Revising Models with Evidence

Marcus pours a cup of sugar into a glass of warm water and stirs. After about a minute, the sugar disappears completely—the water looks perfectly clear. Marcus knows the sugar is still "in there" because the water tastes sweet. He tries to weigh the glass: the water weighed 250 g, the sugar weighed 50 g, and the sugar-water now weighs exactly 300 g.

Marcus then blows air into the water through a straw, and he sees bubbles rise to the top and pop. He wonders: where did those bubbles come from? Is the air in the bubbles part of the water's weight? When the bubbles escape, does the cup get lighter?

To try to explain what's happening, Marcus draws a model showing big chunks of sugar sitting at the bottom of the glass, with air bubbles floating up. But his observations don't match his model—the sugar isn't sitting at the bottom at all! He realizes he needs to revise his model based on the evidence he has gathered.

• If the sugar "disappeared," where did it actually go? What is your evidence?
• Why does the total weight stay the same even though you can't see the sugar anymore?
• If Marcus's first model (sugar chunks at the bottom) doesn't match his observations, how should he change it?

Scientists have spent centuries studying what happens to substances that seem to "disappear" into air or water. The key discovery is that matter is made of particles too small to see, and those particles don't just vanish—they spread out and mix among other particles. This is the Disciplinary Core Idea behind today's lesson: matter is conserved, meaning the total amount (measured by weight) stays the same even when substances dissolve or mix with air.

When we build a model—a drawing, diagram, or description that explains how something works—we should always check it against the evidence. If our observations don't match our model, that's not a failure. It's actually how science works! We revise the model to better fit the evidence.

The Question

When a substance dissolves in water and "disappears," does the total weight of the mixture change?

Materials You Would Need

• A digital kitchen scale (reads in grams)
• A clear plastic cup
• Warm water (about 200 mL)
• One tablespoon of salt
• A stirring stick or spoon
• Notebook for recording data and drawing models

Procedure

When students perform the salt-water investigation, they consistently find the same result: the weight before mixing and the weight after mixing are exactly the same. This is powerful evidence that the salt didn't "disappear" — its particles are still present in the water. The total amount of matter is conserved.

But what about air? Air is trickier because we can't easily see it or feel its weight. Yet scientists have demonstrated through careful measurement that air has mass. When you seal air inside a container and weigh it, then remove the air and weigh again, you can detect the difference. And air can actually dissolve into water—cold water holds more dissolved air than warm water does. That's why you see tiny bubbles form on the walls of a glass as cold tap water warms up: the dissolved air is coming out of solution.

Let's look at what evidence tells us about Marcus's first model compared to a revised model:

Notice how the first model worked for one observation (weight is conserved) but failed to explain three others. The revised model explains all five observations. That's the power of revision: each piece of evidence helps you build a better, more complete explanation of what's happening.

The revised model on the right is a much better match for the evidence. It explains why the mixture is clear (particles are too small to see), why it tastes sweet everywhere (particles are spread evenly), and why the weight stays the same (all particles are still there). This is exactly how science progresses — models get better as we gather more evidence.

The crosscutting concept in this lesson is Scale, Proportion, and Quantity. Many of the changes we've been studying happen at a scale too small for our eyes to see. When sugar dissolves, the particles are still there — they're just too tiny to detect without special tools. When air dissolves into water, the gas particles mix in among the liquid particles at a scale we can't observe directly. Scientists look for patterns in what they CAN observe (like weight staying the same, sweet taste throughout the water, or bubbles forming) to make claims about what is happening at a scale they CAN'T observe.

This same pattern — using large-scale observations to understand small-scale changes — shows up all across science:

In every case, we can't see the particles directly, but we can use evidence from what we CAN see to build and revise our models of what's happening at the particle level. The pattern is the same: observable changes in weight, appearance, or behavior give us clues about invisible changes at the particle scale.

The skill of revising models based on evidence isn't just something scientists do in laboratories — it's a practice that engineers, doctors, weather forecasters, and even chefs use every day. Whenever someone builds an explanation and then checks it against reality, they are using the same thinking process you've been practicing in this lesson.

In all these cases, the process is the same: build a model → test it against evidence → revise if needed → test again. This cycle of revision is one of the most powerful tools in science and engineering.

• Model — A drawing, diagram, or description that represents how something works. Scientists use models to explain things they can't see directly.
• Revise — To change and improve. When new evidence doesn't match a model, scientists revise (update) the model to better explain their observations.
• Evidence — Information gathered from observations, measurements, or experiments that supports or challenges a scientific claim or model.
• Dissolve — When a solid substance (like sugar or salt) breaks apart into particles so small they spread out evenly in a liquid and can no longer be seen.
• Particle — An extremely tiny piece of matter that is too small to see with the naked eye. All matter is made of particles.
• Conservation of matter — The principle that matter is not created or destroyed during changes like dissolving or mixing. The total weight stays the same.
• Dissolved substance — A substance whose particles have spread out evenly in a liquid (or gas) so that it can no longer be seen as a separate material.
• Scale — The size at which something occurs. Particle-level changes happen at a very small scale that we can't see directly, but they cause changes we CAN observe at a larger scale.