Analyze data on physical and chemical properties to identify changes after substances interact - Middle School Physical Science

SECTION 1

Historical Context & Motivation

People have always wondered what happens when substances mix together. Ancient metalworkers noticed that heating certain rocks produced shiny metals. Cooks discovered that mixing ingredients changed their taste and texture. For thousands of years, people observed these changes without fully understanding them.

Over time, scientists developed careful ways to study how substances change. They learned to measure properties (characteristics you can observe or measure) before and after substances interact. This led to one of the biggest ideas in science: matter can change in very different ways.

~400 BCE • Greek Element Theory

Greek philosophers like Empedocles proposed that all matter is made of earth, water, air, and fire. They believed mixing these elements caused all changes.

1661 • Boyle Defines Elements

Robert Boyle argued that true elements cannot be broken down further. He encouraged testing ideas with experiments instead of just guessing.

1789 • Lavoisier and Conservation of Mass

Antoine Lavoisier carefully measured substances before and after reactions. He showed that mass is not created or destroyed — it is conserved.

1869 • Mendeleev's Periodic Table

Dmitri Mendeleev organized elements by their properties. This showed patterns in how different substances behave and interact.

Modern Day • Data-Driven Chemistry

Today, scientists use precise instruments to measure properties like temperature, mass, and pH. Data analysis helps us understand and predict changes in matter.

Here is the big question we will investigate: How can we use data about properties to figure out whether a physical change or a chemical change has occurred? This is exactly what scientists and engineers do every day.

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Anchoring Phenomenon

Imagine you drop a fizzing tablet into a cup of water. Bubbles form, the tablet gets smaller, and the water feels cold. Did the tablet just dissolve, or did something completely new form? How would you use data to answer that question? We will explore this throughout the lesson.

SECTION 2

Core Principles & Definitions

Before we can analyze data, we need to understand the key ideas. Every substance has physical properties (features you can observe or measure without changing what the substance is). Examples include color, shape, melting point, and density. A substance also has chemical properties (features that describe how a substance reacts with other substances). Flammability and reactivity with acid are chemical properties.

Physical Properties

Characteristics you can observe without making a new substance. Think of color, boiling point, mass, density, and state of matter (solid, liquid, gas).

Chemical Properties

Characteristics that describe how a substance can change into a completely new substance. Examples: flammability, ability to rust, and reactivity with acids.

Physical Change

A change where the substance stays the same, even if it looks different. Melting ice is still water. Crushing a can is still aluminum.

Chemical Change

A change where one or more new substances form. Burning wood creates ash and carbon dioxide. Rusting iron creates iron oxide, a new substance.

Evidence of Chemical Change

Clues that a chemical change happened include: color change, gas production (bubbles), temperature change, new smell, or a precipitate (solid forming in a liquid).

When substances interact, we compare properties before and after the interaction. If the properties of the starting materials match the properties of the ending materials, it was likely a physical change. If new properties appear that do not match the starting materials, it was likely a chemical change.

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KEY TAKEAWAY

Think of it like cooking. If you slice a banana, it is still a banana — that is a physical change. But if you bake flour, eggs, sugar, and butter together, you get a cake — something totally new. The ingredients cannot easily go back to what they were. That is a chemical change. You can tell the difference by comparing the properties of what you started with and what you ended up with.

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NGSS Three-Dimensional Learning

DCI (MS-PS1-2): Each pure substance has characteristic physical and chemical properties that can be used to identify it. SEP: Analyzing and Interpreting Data — we use data tables and observations as evidence. CCC: Patterns — we look for patterns in property changes to classify a change as physical or chemical.

SECTION 3

Visual Explanation: Physical vs. Chemical Changes

The diagram below shows how we use property data to classify a change. On the left side, you see a substance before it interacts. On the right side, you see what happens after. By comparing properties, we decide if it was a physical change or a chemical change.

Top row: Melting an ice cube changes its state but not its substance (H₂O). This is a physical change. Bottom row: An iron nail reacting with water and oxygen creates a new substance, iron oxide (Fe₂O₃). This is a chemical change. Notice how many properties changed in the bottom row compared to the top row.

Look at the top row of the diagram. When ice melts, the state changes from solid to liquid. But the substance is still water (H₂O). That is why melting is a physical change. Now look at the bottom row. When iron rusts, the color, texture, and even the chemical formula change. A completely new substance forms. That is a chemical change.

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Crosscutting Concept: Patterns

Notice the pattern: in a physical change, only a few properties change (like state or shape). In a chemical change, many properties change at once. Scientists look for these patterns in data to classify changes.

SECTION 4

How It Works: Using Data to Identify Changes

Scientists do not just guess whether a change is physical or chemical. They collect data (measurements and observations) and compare properties before and after the interaction. Let's walk through the process step by step.

Step-by-Step Data Analysis Process

Record properties BEFORE: Measure and observe properties of each substance before they interact. Include color, state, temperature, mass, smell, and any other relevant data.
Let the substances interact: Mix, heat, dissolve, or combine the substances. Observe what happens during the interaction.
Record properties AFTER: Measure and observe the same properties again. Note anything new, like bubbles, a temperature change, or a different color.
Compare before and after: Make a data table. Put 'before' properties in one column and 'after' properties in another. Look for differences.
Draw a conclusion: If properties show a new substance formed, it is a chemical change. If the same substance remains (just in a different form), it is a physical change.

One important tool is the conservation of mass (the idea that mass does not appear or disappear during a change). In both physical and chemical changes, the total mass stays the same. However, in a chemical change, the arrangement of atoms changes to form new substances.

CONSERVATION OF MASS

Total mass before = Total mass after

This holds true for both physical and chemical changes. If the mass seems to change, some matter escaped (like gas) or entered from the surroundings.

Measuring Temperature as Evidence

Temperature changes can be measured before and after mixing. An exothermic change (one that releases heat) makes the temperature go up. An endothermic change (one that absorbs heat) makes the temperature go down. A significant temperature change often points to a chemical change.

TEMPERATURE CHANGE

ΔT = T_after − T_before

ΔT (delta T) is the change in temperature. If ΔT is positive, heat was released (exothermic). If ΔT is negative, heat was absorbed (endothermic). T is measured in °C.

SECTION 5

Types of Evidence for Chemical Changes

Not all changes are easy to classify. Some physical changes can look a lot like chemical changes. For example, boiling water produces bubbles — but that is a physical change. To be careful scientists, we need to know the specific types of evidence for chemical changes.

This diagram shows five common types of evidence that suggest a chemical change has occurred. Each type is shown with a real-world example. Remember: one clue alone may not be enough. Strong conclusions need multiple types of evidence.

Evidence Type	What You Observe	Example	Caution
Color change	Substance changes to a new color	Silver tarnishing to black	Dissolving food coloring also changes color but is physical
Gas production	Bubbles or fizzing appear	Vinegar + baking soda	Boiling water makes bubbles too, but that is a physical change
Temperature change	Gets hotter or colder without external heating	Hand warmers (iron oxidation)	Dissolving some salts in water also changes temperature
Precipitate	A solid forms when two liquids mix	Mixing silver nitrate + salt water	Sometimes solids just settle out without reacting
New smell	A smell appears that was not there before	Burning wood or food spoiling	Opening a container may release existing smells without a reaction

Five types of evidence for chemical change with cautions

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KEY TAKEAWAY

Think of evidence for chemical change like clues in a detective case. One fingerprint might not solve the mystery. But if you find a fingerprint, a footprint, AND video evidence, you can be much more confident. The same goes for chemical changes — the more types of evidence you find, the stronger your conclusion.

SECTION 6

Worked Example: Fizzing Tablet Investigation

Let's return to our anchoring phenomenon: dropping a fizzing tablet into water. A student collects data before and after the interaction. Let's analyze the data to determine if it was a physical or chemical change.

Analyzing the Fizzing Tablet Data

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Step 1 — Record the Before Data

Before mixing, the student records: The tablet is a white solid with a mass of 3.2 g. The water is a clear, colorless liquid with a mass of 150.0 g and a temperature of 22 °C. Total mass before = 3.2 g + 150.0 g = 153.2 g.

Total mass before = 153.2 g, Temperature = 22 °C
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Step 2 — Observe During the Interaction

The student drops the tablet into the water. Immediately, bubbles form and rise to the surface. The tablet gets smaller and eventually disappears. The water feels colder to the touch.

Observations: gas production (bubbles), tablet dissolving
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Step 3 — Record the After Data

After the tablet fully dissolves, the student measures: The liquid is now slightly cloudy. The temperature dropped to 18 °C. The mass of the open cup and liquid is 152.5 g (some gas escaped into the air).

Mass after (open cup) = 152.5 g, Temperature = 18 °C
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Step 4 — Compare Before and After

Let's compare. Temperature changed: ΔT = 18 − 22 = −4 °C (temperature dropped, so the reaction was endothermic). Gas was produced (bubbles). Mass decreased slightly because gas escaped the open container. The liquid looks different (slightly cloudy).

Evidence: gas production, temperature change, appearance change
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Step 5 — Draw a Conclusion

We found three types of evidence: gas production (a new gas, CO₂, formed), a temperature decrease, and a change in appearance. The tablet's ingredients reacted with water to form new substances. The mass appeared to decrease only because gas escaped — if we used a sealed container, the total mass would still be 153.2 g.

Conclusion: This was a CHEMICAL CHANGE. New substances (including CO₂ gas) were formed.

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SEP Connection: Analyzing and Interpreting Data

Notice how the student used a data table to compare before and after properties. This is exactly what the Science and Engineering Practice of Analyzing and Interpreting Data looks like. You collect evidence, organize it, and use it to support a conclusion.

SECTION 7

Comparing Physical and Chemical Changes

Now that we have seen examples of both types of changes, let's put them side by side. This comparison table will help you quickly tell them apart.

Feature	Physical Change	Chemical Change
New substance?	No — same substance, different form	Yes — one or more new substances form
Reversible?	Usually easy to reverse (melt ↔ freeze)	Usually difficult or impossible to reverse
Properties that change	Shape, size, state, or phase	Color, smell, energy, chemical formula
Mass conserved?	Yes	Yes (atoms rearrange, not created or destroyed)
Everyday examples	Cutting paper, melting butter, dissolving sugar	Burning wood, cooking an egg, rusting iron
Energy change	Often small	Often significant (heat, light, or sound released or absorbed)

Physical vs. Chemical Change Comparison

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KEY TAKEAWAY

Here is a quick test: Can you easily get the original substance back? If you freeze melted water, you get ice again — that is physical. But if you burn a piece of paper, you cannot un-burn it back into paper — that is chemical. The key is whether a new substance formed.

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Tricky Cases!

Dissolving salt in water looks like a big change, but it is actually a physical change. The salt breaks apart into tiny pieces (ions) but can be recovered by evaporating the water. Dissolving sugar is similar. However, dissolving a fizzing tablet involves a chemical reaction because the tablet's ingredients react with water to form new substances like CO₂ gas.

SECTION 8

Connecting to Bigger Ideas in Science

The skills you are learning here connect to much bigger ideas in science. Analyzing data about property changes is the foundation for understanding chemical reactions, energy transfers, and even how living things work.

What You Learn Now	Where It Leads
Identifying physical vs. chemical changes using property data	In high school chemistry, you will write balanced chemical equations and predict products of reactions
Measuring temperature change as evidence	In high school, you will calculate energy changes using the equation q = mcΔT (calorimetry)
Conservation of mass	This becomes the law of conservation of mass, which leads to stoichiometry (balancing equations by counting atoms)
Observing gas production and precipitates	These become formal reaction types: gas-evolving reactions, precipitation reactions, acid-base reactions
Analyzing and interpreting data (SEP)	Data analysis is used in every branch of science and engineering — from biology to physics to environmental science

How this lesson connects to future learning

In high school, you will also learn that atoms are made of even smaller pieces called subatomic particles. The way these particles interact explains why some substances react and others do not. For now, focus on using observable evidence and data to classify changes — this is the skill that makes everything else possible.

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CCC: Cause and Effect

When two substances interact, the interaction is the cause. The changes in properties are the effect. By carefully measuring the effects (property changes), we can figure out what kind of cause (physical or chemical change) produced them. This cause-and-effect thinking is a crosscutting concept that connects all areas of science.

SECTION 9

Practice Problems

Now it is your turn to analyze data and determine whether physical or chemical changes occurred. Read each scenario carefully and use evidence to support your answer.

Question 1: CONCEPTUAL

Which of the following is a chemical property of iron? A) It is a silver-gray solid. B) It has a density of 7.87 g/cm³. C) It melts at 1,538 °C. D) It reacts with oxygen to form rust.

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D) It reacts with oxygen to form rust. A chemical property describes how a substance interacts with another substance to form something new. Options A (color/state), B (density), and C (melting point) are all physical properties because you can observe or measure them without creating a new substance. Reacting with oxygen to form rust (iron oxide) is a chemical property because it describes how iron changes into a completely different substance.

Question 2: BASIC

A student heats sugar in a pan. The white sugar turns into a brown liquid, and a sweet smell fills the room. The student measures the temperature change: ΔT = 170 °C − 22 °C = 148 °C. Based on this data, what type of change occurred? A) Physical change — the sugar just melted. B) Chemical change — new substances formed. C) Physical change — heat always causes physical changes. D) Neither — no change happened.

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B) Chemical change — new substances formed. The evidence includes a color change (white to brown) and a new smell. These are signs that new substances formed through caramelization. Option A is wrong because simple melting would not change color or produce a new smell. Option C is wrong because heat can cause either physical or chemical changes. Option D is wrong because the data clearly shows multiple changes in properties.

Question 3: INTERMEDIATE

A student mixes two clear, colorless liquids in a beaker. She records the following data: • Before: Liquid A mass = 50.0 g, Liquid B mass = 50.0 g, Temperature = 21 °C • After: A yellow solid appears at the bottom of the beaker, the liquid above is still clear, Temperature = 23 °C, Total mass = 100.0 g What is the best conclusion? A) A chemical change occurred because a precipitate formed and the temperature increased. B) A physical change occurred because mass was conserved. C) A chemical change occurred because mass was conserved. D) A physical change occurred because the liquid stayed clear.

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A) A chemical change occurred because a precipitate formed and the temperature increased. Two types of evidence point to a chemical change: a precipitate (yellow solid) formed, and the temperature increased slightly (from 21 °C to 23 °C, meaning ΔT = +2 °C, exothermic). Option B is wrong because conservation of mass happens in both physical and chemical changes — it does not tell you which type occurred. Option C is wrong for the same reason — conservation of mass is not evidence for a specific type of change. Option D is wrong because the formation of a solid in a liquid is evidence of a chemical change, not a physical one.

Question 4: APPLIED

A park ranger notices that iron chains on a dock have changed from shiny silver to rough reddish-brown over the summer. She weighs one chain and finds it is 2.3 g heavier than when it was installed. The chain is near salty ocean water and humid air. Based on this evidence, what happened and why did the mass increase? A) Physical change — the salt water stained the chains a different color. B) Chemical change — iron reacted with oxygen and water to form iron oxide; the extra mass came from oxygen atoms bonding to the iron. C) Physical change — the chain absorbed water, making it heavier. D) Chemical change — the chain lost iron atoms, and the remaining atoms weigh more.

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B) Chemical change — iron reacted with oxygen and water to form iron oxide; the extra mass came from oxygen atoms bonding to the iron. The color change (silver to reddish-brown), texture change (shiny to rough), and mass increase all point to a chemical change. Iron (Fe) reacted with oxygen (O₂) and water (H₂O) to form iron oxide (rust, Fe₂O₃). The extra 2.3 g came from oxygen atoms that bonded to the iron. Option A is wrong because staining does not change the chemical makeup of the chain. Option C is wrong because absorbing water would make the chain wet, not rough and reddish-brown. Option D is wrong because atoms were gained (oxygen), not lost, and individual atoms do not change mass.

Question 5: CRITICAL THINKING

A student places a sealed plastic bag on a balance. Inside the bag are a small cup of vinegar and a small cup of baking soda. The balance reads 45.8 g. The student tips the bag to mix the vinegar and baking soda. Bubbles form, the bag puffs up, and it feels cold. The balance now reads 45.8 g. Another student says, "The mass did not change, so it must be a physical change." Is this student correct? Explain using at least two types of evidence. A) Yes — if mass does not change, it cannot be a chemical change. B) No — mass is conserved in both types of changes; the bubbles, cold temperature, and new gas are evidence of a chemical change. C) Yes — the bag puffing up means gas was released, which proves it is a physical change. D) No — the mass should have decreased, so the balance must be broken.

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B) No — mass is conserved in both types of changes; the bubbles, cold temperature, and new gas are evidence of a chemical change. The second student made a common mistake. Conservation of mass applies to ALL changes — physical and chemical. The mass stayed at 45.8 g because the bag was sealed and no matter could escape. The evidence for a chemical change includes: (1) gas production — bubbles and the bag puffing up show CO₂ gas formed, (2) temperature decrease — the cold feeling means the reaction was endothermic, absorbing energy from the surroundings. These are two strong types of evidence that new substances formed. Option A is wrong because conservation of mass does not indicate what type of change occurred. Option C is wrong because gas production is evidence of a chemical change, not a physical one. Option D is wrong because the balance is working correctly — mass is conserved in a sealed system.

SUMMARY

Lesson Summary

Summary

Every substance has physical properties (like color, mass, density, and melting point) and chemical properties (like flammability and reactivity). When substances interact, you can compare these properties before and after to determine what type of change occurred. A physical change keeps the same substance in a different form. A chemical change produces one or more new substances with different properties.

The five main types of evidence for a chemical change are color change, gas production, temperature change, precipitate formation, and new smell. Always look for multiple types of evidence to support your conclusion. Remember that conservation of mass applies to both physical and chemical changes — mass is never created or destroyed, only rearranged. Scientists use the practice of analyzing and interpreting data to identify patterns in property changes and draw evidence-based conclusions.