Resource: Disappearing Milk

Most substances exist in one of the three main states of matter—solid, liquid, or gas. Each of these states is characterized by certain observable properties. For example, a solid has a fixed volume and holds its shape, whereas a liquid has a fixed volume but takes on the shape of its container. A solid is rigid, while a liquid is fluid, meaning it can flow and move.

However, some substances have properties that make them difficult to categorize. For example, a gel seems to have components of both solids and liquids—it sort of holds its shape, but can also take on the shape of its container. So is it a solid or a liquid? As it turns out, a gel is actually a combination of both states; it is a mixture of a solid and a liquid.

A super absorbent polymer (a polymer is a long chain composed of repeating units of molecules) is a type of chemical able to create a gel by absorbing large amounts of water. One of the most common super absorbent polymers is sodium polyacrylate, which can absorb up to 800 times its weight in distilled water.

Sodium polyacrylate is usually found in powdered or granular form, where each strand of the polymer is tightly coiled. However, when the polymer is mixed with water, the water molecules are absorbed inside the polymer. The water molecules bond to the strands, causing them to swell and uncoil. However, the polymer has cross-links, meaning that the polymer strands form a web of intertwined molecules that acts like a sponge.

This web of molecules is what gives hydrated sodium polyacrylate the consistency of a gel. When the polymer is combined with water, the resulting mixture is mostly liquid held in a network of solid strands. In this video, the sodium polyacrylate starts off as a solid in powder form, but when mixed with the liquid milk, the result is a gel. This characteristic of super absorbent polymers makes them useful in many applications, such as in disposable baby diapers.

To learn more about polymers, check out The Impact of Technology: Nylon.

To learn more about another substance with unusual properties, check out Mystery Mud: Exploring Changes in States of Matter.

To learn more about the structure of matter, check out Atoms: The Space Between.

• What are the properties of the milk before it was poured into the glass, and what are the properties after? What changed? What didn't change?
• Can you imagine a way to make the milk turn quickly from a liquid to a solid?
• Was this a chemical reaction or a physical change? (Or both?) How could you support this theory? Could you change the solid back into a liquid? If so, how?
• What useful products might be made or tasks accomplished using the properties of the powder in the glass? (Diapers are one example.)
• Do you think you could substitute water for milk and get the same or a similar reaction? Why or why not?