Matter
A student says, Air cannot be matter because I cannot see or touch it.
Air is matter because it has mass and takes up space (has volume), even though it cannot normally be seen.
A student says, “The particles in a solid are not moving because a solid keeps a fixed shape.” Is the student correct? Explain what the particles are actually doing and why the solid maintains its shape.
No. The particles in a solid vibrate in place. They do not have enough energy to move past one another, so they remain tightly packed in fixed positions. This gives a solid a fixed shape and fixed volume.
A scientist observes that a substance has a strong odor, conducts electricity, and dissolves in water. The scientist does not change the substance into a new substance while making these observations. What type of information is the scientist collecting, and why?
The scientist is observing physical properties because these characteristics can be observed or measured without changing the substance into a different substance.
A clear liquid is mixed with another clear liquid. Within seconds, bubbles form, the mixture becomes a different color, and heat is released. Why are these observations strong evidence that a chemical change occurred?
These are clues that new substances formed. The production of gas (bubbles), color change, and release of heat are common evidence of a chemical reaction.
A student claims: “Every molecule is a compound, and every compound is an element.” Identify both errors in the statement and correct it.
Error 1: Not every molecule is a compound. O₂ is a molecule but contains only one element.
Error 2: A compound is not an element. A compound contains two or more different elements chemically combined.
A molecule consists of two or more atoms, while a compound contains two or more different elements chemically combined.
A substance contains only one type of atom. It cannot be broken down into simpler substances through chemical means. What is it?
Element. An element is a pure substance made of only one type of atom and cannot be chemically separated into other substances.
Water can flow and take the shape of its container, but a fixed amount of water still occupies approximately the same volume. What does this tell you about how the particles in a liquid move?v
The particles have enough energy to move and slide past one another, allowing the liquid to change shape. However, they remain relatively close together because of attractive forces, so the liquid maintains a fixed volume.
A block of ice melts into liquid water and then boils into water vapor. Has the water become a new substance? Explain why these are physical changes.
No. The substance remains water (H₂O). Melting and boiling change the state and physical properties of the water but do not change its chemical properties. Therefore, both are physical changes.
A student says, “Flammability is a physical property because I can observe whether an object burns.” Explain why this statement is incorrect.
Flammability is a chemical property because it describes a substance's ability to undergo a chemical change when it burns. During burning, the substance combines with oxygen and forms new substances.
A student observes that a substance changes from a solid to a liquid. Another student observes that a different substance produces bubbles, releases heat, and changes color when mixed with another substance. Which observation provides stronger evidence of a chemical change?
The bubbles, heat, and color change provide stronger evidence of a chemical change because they indicate that new substances may have formed. A solid changing into a liquid is only a physical change because the substance itself has not changed into a different substance
O₂ and CO₂ are both molecules. However, only one of them is a compound. Which one is the compound, and why?
CO₂ is the compound
A sealed container is opened and a gas quickly spreads throughout the room. Explain this behavior using particle motion, energy, and spacing.
Gas particles have enough kinetic energy to overcome the attractive forces between them. They are far apart and move rapidly, slipping past one another and spreading out. Therefore, a gas has neither a fixed shape nor a fixed volume and fills the available space.
A student freezes liquid water into ice and claims, “Freezing must be a chemical change because the water looks completely different.” Is the student correct? Explain.
No. Freezing is a physical change because the water changes from liquid to solid without becoming a different substance. The ice can also be melted back into liquid water, demonstrating that the change can be reversed.
An iron nail is silver-colored and hard. After being exposed to oxygen for a long time, it becomes covered with a reddish-brown, flaky substance. Is this a physical or chemical change? What chemical property of iron is demonstrated?
It is a chemical change because iron combines with oxygen and forms the new substance iron oxide. The chemical property demonstrated is reactivity/ability to oxidize (rust).
A mysterious substance has these characteristics:
It is silver-colored.
It conducts electricity.
It reacts with oxygen and forms a reddish-brown substance.
It burns when exposed to sufficient heat.
Its reddish-brown product has different properties from the original substance.
Which observations are physical properties, which are chemical properties, and which demonstrate chemical changes?
Silver color → physical property.
Ability to conduct electricity → physical property.
Ability to react with oxygen → chemical property.
Ability to burn → chemical property (flammability).
Actually reacting with oxygen and forming the reddish-brown substance → chemical change.
A chemical reaction occurs inside a completely sealed container. Before the reaction, the substances have a total mass of 150 grams. What should the total mass be after the reaction?
150 grams
Two substances are touching. Substance A is at a higher temperature than Substance B. Predict the direction of thermal energy transfer and explain what happens at the particle level.
Thermal energy transfers from Substance A (warmer) to Substance B (cooler). Faster-moving particles in the warmer substance collide with slower-moving particles in the cooler substance and transfer some of their energy. This continues until they reach the same temperature.
Two objects have the same volume, but Object A has more mass than Object B. Which object has greater density? Explain what this tells you about the particles.
Object A has greater density. Because the objects have the same volume, the one with more mass has more matter packed into the same amount of space. This indicates that its particles are more closely packed.
Wood burns and produces ash, gases, and heat. The ash cannot simply be turned back into the original wood. What type of reaction is this, and why?
It is an irreversible chemical reaction. Burning changes the wood into new substances, and the products cannot simply change back into the original wood. It is also a combustion reaction.
A student pours 75 g of water into a cup containing 25 g of salt. After stirring, the salt dissolves completely. Assuming no material spills or leaves the cup, what is the total mass of the mixture? Explain what happened to the mass.
The salt did not disappear—it transferred into the water and became part of the mixture. Since no matter left the system, the total mass is conserved: 75 g + 25 g = 100 g.
A metal ball is heated. Its particles begin moving faster, and the ball becomes slightly larger. Explain the connection between temperature, kinetic energy, particle motion, and expansion.
As temperature increases, particle kinetic energy increases. The particles move or vibrate more rapidly. Because of this increased motion, the particles generally spread slightly farther apart, causing the substance to expand when heated.
A student cuts a piece of metal into four smaller pieces. Another student melts the same metal and changes it from a solid into a liquid. A third student dissolves salt in water. What do all three changes have in common, and why are they classified as physical changes?
All three are physical changes because the substances do not become entirely different substances. Cutting changes size, melting changes state, and dissolving changes how the substance is distributed. The chemical properties of the substances remain unchanged.
A substance is tested and scientists discover that it burns when exposed to oxygen, reacts strongly with water, and produces a new substance when it burns. Are these observations describing physical properties or chemical properties? Explain how you know.
They describe chemical properties because they show how the substance behaves when it undergoes chemical changes. Flammability, reactivity with water, and the ability to react with oxygen all involve the formation of new substances.