Mixing Substances: What Really Happens?

The Phenomenon

Here's what's strange: you can't simply "un-mix" the cake to get back the flour, eggs, and milk. The original ingredients seem to have vanished. Yet if you weigh all the ingredients before mixing and then weigh the finished cake, the numbers are remarkably close. The stuff is still there — but it has changed into something new.

Not all mixing works this way, though. If you stir sand into a jar of water, you can let it settle and separate it right back out. The sand doesn't change at all. So what makes one type of mixing different from another?

• Why can't you separate the cake back into eggs, flour, and milk?
• If you stir sand into water, can you separate them again? What makes that different from baking a cake?
• If the total weight of the ingredients is about the same as the weight of the cake, where did the original substances go?

What Scientists Know

When you combine two or more substances, one of two things can happen. Sometimes the substances just get mixed together but each one keeps its own properties — you can still tell them apart and separate them. Other times, the substances interact to form entirely new substances with different properties. Understanding the difference between these two outcomes is one of the most important ideas in the study of matter.

Let's Investigate

Scientists don't just guess about what happens when substances are mixed — they plan and carry out investigations. That means deciding on a clear question, choosing materials carefully, measuring before and after, making detailed observations, and recording data so the results can be compared. Let's look at an investigation designed to answer this question: When we mix different pairs of substances, which combinations create a new substance and which create just a mixture?

Testing Four Substance Combinations

Question: Which combinations of substances show evidence that a new substance has formed?

Materials:

• 4 clear cups labeled A, B, C, D
• Baking soda, vinegar, salt, water, sand, lemon juice
• A kitchen scale (in grams)
• A thermometer
• Safety goggles and a data notebook

Procedure:

• Weigh each substance separately before mixing. Record the individual weights and calculate the total starting weight for each pair.
• Measure the starting temperature of each liquid.
• Mix each pair of substances in its labeled cup and observe what happens immediately. Record any changes in color, temperature, bubbling, smell, or solid formation.
• After 5 minutes, weigh each cup with its contents. Record the final weight.
• Try to separate the substances — can you get the original materials back?

Fair Test: Use the same amounts (50 mL liquid, 10 g solid) for every trial and measure everything the same way. This ensures a fair comparison across all four combinations.

What We Discovered

The investigation revealed a clear pattern. In Cups A and D (baking soda + vinegar, and baking soda + lemon juice), the substances didn't just sit side by side — they reacted. The vigorous bubbling was evidence that a gas was being produced, and the temperature drop showed energy was being absorbed during the reaction. These observable changes are strong evidence that new substances formed that were not present before mixing.

In Cups B and C (salt + water, and sand + water), there were no dramatic changes. The salt dissolved into the water but didn't change into anything new — you could boil the water away and recover the salt crystals. The sand simply sank to the bottom and could be filtered out. In both cases, the original substances kept their own properties, which tells us these were mixtures, not reactions that formed new substances.

What about weight? Look at the data below from a careful measurement:

*The small weight loss in Cups A and D happened because the gas (carbon dioxide) that formed during the reaction escaped into the air. If you sealed the cup so no gas could leave, the weight would stay exactly the same.

This is a crucial finding: the total weight of matter is conserved. Matter was not destroyed — it just changed form. Some of it became an invisible gas that floated away. If scientists could capture that gas and weigh it along with the liquid left behind, the total would match the starting weight perfectly. This principle — conservation of weight — holds true whether you make a simple mixture or create brand-new substances.

Patterns and Connections

One of the most powerful tools scientists have is recognizing patterns. When you look at all the investigations involving mixed substances, a reliable pattern emerges: certain observable clues consistently indicate that a new substance has formed. This pattern doesn't just show up in one experiment — it appears across many different situations in science.

Scientists call this crosscutting concept Cause and Effect. When two substances are combined (the cause), the result (the effect) depends on the specific properties of those substances. Some combinations produce dramatic effects — bubbling, color changes, temperature shifts — while others produce no change at all. By carefully observing the effect, scientists can determine what kind of interaction occurred.

Notice the pattern: whenever a new substance forms, there are observable changes in properties (color, temperature, gas production, smell). Whenever substances simply mix, each one keeps its original properties. This pattern is consistent and predictable, which is why scientists can use these clues to draw conclusions across many different fields of study.

Real-World Connections & Engineering

Understanding what happens when substances mix isn't just a classroom exercise — it's essential knowledge used by scientists, engineers, chefs, doctors, and many others every single day.

Design the Best Fizzy Reaction

Problem: A toy company wants to create a "fizzy bath bomb" that produces the most bubbles when dropped in water.

Your challenge: Using what you know about substances that react versus simply mix, how would you design an investigation to test different combinations of ingredients? Consider: Which substances would you test? How would you measure "most bubbles"? How would you keep the test fair? What would you change between trials, and what would you keep the same?

This is exactly how engineers work — they use scientific knowledge about how substances interact to design products that solve real problems.

Key Vocabulary Review

• Substance — A type of matter that has specific, identifiable properties (such as color, texture, melting point, or density). Water, salt, and iron are all substances.
• Mixture — A combination of two or more substances in which each substance keeps its own properties and can be separated by physical means (filtering, evaporating, picking apart).
• Chemical reaction — A process in which substances interact and rearrange to form one or more new substances with different properties than the originals.
• Properties — Characteristics of a substance that can be observed or measured, such as color, hardness, texture, temperature, smell, and state of matter.
• Conservation of weight — The principle that the total weight of substances before mixing equals the total weight after mixing, as long as no matter escapes (like gas floating away).
• Precipitate — A solid substance that forms when two liquids are mixed together; one sign that a new substance has been created.
• Fair test — An investigation in which only one variable is changed at a time while all other conditions are kept the same, ensuring the results are reliable.
• Evidence — Observations and data collected during an investigation that support or refute an explanation.

Practice: Test Your Understanding

What's Next?