Designing a Salt Water/Fresh Water Experiment

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SWBAT design an experiment to test whether all plants can grow in salt water.

Big Idea

By testing a hypothesis, students gain a clearer understanding of how plants must adapt to their environment

Teacher Background

The Next Generation Science Standards require that students design an experiment to see if plants can grow with and without sunlight. This lesson has students testing salt and fresh water, but to meet this standard it would be easy to also place some of the plants in darkness and some in the light. Students could then test for all 4 variables at the same time.  The lesson is only written for types of water, but the sunlight/darkness piece is one that could also contribute to the understanding of how living things adapt to their environments, and how certain living things are better suited for one habitat or another, such as needing a location with more or less light in order to grow.

This lesson includes several weeks of observing the growing plants. The initial lesson and final conclusions are full science days. The lessons in between are just quick check ins to observe and record the growth.

I recommend a simple plant such as string beans for this experiment because they grow quickly and are hardy.

I Can Statement and Introduction

10 minutes

I ask students, "do you remember in our last lesson when we looked at wetland plants, we noticed that some plants grew in what kind of water? (salt water) Today we are going to create an experiment that over the time will help us see if plants from other habitats can survive in salt water. Let's read the I Can Statement together." We read, "We will try to find out if other plants can survive in salt water."

"What do you think we need to do in order to find out if plants grow in salt water the way they do with regular water?" (water them with salt water, put salt water on them, etc.) "Right and as a scientist what is it called when we try to figure something out?" (an experiment) "Does anyone remember what we need to create an experiment?" (A question). "Right we need to begin with a question. Can anyone suggest a question that we could ask about how plants grow in salt water?" 

I let students suggest questions and I jot them down on the board. We talk about our purpose to test if plants can grow with salt water and then I hand out the student journal page and ask students to write their question on their papers. 

Designing the Experiment

20 minutes

Once students have written their questions on their journal page, I ask them, "what is the next part of creating an experiment?" (the hypothesis). "Yes, the next part is the hypothesis (if students don't remember this, I ask questions to lead them towards the idea that we need to predict, or make a good guess about what will happen.) "Would you write a hypothesis about what you think the answer to your question is?" I give students a few minutes to write a question. My students are not at a point where they can create a full experimental design without scaffolding. I help them walk through the steps and support them in reviewing the important parts of creating an experiment.

"When you have written your hypothesis, put your hand on your head and I will know you are ready to go on." When students have created a hypothesis I ask, "What do we need now to design our experiment?" (What we are going to do, directions, etc.). Again I support students by scaffolding their thinking about how we have a question and a guess now before we can answer our question, what do we need to do?

I say, "today we will design the experiment together. What might we need first?" (several plants)  "Right so step 1 might be to get or plant plants. For this we will start with seedlings. Do you know what a seedling is?" (A baby plant). "So for step 1 what might you say?" (Get seedling, baby plants). "Ok now what do plants need in oder to live?" (water, sunlight, dirt,). "Our plants are already in dirt but they do need sunlight so what might we write?" (put plants in sunlight) "Would you write that in your journal?" "Now how can we test if the plants can grow with salt water?" (water them all with salt water). "How will we know if these plants grow with regular water if we water them all with salt water?" (we could water some with salt water and some with plain water). "Ok, that sounds like a good idea. Could you write we will water half with salt water and half with fresh water?"

After students have written that I ask, "how will we know which plants are which?" (we need to label them)."Right, so would you write label the plants fresh and salt water." "Do we just water the plants one time?" (no) "So what should we write?" (water the plants every day, water the plants every 2 days..) "OK, so add that to your journal." "How will we know if the plants have grown?" (we can measure them, look at them, etc.) "Right so you have 1 more step to write in your journal, observe and measure the plants."

"We have designed a good experiment. What happens next?" (we do the experiment and see what happens.) "Do we need to keep track of our experiment?" (Yes we need to record what we see). fresh:salt journal.pdf


Observing Our Plants

15 minutes

This experiment will take some time to carry out. Students will set up the seedlings today, label them, measure them and put the initial fresh or salt water on the plants. I will designate waterers for each day and have a small cup for each type of water with the plants. Each morning at circle time I will designate 2 children to report out on what they are seeing. We will keep a class observation chart of the growth of the 2 seedlings.

"Today we will set up our seedlings as you have written in your journal. I will ask each partnership to label one plant salt, one fresh and put their math numbers on the label (math numbers are just numbers assigned to students at the beginning of the year putting the students in alphabetical order. It is a way to have students keep track of their work without having to write their whole names on something). Your plant pot should have one math numbers and say salt and other one will have the other math number on it and will say fresh. Would you take a ruler and set it on the dirt and measure the height to the nearest inch of the height of your seedling and record it on the class chart above your math number? Also note in your own journal if you notice any leaves, flowers, buds, brown spots, etc."

I give students about 15 minutes to label and record about their plants. When everyone has recorded their initial sizes and observations I say, "Now if you have a fresh pot, I would like you to place it over here on the side of the table labeled fresh." When they are done, I repeat the directions for the salt water plants. 

"Now I have this small cup for fresh water and this one for salt water. I will ask the math numbers 1 and 2 to come up and pour a little water on each plant of their kind." One child waters all the fresh plants and one waters all the salt water plants. 

"I have posted a schedule and each day 2 math numbers will water the plants reading which kind to water on the chart. You will also tell us about the plants at Morning Meeting. Every couple of days we will record our observations in our own journals as well, with you finding your own plant and writing down its size and what you notice."

Follow Up

5 minutes

At the beginning of science lessons every 3rd day, I ask students to find their plants, measure the height and record any things they notice that are changes in their plants, i.e. buds, brown leaves, etc.

We do this until there are noticeable differences in the 2 sets of plants, or for 3 weeks until growth of all plants is obvious.

Drawing Conclusions

20 minutes

Now that we have seen what happens when we water plants with salt and fresh water, I invite students to bring their journals and come to the rug. I bring the plants over so students can see them. 

"We have been watching the plants for several weeks. What have you noticed?" I let students talk about the changes they have seen, about which plants have grown the most, about any changes such as buds or browning leaves, about which set of plants are the tallest, healthiest, etc.

"This has been an interesting experiment. Now we need to record our conclusions. Look back at your hypothesis. Did you predict that the plants would all grow the same, or that there would be differences? Now you need to write your conclusions. You can refer back to your observation notes and your hypothesis as you tell now about what happened. You will notice a new part to the conclusion. It says, "I think this happened because.." I want you to tell in your conclusion what happened in our experiment and then tell why you think this happened."

I send students back to their seats to write in their journals. I collect the journals to review their understanding of the scientific process, and to see if they were able to draw logical conclusions about the experiment.

Connection to Diversity and Adaptations to a Habitat

15 minutes

I invite students to the rug. I read aloud some of the conclusions that students put in their journals. I say, "do you think there is a reason that some plants or animals live in one habitat and some live in another?" I see if students come up with the idea (probably not the term) of adapting to the habitat. If they do not I lead them towards the idea by asking questions such as, " why can't all plants or animals live in the same habitat? Why is it that we saw plants when we studied the marsh that were living in salt water, but the plants that we had didn't do as well with the salt water? Do you think certain animals live better in one habitat than another?" 

The experiment and this final discussion should help students have a clearer understanding of diversity and adaptations to habitats.