Sinking, Rising, and the Role of Cold Water: A Hands-on Experiment

Mangroves thrive in brackish water—a mix of salt and fresh water. But what makes brackish water unique? How does it form? And doesn’t salt water always mix right away when added to fresh water? These were the questions we set out to explore in our latest hands-on experiment as part of our ongoing MDP on mangroves.

We began with a fish tank filled with cold fresh water. Then we prepared two bottles—one containing salt water mixed with food coloring and the other containing fresh water with a different food coloring. In this case, the orange-colored water was salt water, and the blue-colored water was fresh water so we could observe its movement. But here’s the fun part—I hadn’t tried this experiment before either! When the children asked, “Have you done this before?” I told them, “No, I’m learning along with you.” It was important for them to see that I don’t always have to know the answers beforehand. Learning is about curiosity, and I was just as eager as they were to see what would happen.

When we released the orange-colored salt water, it sank straight to the bottom. Here's the twist: the children didn’t know that yet. They weren’t told which bottle contained salt water and which had fresh water—they had to figure it out based on what they observed.

That’s when the room filled with chatter. The children split into two groups:

One group believed the orange-colored water was salt water because it came out easily and settled at the bottom.

The others thought the blue-colored fresh water was actually salt water because it struggled to come out, making them think it must be denser and heavier.

They took time to observe, think it through, and discuss their ideas within their teams before settling on their answers. Slowly, the pieces started falling into place.

Then, we asked an important question: Why didn’t the orange-colored water mix with the rest right away? That’s when we considered the role of the cold water in the tank. Usually, when you mix salt water with fresh water, it should blend. But here, it didn’t.

Through discussion, the children realized that temperature affects density. The cold fresh water influenced how the salt water behaved, preventing it from immediately mixing and instead making it sink.

To make things even clearer, we stirred up the water and repeated the experiment—this time switching the colors. Now, the blue-colored water represented salt water. This second trial made the sinking effect even more visible. Once again, the salt water came out and sank to the bottom, reinforcing the concept in a way the children could clearly see.

By the end of the activity, they understood that brackish water is denser than fresh water but not as dense as seawater, and its formation is influenced by factors like salinity, temperature, and movement.

Beyond the science, this experiment was a joy to watch. The children were completely engaged—not just with the water, but with their own thoughts, reasoning, and teamwork. They debated, reconsidered, and ultimately connected their observations to how mangroves survive in these dynamic water conditions.

It was a reminder that hands-on exploration isn’t just about finding the right answer—it’s about thinking, questioning, and discovering together.