The Phenomenon: A Kitchen Mystery
Both situations involved mixing two or more things together — yet one mixture left everything unchanged, while the other produced an entirely new substance that wasn't there before. What makes these two events so different?
💭 Thinking Questions
- What clues tell you that something new was created when baking soda met vinegar?
- Why can you pick the croutons out of a salad, but you can't "un-fizz" the vinegar and baking soda?
- What evidence would you look for to figure out whether mixing two things creates a new substance?
What Scientists Know
When you combine two or more substances, you create a mixture. But not all mixtures behave the same way. Scientists divide mixing events into two main categories based on a powerful question: Did the substances keep their original properties, or did something entirely new form?
The answer depends on whether a chemical reaction took place. A chemical reaction is a process in which the original substances (called reactants) rearrange at the particle level and produce one or more new substances with different properties. When no chemical reaction occurs, the substances simply sit together — they can be separated again, and nothing new is created.
Mixtures Without New Substances
Mixtures That Produce New Substances
Evidence of a New Substance
Weight Is Conserved
Let's Investigate
Testing Mixtures for New Substances
What scientists do: Scientists plan and carry out investigations to collect evidence. They compare properties before and after mixing to determine whether a new substance formed. They also measure weight to check whether matter is conserved.
Investigation question: When we mix different pairs of substances together, which pairs create a new substance and which do not?
Materials:
- Baking soda
- Vinegar
- Sand
- Water
- Milk
- Lemon juice
- A balance/scale
- Clear cups, spoons
Procedure: For each pair of substances, record the properties before mixing (color, smell, texture, temperature, weight). Then mix them, observe carefully for any signs of change, and record the properties again. Weigh the result on the balance to compare total weight before and after.
What to observe: Look for these signs of a new substance: unexpected bubbles, color change, temperature change (feel the cup), new smell, or a solid forming in a liquid.
What We Discovered
When scientists carry out this type of investigation, they find a clear pattern in the data. Some pairs of substances can be mixed and then separated again with no lasting change. Others combine and produce new materials that have completely different properties. Let's look at what the results might show:
| Substances Mixed | Observations After Mixing | New Substance? | Weight Change |
|---|---|---|---|
| Sand + Water | Sand sinks; water stays clear above it. No temperature change. | No | 0 g change |
| Baking soda + Vinegar | Fizzing, bubbles (gas), temperature drops slightly. Cannot get original substances back. | Yes | 0 g (in closed container) |
| Salt + Water | Salt dissolves; clear liquid. Can evaporate water to recover salt. | No | 0 g change |
| Milk + Lemon juice | Milk curdles — white lumps (solid curds) form in liquid. New texture and smell. | Yes | 0 g change |
Notice that in every single row, the weight stayed the same. That's the conservation of matter in action — no matter disappeared and no new matter was created from nothing. The matter simply rearranged itself.
The key difference is in the properties of what you end up with. When sand is stirred into water, the sand is still sand and the water is still water — you can let the sand settle, pour off the water, and recover both. But when baking soda reacts with vinegar, the fizzing gas is carbon dioxide — a completely different substance that wasn't there before. The liquid left behind is mostly water with a dissolved salt called sodium acetate, which is also a new substance. The original baking soda and vinegar no longer exist in their original form.
The investigation reveals that whether or not new substances form depends on whether the particles of the original materials rearrange into new combinations. In a simple mixture, the particles just sit side by side without changing. In a chemical reaction, the particles break apart and reconnect in new ways, creating substances with entirely different properties — different colors, textures, smells, or states of matter.
Patterns and Connections
The crosscutting concept at work in this lesson is Cause and Effect. Scientists design tests to identify causes — in this case, what causes a mixture to produce a new substance versus leaving the original substances unchanged? The cause is a chemical reaction at the particle level, and the effects are the observable changes we can detect.
This same pattern of cause and effect appears throughout science. Whenever you see a change in properties, you can ask: What caused this? Is the change reversible or not? Let's look at examples across different areas of science:
| Area of Science | Cause | Effect (Observable Change) | New Substance? |
|---|---|---|---|
| Physical Science — Baking soda + vinegar | Chemical reaction between an acid and a base | Bubbles, temperature change, gas produced | Yes |
| Physical Science — Salt + water | Salt dissolves (particles spread out) | Salt "disappears" into water; clear liquid | No |
| Life Science — Food digestion | Enzymes break down food chemically | Food changes color, texture, and form in the stomach | Yes |
| Earth Science — Iron + water + air | Chemical reaction (rusting/oxidation) | Shiny iron becomes flaky orange rust | Yes |
| Earth Science — Rocks in a river | Physical weathering (grinding) | Rocks get smaller and smoother over time | No |
Do you see the pattern? When the cause is a chemical reaction (particles rearranging), the effect is a new substance with different properties. When the cause is only a physical change (particles moving around but not rearranging), the original substances keep their properties. This cause-and-effect relationship is the same whether we're talking about a kitchen experiment, digestion in your body, or rust forming on a bike.
Real-World Connections & Engineering
Understanding which mixtures create new substances — and which don't — is at the heart of many real-world technologies and daily activities. Here are some ways this science shows up in life:
🍞 Baking Bread
♻️ Recycling and Separation
💊 Medicine
🚗 Rust Prevention
In each of these examples, engineers use the same scientific understanding you've been building in this lesson: some mixtures react and form new substances, while others don't. Knowing which is which — and being able to test for it — is a powerful tool for solving real-world problems.
Key Vocabulary Review
- Mixture — A combination of two or more substances where each substance keeps its own properties. The substances can usually be separated again.
- Chemical Reaction — A process where substances interact and rearrange at the particle level to form one or more new substances with different properties.
- New Substance — A material that has different properties (color, smell, texture, etc.) from the starting materials. It forms as the result of a chemical reaction.
- Properties — Characteristics of a substance that can be observed or measured, such as color, smell, texture, hardness, or melting point.
- Conservation of Matter — The principle that the total weight (mass) of substances before a change equals the total weight after the change. Matter is neither created nor destroyed.
- Evidence — Observations or data that support or disprove a scientific claim. Signs of a chemical reaction (bubbles, color change, temperature change) are evidence of a new substance forming.
- Reversible Change — A change that can be undone, like dissolving salt in water (you can evaporate the water to get the salt back). Simple mixtures involve reversible changes.
- Irreversible Change — A change that cannot be easily undone, like burning wood or mixing baking soda and vinegar. Chemical reactions typically produce irreversible changes.