonalities between students' models could include:

the oil and water were in layers with the oil on the top
the cocoa powder/pepper was floating on top of the oil but some of the chunks also sunk to the bottom
the soap made the oil move to the sides of the dish

Guidance: If you are working with younger children, you might want them to draw their model and then explain what they think is going on through discussion.

Tell us what you know...

Encourage your students to explain what they know (or think they know) about why soap seemed to move the oil and how that helps them figure out how soap works to get things clean. Ask them to explain the science of why soap seems to move the oil. Students will attempt many varieties of explanations, but our goal here is not to distinguish between right and wrong answers or ideas. Rather, we want to foster discussion about how or why these things happen.

Accessing Prior Knowledge

Students may call on knowledge from previous grade levels during this part of the discussion.

Early elementary students (grades K-2) may mention they know they need to use soap to make things clean, like when they wash their hands. They may also know that different soaps are used to wash their bodies, hair, clothes, floors, etc. Student may also bring up some soaps make bubbles and others do not.
Upper elementary students (grades 3-5) may mention that not all soap is the same - they may share soap comes in different forms - liquid, foam, bar, powder - and that different soap is used for different things. They may also bring up ideas from things they have seen on TV commercials such as dish soap cuts grease and some soaps can be used to clean oil off of animals.
Middle or high school students may talk about the different properties of the substances used in the investigation(s). For example, they may say oil floats on water because it is less dense than water or that oil and water separate in things like salad dressing and have to be shaken up to get them to mix. They may also mention that the mixed oil and water will eventually separate again. Students at this age may also bring up the idea of chemical reactions and suggest that the soap causes some kind of chemical change in the oil.

All of these connections to ideas and learning opportunities at previous grade levels should be encouraged by asking follow up questions such as:

“Can you tell me more about that?”

“How do you know that?”

What questions do you have?

Tell students to think about their investigations (and/or video) and the initial models they developed. Ask them to share any questions they have about what they observed when the soap was added to the water and oil mixture. Common questions may include:

Did the soap change the oil?
Why did the oil layer move to the sides of the dish?
Did the soap do something to the sides of the dish to make it attract the oil?
Why did the powder move too?
Why didn't the soap move the water?
Will the oil eventually move back?

Pursuing Common Questions

Explain that not all soap is the same but all soap works in a similar way. For example, the soap used in this investigation(s) is a liquid dish soap that makes bubbles when you use it but soap used in a dishwasher is usually a powder and does not make bubbles. However, both of these soaps are used for cleaning dishes. Tell students there are also special soaps that are used for cleaning different things. Ask students about the different kinds of soap they are familiar with or might have seen; these soaps could include:

laundry
hand
body/face
shampoo
'cleaning' soap - like for floors

Acknowledge that their are many different kinds of soap and ask them if they think all these soaps would make the oil move like the dish soap did.

Extension Opportunity: If time permits, consider having students work in small groups to investigate different kinds of soap to see if they all make the oil move in the same way as the dish soap did.

Developing Explanations

Explain to students that oil and water don't mix because they are made up of very different molecules (or particles if working with younger students). Water molecules act like magnets (or share that water molecules are polar if working with older students) and are attracted to each other. Oil molecules are not attracted to water so oil molecules stay next to other oil molecules. However, when soap is added to the oil and water, it breaks the oil into very small drops so it can mix with the water.

Depending on the age of your students this may be enough of an explanation. However, for high school-age students you can introduce the idea of polar molecules or have them use polar property of water molecules to explain the phenomenon if they are already familiar with the idea. Students may offer a more complex explanation if they've already had chemistry.

Next, have students watch the video and read the article "Dear Science: How does soap make things clean?" from the Washington Post to help them develop an explanation for how soap works.

Note: You may need to copy and paste the link below into Chrome (browser) to access the video and article.

https://www.washingtonpost.com/news/speaking-of-science/wp/2017/03/20/dear-science-how-does-soap-make-things-clean/

After students read the article and watch the video, revisit their initial questions:

Did the soap change the oil?
Why did the oil layer move to the sides of the dish?
Did the soap do something to the sides of the dish to make it attract the oil?
Why did the powder move too?
Why didn't the soap move the water?
Will the oil eventually move back?

Have a discussion about what questions they can answer now based on the information they gathered from the article and video. Students will have figured out:

Next, have students revisit their initial model to make revisions based on what they now know about how soap makes dishes clean. When students have make their revisions have a discussion about the new information they added to their models and how they can use their models to explain how soap works to make dishes - and many other things- clean.

Additional Guidance: Having students revisit and revise their model reinforces the idea that it is okay to change your thinking base on new information. Explain to students this is how science works; sometimes scientists change their minds when they gain new information or gather more data from additional investigations. Revisions also allows students to see the value of perseverance and that it is okay to document ideas before have all the information they need.

How does soap make our dishes clean?

Now that we understand more about how soap works to make our dishes clean, it makes us wonder about other things that happen in the kitchen. If you and your students would like to pursue another activity connected to this Dinner Table Discussion, check out Why does some corn pop? Daily Do.

NSTA Collection of Resources for Today's Daily Do

NSTA has created a How does soap clean our dishes? collection of resources to support teachers and families using this task. If you're an NSTA member, you can add this collection to your library by clicking Add to my library located near the top of the page.

Check Out Previous Daily Dos from NSTA

The NSTA Daily Do is an open educational resource (OER) and can be used by educators and families providing students distance and home science learning. Access the entire collection of NSTA Daily Dos.