Mixing and Changing: Evidence of New Substances

Before the mixing, you had two substances with very specific properties — vinegar was a clear, sour-smelling liquid, and baking soda was a dry, white powder. But after mixing, the fizzing, bubbling substance in the volcano seems to be something entirely new. The properties have changed in ways that can't be undone just by straining or filtering.

How do scientists figure out whether mixing has created a brand-new substance, or whether the original materials are just jumbled together? The answer lies in carefully comparing properties before and after mixing.

• What properties of the vinegar and baking soda changed after they were mixed together?
• Why do you think bubbles formed? Where did the gas come from?
• If you could measure the temperature before and after mixing, what evidence would that give you about whether a change occurred?

Every substance in the world has properties — characteristics that help us identify what it is. Think of properties as a substance's "fingerprint." Just like your fingerprint is unique to you, the set of properties for any substance is unique to that substance. When scientists want to know whether mixing two substances created something new, they use a powerful strategy: compare the properties before mixing to the properties after mixing.

If the properties after mixing are the same as what you started with — like mixing red and blue marbles in a jar — you just have a mixture where the original substances haven't really changed. But if the properties after mixing are completely different from what you started with — like new colors, new smells, gas bubbles, or temperature changes — that's evidence that a new substance has formed.

When scientists want to determine whether mixing has produced a new substance, they don't just guess — they plan a fair investigation and collect data. A fair investigation means you observe and measure properties carefully both before and after mixing, keeping everything else the same so you can be confident the changes are caused by the mixing itself.

Investigation question: When baking soda and vinegar are mixed, do the properties of the resulting mixture differ from the properties of the original substances?

Materials scientists might use:

• Baking soda (measured amount)
• Vinegar (measured amount)
• Thermometer (to measure temperature)
• Balance or scale (to measure weight)
• Sealed bag or closed container (to capture gas)
• Observation notebook

What they would observe: Vigorous fizzing and bubbling (gas production), a drop in temperature (the mixture gets cooler), the disappearance of the white powder, and a liquid that tastes and smells different from vinegar. If the mixture is sealed in a bag, the bag inflates with gas — but the total weight on the scale stays the same.

Notice how the investigation focused on measuring specific properties both before and after mixing. This is the science practice of planning and carrying out investigations — one of the most important things scientists do. By collecting data on properties like temperature, color, smell, and gas production, scientists gather the evidence they need to draw conclusions about whether a new substance was produced.

Let's look more carefully at the data from our investigation. When we compare properties before and after mixing, we're looking for patterns in the evidence that help us determine what happened. Not every change in properties means a new substance was formed — but certain types of changes are especially strong evidence.

For example, if you mix salt and water, the salt seems to disappear. But if you taste the water, it's salty — and if you let the water evaporate, the salt comes back. The salt didn't actually change into something new; it just dissolved. You can still get it back. Compare this to the baking soda and vinegar reaction: once they've mixed, you can't separate them and get back the original baking soda. That's because a new substance was truly created.

Scientists look at multiple pieces of evidence together, not just one property change. The more properties that change, the stronger the evidence that a new substance was formed. Here's what our investigation data shows:

The data shows five different properties that changed after mixing. That's strong evidence that mixing baking soda and vinegar produced one or more new substances. But notice that the total weight did not change. This is a fundamental principle: when substances are mixed and new substances form, the total amount of matter is conserved. Nothing was created from thin air, and nothing disappeared — the matter simply rearranged itself into new substances with new properties.

Scientists don't just study one experiment — they look for patterns that repeat across many different situations. The crosscutting concept that connects our mixing investigation to the rest of science is Cause and Effect: events have causes that generate observable patterns. When two specific substances are mixed, the pattern of property changes is predictable and repeatable.

Mixing baking soda and vinegar always produces gas bubbles and a temperature drop. It happens every time, not just once. That pattern tells scientists this is a reliable cause-and-effect relationship: the mixing (cause) produces a new substance with different properties (effect). Scientists design fair tests to identify these cause-and-effect patterns — they change one thing at a time to make sure the mixing itself is what caused the property changes.

This same idea of comparing properties before and after to look for evidence of change shows up all across science:

Notice the pattern: in the first three examples, the property changes are dramatic, irreversible, and include things like gas production, color change, and temperature change — all strong evidence that new substances formed. In the last example (dissolving salt), the change is reversible and doesn't produce gas or temperature changes, which tells us it's not the formation of a new substance.

Understanding how to identify new substances by comparing properties isn't just a classroom exercise — it's a skill that scientists and engineers use every day in the real world.

Food scientists compare properties before and after mixing ingredients to develop new recipes and ensure food safety. When milk is mixed with certain acids (like lemon juice), it curdles — the liquid turns into lumps. That property change tells the food scientist that a new substance has formed, which is exactly how cheese is made! By carefully controlling which substances are mixed and in what amounts, food scientists engineer products with the exact texture, flavor, and appearance they want.

Environmental engineers test water quality by mixing water samples with special chemicals called indicators. If the water's color changes when the indicator is added, it reveals the presence of pollutants or harmful substances. The property change (color) is evidence of a specific substance in the water.

Forensic scientists (the people who solve crimes using science) compare the properties of unknown substances found at crime scenes. By mixing an unknown powder with specific liquids and observing property changes — does it fizz? does the color change? does the temperature shift? — they can identify what the substance is. Every substance has a unique "fingerprint" of properties.

• Property — A characteristic of a substance that can be observed or measured, such as color, temperature, smell, texture, or state of matter. Properties help us identify and compare substances.
• Substance — A specific type of matter that has a unique set of properties. Water, baking soda, and vinegar are each different substances.
• Mixture — A combination of two or more substances where each substance keeps its own properties and can usually be separated. Trail mix is a mixture because you can pick out each ingredient.
• New substance — A substance that forms when two or more substances are mixed and the result has different properties than the starting materials. The new substance cannot easily be separated back into the original materials.
• Evidence — Observations, measurements, or data that support or disprove a claim. Scientists use evidence from property comparisons to determine whether mixing created something new.
• Conservation of weight — The principle that the total weight of substances before mixing equals the total weight after mixing, even when new substances form. Matter is not created or destroyed.
• Precipitate — A solid that forms inside a liquid when two liquids are mixed. The appearance of a precipitate is evidence that a new substance may have been created.
• Gas production — The formation of bubbles or fizzing when substances are mixed. Gas production is one of the strongest pieces of evidence that a new substance has formed.