Mixing It Up: Changes When Substances Combine

The Phenomenon

Meanwhile, at the snack table, someone stirs a spoonful of sugar into a cup of warm water. The sugar crystals slowly disappear. The water looks exactly the same — clear and colorless — but when you sip it, the water tastes sweet. If you leave the cup on a windowsill for a week, the water evaporates and the sugar crystals reappear.

Both of these are examples of mixing substances together. But look closely — the results are very different. One mixture produced something entirely new. The other just blended the original ingredients.

• Why did the baking soda and vinegar produce bubbles and gas, while the sugar simply disappeared into the water?
• How could you figure out if a new substance was really created, or if the original substances just mixed together?
• What kinds of clues — things you can see, feel, hear, or smell — would tell you that a change happened when two substances were mixed?

What Scientists Know

When you combine two or more substances, the result isn't always the same kind of change. Scientists have discovered that mixing substances can cause either a physical change or a chemical change — and learning to tell the difference is a key skill in understanding how matter behaves. The clues are all around you if you know what to look for.

Let's Investigate

Fair Test: Which Mixtures Create New Substances?

Scientists use a Science and Engineering Practice called planning and carrying out investigations. They design fair tests — experiments where they change only one thing at a time and keep everything else the same — to gather evidence about how the world works. Here's an investigation you could carry out to study what happens when different substances are mixed.

Research Question: When we mix different pairs of substances, which mixtures show evidence of a new substance forming?

Materials:

• 4 clear cups
• Baking soda, vinegar, salt, warm water, lemon juice, milk
• A kitchen scale (to measure weight before and after)
• A thermometer (to check temperature changes)
• Observation chart

Procedure:

• Weigh each substance separately and record the total weight of both substances before mixing.
• Mix each pair of substances in a cup and immediately observe: Do you see bubbles? A color change? Does the temperature change? Is a new smell produced?
• After each mixture settles, weigh the cup and its contents. Record the final weight.
• Try to separate the substances back to their original forms. Can you do it?
• Record all observations on your chart.

What to expect: Some mixtures (like salt + water) will show no dramatic clues — the salt dissolves but no new substance forms. Other mixtures (like baking soda + lemon juice) will fizz, produce gas, and change temperature — strong evidence that a chemical change occurred.

What We Discovered

When scientists carried out investigations like the one described above, they discovered a clear pattern in the evidence. Some mixtures show dramatic, observable clues — bubbling, temperature changes, color changes, new odors, or solid clumps forming — while other mixtures show no such clues at all. The presence or absence of these clues tells us what kind of change occurred.

When baking soda and vinegar were mixed, the vigorous fizzing was evidence that a gas (carbon dioxide) was being produced — a substance that wasn't there before. The temperature drop confirmed that a chemical reaction had taken place. When lemon juice was added to milk, the proteins in the milk changed structure and formed solid curds — another new substance. These are both chemical changes because new substances with new properties were created.

By contrast, when salt or sugar dissolved in water, there were no dramatic clues. The dissolved salt and sugar were still present — just spread out invisibly among the water particles. The evidence? You could get the original substances back by simply evaporating the water. These are physical changes — the substances were mixed, but their identity didn't change.

One critical observation held true across every single test: when scientists carefully measured the total weight before and after mixing, the weight was always the same — as long as they captured any gas that escaped. Matter wasn't appearing from nowhere or vanishing into thin air. It was simply rearranging itself.

* The weight decreased only because CO₂ gas escaped into the air. If the cup had been sealed, the weight would have stayed the same.

Patterns and Connections

One of the most powerful ideas in science is the crosscutting concept of Cause and Effect. Scientists know that events don't happen randomly — they have causes, and those causes produce predictable, observable effects. When you learn to identify the effect (what you see, feel, or measure), you can trace it back to the cause (what made it happen). This pattern works across every area of science.

In our investigation, the cause was mixing two specific substances together. The effect was the set of observable changes — or the lack of changes. Scientists use these cause-and-effect relationships to predict what will happen in new situations. If you know that acids (like vinegar and lemon juice) react with bases (like baking soda), you can predict that mixing any acid with any base will produce a chemical change.

Cause and Effect Across Science

Do you see the pattern? In every case, there is a specific cause that leads to a predictable, observable effect. Scientists design investigations to test cause-and-effect relationships, and engineers use these relationships to design solutions that work reliably.

Real-World Connections & Engineering

Understanding what happens when substances are mixed isn't just a classroom activity — it's knowledge that people use every single day to solve problems, create new materials, and keep us safe.

Design Challenge: Cold Packs

Have you ever used an instant cold pack on a sports injury? Inside the pack, two substances are kept separate by a thin barrier. When you squeeze the pack, the barrier breaks, the substances mix, and a chemical change causes the temperature to drop dramatically — creating an ice-cold pack without a freezer.

If you were an engineer designing a cold pack, you'd need to think about: Which substances produce the biggest temperature drop when mixed? How long does the cold effect last? Is the reaction safe to hold against skin? These are all questions that require understanding what happens when substances are mixed — exactly what you learned in this lesson.

Key Vocabulary Review

• Substance — A type of matter that has a specific set of properties (such as color, smell, melting point, and density) that can be used to identify it.
• Physical change — A change in the appearance or form of a substance that does NOT create a new substance. The original substance can usually be recovered.
• Chemical change (chemical reaction) — A process in which one or more substances interact to produce one or more new substances with different properties.
• Precipitate — A solid substance that forms when two liquids are mixed together. It's one clue that a chemical change has occurred.
• Conservation of matter — The principle that the total amount (weight) of matter stays the same before and after any change — matter is not created or destroyed.
• Observable evidence — Things you can see, smell, feel, hear, or measure that give you information about what is happening in a mixture or reaction.
• Properties — Characteristics of a substance that can be observed or measured, such as color, texture, hardness, temperature, and solubility.

Practice: Test Your Understanding

What's Next?