Most second-grade students have probably noticed that change sometimes affects familiar objects in their world. For example, they may have watched sugar "disappear' in a glass of lemonade or observed ice melting on a hot summer day. They may have wondered why bicycles rust in the rain or noticed that yellow paint turns orange when they add red paint to it.

Changes, a 16-lesson unit designed for second-graders, helps students investigate examples of changes that affect their daily lives. Specifically, students examine some changes that occur when solids and liquids are mixed or change state. They consider how water freezes, melts, evaporates, and condenses. They observe the properties of solids, liquids, and gases and describe some of the changes that take place when substances are combined or separated. Over the course of the unit, they notice that some changes occur quickly and that others take longer.

Lesson 1 serves as a pre-unit assessment of students' knowledge. The lesson begins with a brainstorming session in which students share what they know about the concept of change and about solids and liquids. Then, looking at cards that illustrate everyday scenes, students identify solids and liquids and predict how the materials pictured might change over time. Students have the opportunity to observe change taking place as they add an effervescent tablet (such as Alka-Seltzer¨) to a cup of water. The discussion that follows this investigation helps the teacher assess students' knowledge of solids, liquids, and gases and some of the changes that can take place when solids and liquids are mixed. Finally, students prepare for Lesson 2 by watching their teacher fill ice cube trays with water. The trays are put in a freezer for the next lesson.

In Lessons 2 and 3, students investigate how water changes state. They begin by describing how the water in the ice cube trays has turned to ice. Students then design a method to melt the ice in the quickest time possible. At the close of the lesson, they place the melted ice in a petri dish. Over the next few days they observe changes in the water as it evaporates over time. In Lesson 3, students discuss these changes caused by evaporation and then set up an investigation in which they observe condensate forming inside the cover of a cup of warm water. This activity sets the stage for later lessons, in which students use evaporation to separate a salt-and-water solution.

Lesson 4 begins a series of lessons in which students investigate changes involving mixtures. To grasp the concept of "mixture," students first put two solids--salt and gravel--in a cup, observing how the component parts intermingle as they are dispersed throughout the cup. Using a sieve, students then discover that they can separate their mixture again into its individual parts, and they develop a basis for understanding the mixtures they create in later lessons.

In Lesson 5, students mix a liquid--water--with three different solids--gravel, toilet tissue, and salt--and observe and compare the changes that result. As students observe the three different mixtures and ask, "Where did the salt go?" they are preparing for Lesson 6, in which they use filtration to attempt to separate the three mixtures. In Lesson 6, students find that they can separate two of their mixtures using filtration, but they are unable to separate their salt-and-water solution with this process. This leads to a discussion of the concept of dissolving, and they set up an experiment to use the process of evaporation, learned in Lesson 3, to separate the salt from the water. They will review the results of this experiment in Lesson 9.

Lessons 7 and 8 give students the opportunity to explore the process of dissolving further. In these lessons, they conduct "dissolving races" with sugar and water. In the race in Lesson 7, they compare how two different forms of sugar--a cube and granulated sugar-dissolve in water at room temperature. In the race in Lesson 8, students experiment with one form of sugar, granulated, in water at two different temperatures--very warm and cold. A discussion of their observations helps students apply to their own lives what they have learned about dissolving.

In Lesson 9, students take another look at the petri dishes of salt solution they set up in Lesson 6. They observe some dramatic changes--the formation of large salt crystals and the disappearance of the water. This experience enables students to add evaporation to their list of methods for separating mixtures. It also helps them see that, although some changes occur quickly, others take more time. Lesson 10 introduces students to another method for separating mixtures: chro-matography. Using ink, which is an already-existing liquid mixture, students see that water and filter paper can be used to separate the ink into its original component colors.

Lesson 11, an embedded assessment, completes a series of lessons on mixing and separating. Given a mystery mixture (salt and pepper), students are asked to identify and separate the components of the mixture. Using skills and concepts that they learned in Lessons 1 through 10, students add water to the mixture and then use a sieve, filtration, and evaporation to separate the mixture into its original solids. This embedded assessment enables the teacher to evaluate students' understanding of the properties of mixtures and to assess the skills they have developed in separating and identifying the components of mixtures.

Lesson 12 begins a series of investigations in which students consider changes that result when substances react chemically with one another. As students discuss these changes in the next few lessons, they begin to recognize some characteristics of chemical reactions: some combinations of substances result in the creation of visible new substances (such as gas or rust), while other reactions produce a change in odor or color.

In Lesson 12, students mix baking soda with water and then with vinegar. By observing and comparing the changes that occur, students recognize that mixing some substances, such as vinegar and baking soda, results in the formation of a new substance--a gas that was not present originally. Students continue their investigation of chemical reactions in Lesson 13 as they return to the solid and liquid they had used in Lesson 1--an effervescent tablet and water--to capture and explore the properties of the gas produced. In Lesson 14, students observe how rust can form. They set up a rusting experiment in which they discover that, in some instances, rust develops much more quickly than in others. Students also focus on other signs of a chemical reaction, such as a change in color or a change in temperature. These lessons reinforce students' understanding that some changes happen quickly and others can take a long time.

Lessons 15 and 16 serve as a second embedded assessment. In Lesson 15, selecting substances previously used in the unit or new substances with properties similar to those used earlier, students create their own recipes to bring about a specific change. In Lesson 16, students exchange recipes and demonstrate the changes that occur in each case. This embedded assessment allows students to analyze the changes they have observed during the unit and apply what they have learned earlier.

Following Lesson 16 is a post-unit assessment that is matched to the pre-unit assessment in Lesson 1. Additional assessments provide further opportunities for evaluating student progress.