Mixtures, Reactions, and New Substances - 5th Grade Science

SECTION 1

The Phenomenon: A Kitchen Mystery

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🔍 Anchoring Phenomenon

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?

Side-by-side comparison: a salad (simple mixture) versus baking soda and vinegar (chemical reaction)

💭 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?

SECTION 2

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

In a simple mixture, each ingredient keeps its own properties. Trail mix, salt water, and a handful of different coins are all mixtures — but no new substance is made. You could pick out the nuts, evaporate the water, or sort the coins. This helps us explain why a salad can always be taken apart again.

Mixtures That Produce New Substances

When a chemical reaction occurs, the starting materials transform into different substances with different properties. The fizzing of baking soda and vinegar produces carbon dioxide gas, water, and a new salt. You can't simply reverse the process to get the baking soda and vinegar back.

Evidence of a New Substance

Scientists look for observable clues that a new substance has formed: a color change that won't reverse, gas bubbles, a temperature change (the mixture gets hot or cold on its own), a new smell, or a solid forming in a liquid. These are signs that a chemical reaction happened.

Weight Is Conserved

Whether or not a new substance forms, the total weight before mixing equals the total weight after mixing. Matter is not created or destroyed — it just rearranges. If gas escapes into the air, it might seem like weight disappeared, but the gas still has mass. This is the law of conservation of matter.

SECTION 3

Let's Investigate

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🔬 Investigation Spotlight

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.

Flowchart: How to determine if a mixture produced a new substance

SECTION 4

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.

Simple Mixture vs. Chemical Reaction — particle-level comparison

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.

SECTION 5

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.

SECTION 6

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

When you mix flour, water, yeast, and sugar, the yeast feeds on the sugar and produces carbon dioxide gas — a brand-new substance. Those gas bubbles get trapped in the dough, making the bread rise. This is a chemical reaction that bakers rely on every day. You can't "un-bake" bread back into flour and water!

♻️ Recycling and Separation

Recycling plants depend on the fact that many waste materials are simple mixtures — metals, plastics, glass, and paper mixed together in a bin. Because no chemical reaction happened, each material still has its original properties, so engineers can design systems to separate them using magnets, density, and air jets.

💊 Medicine

Pharmaceutical engineers combine chemicals in precise amounts to cause specific chemical reactions that create new medicines. They must understand exactly which mixtures create new, helpful substances and which would just sit there without reacting. Getting this wrong could mean the medicine doesn't work — or is dangerous.

🚗 Rust Prevention

Engineers who design cars and bridges know that iron mixed with water and oxygen causes a chemical reaction that produces rust — a new, weaker substance. To prevent this, they coat metal with paint or use alloys (mixtures of metals that resist reaction). Understanding which mixtures react helps engineers solve problems and make things last longer.

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.

SECTION 7

Key Vocabulary Review

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📖 Key Vocabulary

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.

SECTION 8

Practice: Test Your Understanding

Question 1: Problem 1

A student stirs a spoonful of sugar into a glass of warm water. The sugar disappears, and the water tastes sweet. Which statement best explains what happened?

A. The sugar and water reacted to form a new substance that tastes sweet.
B. The sugar dissolved and spread throughout the water, but no new substance was formed.
C. The sugar was destroyed when it was stirred into the water.
D. The water changed into a completely new liquid with different properties.

Show answer

B is correct. The sugar dissolved in the water, meaning it broke into pieces too small to see and spread evenly throughout the liquid. The sweet taste shows the sugar is still there — it was not destroyed or changed into something new. You could even get the sugar back by letting the water evaporate. Since the sugar kept its original properties (sweetness), no new substance was formed. A is incorrect because a sweet taste does not prove a new substance formed — it proves the sugar itself is still present. C is incorrect because the sugar was not destroyed; it is still in the water. D is incorrect because the liquid is still water with sugar mixed in, not a completely new liquid.

Question 2: Problem 2

A student drops a fizzing tablet into a cup of water. Bubbles form rapidly, and after the tablet is completely gone, the water looks clear. Which observation is the strongest evidence that a new substance was formed?

A. The water feels slightly cooler than before.
B. The tablet got smaller as it dissolved in the water.
C. Gas bubbles formed and escaped into the air during the process.
D. The water in the cup stayed clear after the tablet was gone.

Show answer

C is correct. The gas bubbles are strong evidence that a chemical reaction occurred. The bubbles are carbon dioxide gas — a substance that was not present as a separate material before the tablet was mixed with water. The reaction between ingredients in the tablet and the water produced new substances, including this gas. The formation of a gas that was not there before is one of the key signs that new substances have been created. A is incorrect because a temperature change by itself can happen during both physical and chemical changes, so it is not the strongest evidence. B is incorrect because the tablet getting smaller could mean it is simply dissolving (a physical change) or reacting (a chemical change) — size change alone cannot tell you which one happened, so it is not the strongest evidence. D is incorrect because the water staying clear does not tell us whether a new substance formed; it only describes the appearance of the liquid.

Question 3: Problem 3

A student shakes a bottle containing oil and vinegar. The two liquids mix together and look like one liquid. After sitting for ten minutes, the oil floats back to the top and the vinegar sinks to the bottom because oil is lighter than vinegar. What does this tell you about the mixture?

A. The oil and vinegar reacted and formed a new substance that later separated.
B. The oil and vinegar mixed together but no new substance was formed.
C. The vinegar dissolved the oil, creating a new type of liquid.
D. The shaking caused a chemical reaction that wore off after ten minutes.

Show answer

B is correct. After sitting, the oil and vinegar separated back into the same two original liquids, each with its original properties — the oil still looks and behaves like oil, and the vinegar still looks and behaves like vinegar. There was no evidence of a new substance: no color change, no gas bubbles, no heat, and no new material appeared. The two liquids simply separated because oil is lighter than vinegar and floats on top. This is a physical mixture, not a chemical reaction. A is incorrect because there is no evidence that a new substance formed — no color change, gas, heat, or new material appeared, and both original substances returned with their original properties. C is incorrect because the oil was not dissolved; it separated right back out with no change in its properties. D is incorrect because chemical reactions do not "wear off" — once new substances form, the original materials do not simply reappear on their own.

Question 4: Problem 4

A student leaves a shiny iron nail outside for several weeks. When she comes back, the nail is covered in a rough, reddish-brown coating. She tries to scrub the coating off, but the shiny iron underneath is partly gone. What best explains what happened to the nail?

A. Dirt from the ground stuck to the nail and stained it reddish-brown.
B. A new substance called rust formed on the nail, which has different properties than the original iron.
C. The nail simply changed color because of the sunlight shining on it.
D. The iron melted slightly in the rain and then hardened into a different shape.

Show answer

B is correct. The reddish-brown coating is rust, which is a new substance with different properties than the original shiny iron nail. The rust is a different color (reddish-brown instead of silver), a different texture (rough and flaky instead of smooth and hard), and it cannot simply be turned back into shiny iron by scrubbing. The fact that some of the original iron is gone shows that the iron itself was changed into this new material. These are all signs that a new substance was formed. A is incorrect because if dirt had simply stuck to the nail, scrubbing would remove it completely and the original shiny iron would still be fully underneath — but the student found that some of the iron itself was gone. C is incorrect because sunlight does not change iron's color; the nail has a rough, flaky coating with completely different properties, which is much more than a color change from light exposure. D is incorrect because iron does not melt in rain — iron requires extremely high temperatures to melt, far hotter than any weather conditions.

Question 5: Problem 5

A student pours baking soda into a bowl of vinegar. The mixture fizzes and bubbles up. After the fizzing stops, the liquid in the bowl looks and tastes different from the original vinegar. Which statement best explains why this mixture resulted in new substances?

A. The baking soda dissolved in the vinegar, just like sugar dissolves in water.
B. The bubbling and the change in taste show that new substances with different properties were produced.
C. The vinegar got weaker because the baking soda soaked it up like a sponge.
D. The fizzing was caused by air trapped in the baking soda powder being released.

Show answer

B is correct. Two important observations tell us new substances were produced: the gas bubbles that formed during fizzing and the change in taste of the remaining liquid. The gas bubbles are carbon dioxide, a substance that was not present as a separate material before mixing. The liquid left behind has different properties than the original vinegar, which means it is not simply vinegar anymore. When mixing two materials produces gas, changes in taste, or other new properties, that is strong evidence that a chemical reaction occurred and new substances were formed. A is incorrect because when sugar dissolves in water, no gas is produced and the sugar keeps its sweet taste — it is still sugar. Here, the fizzing and change in taste show something very different happened. C is incorrect because baking soda is not a sponge — it does not soak up liquids. It reacted with the vinegar to create new substances. D is incorrect because the amount of gas produced is far more than the tiny bit of air that could be trapped between grains of powder. The gas was created by the reaction between the baking soda and the vinegar.

SECTION 9

What's Next?

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🔮 What's Next?