Matter - LectureHomework Part A

A scientist discovers a new substance and wants to determine if it is an element. Using your understanding of the characteristics of matter, describe the reasoning process the scientist should use to decide if the substance is an element, and explain what evidence would confirm it.

• The scientist should check if the substance can be broken down into simpler substances by chemical means; if it cannot, and all atoms have the same number of protons, it is an element.

• The scientist should measure the mass of the substance; if it is heavier than water, it is an element.

• The scientist should see if the substance reacts with oxygen; if it does, it is an element.

• The scientist should check if the substance is magnetic; if it is, it is an element.

Given two substances, Substance X and Substance Y, both of which are pure and cannot be broken down by chemical means. Substance X has atoms with 6 protons, while Substance Y has atoms with 8 protons. Using strategic reasoning, explain how you would classify these substances and what this tells you about the difference between elements.

• Substance X and Substance Y are different elements because they have different numbers of protons.

• Substance X and Substance Y are the same element because they are both pure substances.

• Substance X and Substance Y are mixtures because they cannot be broken down by chemical means.

• Substance X and Substance Y are isotopes of the same element because they are both pure substances.

Imagine you are given a sample of matter and asked to identify its fundamental particles. Describe the reasoning process you would use to identify these particles and explain why each is important for the structure of matter.

• I would look for protons, electrons, and neutrons, as these are the fundamental particles that make up all matter.

• I would look for molecules and compounds, as these are the fundamental particles of matter.

• I would look for only protons and electrons, as neutrons are not important for the structure of matter.

• I would look for atoms and ions, as these are the only fundamental particles of matter.

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A student claims that the mass or behavior of an atom determines which element it is. Using evidence and reasoning, explain whether this claim is correct or incorrect.

• The claim is incorrect; each element is defined by its atomic number (number of protons), not its mass or behavior.

• The claim is correct; the mass of an atom always determines the element.

• The claim is correct; the behavior of an atom in chemical reactions determines the element.

• The claim is incorrect; elements are defined by the number of neutrons in their nuclei.

Evaluate the difference between a molecule and a compound by providing an example of each and explaining how their definitions apply to your examples.

• A molecule is always a compound, and a compound is always a molecule; for example, O2 and H2O are both compounds.

• A molecule can consist of the same element, like O2, while a compound must consist of different elements, like H2O.

• A molecule is a group of atoms held together by metallic bonds, while a compound is held together by covalent bonds.

• A molecule is a substance made of ions, while a compound is made of neutral atoms.

Strategically determine why the ratio of elements in a compound must be fixed, and predict what would happen if the ratio were not fixed, using water (H2O) as an example.

• The properties of the compound would remain unchanged regardless of the ratio.

• The compound would not have consistent properties, and it would not be considered a true compound.

• The compound would become a mixture, not a pure substance.

• The atoms would not bond at all if the ratio changed.

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A scientist is given three unknown substances. Substance A maintains both its shape and volume regardless of the container, Substance B maintains its volume but takes the shape of its container, and Substance C changes both its shape and volume to fit any container and is highly compressible. Based on this information, classify each substance as a solid, liquid, or gas, and justify your reasoning.

• Substance A is a solid, Substance B is a liquid, Substance C is a gas, because solids have fixed shape and volume, liquids have fixed volume but indefinite shape, and gases have indefinite shape and volume and are highly compressible.

• Substance A is a gas, Substance B is a solid, Substance C is a liquid, because gases have fixed shape and volume, solids have fixed volume but indefinite shape, and liquids have indefinite shape and volume and are highly compressible.

• Substance A is a liquid, Substance B is a gas, Substance C is a solid, because liquids have fixed shape and volume, gases have fixed volume but indefinite shape, and solids have indefinite shape and volume and are highly compressible.

• Substance A is a gas, Substance B is a liquid, Substance C is a solid, because gases have fixed shape and volume, liquids have fixed volume but indefinite shape, and solids have indefinite shape and volume and are highly compressible.

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Given the definition and characteristics of pure substances, devise a method to distinguish between a pure substance and a mixture using their physical and chemical properties. Explain your reasoning.

• By observing if the substance has a unique set of properties and a fixed composition.

• By checking if the substance can be separated by physical means only.

• By determining if the substance changes color when heated.

• By measuring the mass of the substance.

Suppose you are given two samples: one is a shiny metallic liquid (like mercury), and the other is a crystalline solid. Using your understanding of pure substances, explain how you would classify each sample as either an element or a compound, and justify your classification.

• The metallic liquid is an element because it has a unique set of properties, while the crystalline solid could be a compound if it consists of more than one element.

• Both are elements because they are pure substances.

• The metallic liquid is a compound because it is shiny, and the crystalline solid is an element because it is solid.

• Both are compounds because they have a fixed composition.

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NGSS.MS-PS1-1

A chemist is given two samples of a substance and is told that one is a pure substance and the other is a mixture. Using the concept of fixed composition, how could the chemist determine which sample is the pure substance? Explain your reasoning.

• By checking if both samples have the same color.

• By measuring the proportion by mass of the elements in different portions of each sample; the pure substance will have a constant composition throughout.

• By observing if the samples react with water.

• By checking if both samples have the same melting point.