Place an egg into a bowl filled with vinegar overnight. After 24 hours, the eggshell will dissolve as calcium carbonate reacts with the acidic vinegar. You and your first grade scientists can use a flashlight to examine the yolk of an intact egg. Let your students take the lead, but consider having a book or diagram with the labeled parts of the yolk and membrane to compare.
Students will be amazed when you demonstrate the three different states of matter that water can take. Start with water in its liquid state and discuss the properties of a liquid, such as that it takes the shape of its container. Let students pour water into an ice cube tray and watch as it takes the shape of the tray. Put the tray into the freezer. While you are waiting for the water to freeze, pour water into a teapot and place on the stove until it boils and starts to emit steam (water in its gas form). Let some of the steam collect on a bottle of cold water to show students how the steam turns back to water droplets when it cools. Pull the ice cubes out of the freezer to show water in its solid form--ice. Place the ice cubes in a glass to demonstrate that ice cubes will also turn back to water as they warm up.
This experiment goes by many names, but the result is always the same. Mingle a solid with a liquid to make a whole new state of matter that will fascinate your first graders, especially when they are allowed to dig in with their hands. Pour cornstarch into a shallow pan large enough for several students to share. Let students add a little bit of water at a time, mixing with their hands. Allow them to press the mixture to see how it feels like a solid and then pour it to see how it seems like a liquid.
There is no better way to make science interesting to a first grader than to do a science project they can eat. Experience the fun of baking as chemistry with your student by creating three small cakes, each with a missing ingredient, to discuss the properties of each substance. Find your favorite cake recipe and then divide all the ingredients by three. Prepare three small cake pans and then mix one-third of the cake ingredients in three separate bowls, leaving out one key ingredient of each cake. Leave the egg out of the first, the oil out of the next and the baking powder out of the third. This is a good time to make predictions in your science journals if your students use them. Bake the cakes in the oven, let cool, and then cut open the cakes to discuss the physical properties. The first cake should be less firm without the protein of the egg, the second should be dry without the oil and the third should be denser without the baking powder to create gas bubbles in the batter. Let students taste the cakes and discuss and record their observations.