How are chemical formulas used to represent substances?

• By using colors to indicate different elements

• By using symbols for elements and subscripts to indicate the number of atoms in a compound

Answer explanation

Chemical formulas use symbols to represent elements and subscripts to show the number of atoms in a compound, making it clear how many of each type of atom are present. This is the correct method for representing substances.

What is a ball-and-stick model?

• A model that represents atoms as balls and bonds as sticks, showing the structure of molecules

• A model that uses colors to represent different elements

Answer explanation

A ball-and-stick model visually represents atoms as balls and chemical bonds as sticks, effectively illustrating the structure of molecules. This makes it easier to understand molecular geometry and connectivity.

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NGSS.MS-PS1-1

Describe the atomic composition of a water molecule.

• Two hydrogen atoms and one oxygen atom

• One hydrogen atom and two oxygen atoms

Answer explanation

A water molecule consists of two hydrogen atoms and one oxygen atom, represented as H2O. This composition is essential for its unique properties, making the choice 'Two hydrogen atoms and one oxygen atom' the correct answer.

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NGSS.MS-PS1-1

What is the difference between an element and a compound in terms of chemical formulas?

• Elements have a single symbol, while compounds combine symbols

• Elements have multiple symbols, while compounds have a single symbol

Answer explanation

Elements are represented by a single symbol (e.g., H for hydrogen), while compounds are formed by combining symbols of the elements (e.g., H2O for water). This distinction highlights the structural difference between elements and compounds.

How can ball-and-stick models help classify substances?

• By visually demonstrating whether a substance is an element or a compound

• By showing the temperature at which the substance melts

Answer explanation

Ball-and-stick models visually represent the arrangement of atoms in a substance, helping to distinguish between elements (single type of atom) and compounds (multiple types of atoms), making it clear whether a substance is an element or a compound.

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How is the molecule CO₂ represented in chemical and model form?

• The chemical formula is O₂C, and the model shows two oxygen atoms bonded to one carbon atom

• The chemical formula is CO₂, and the model shows one carbon atom bonded to two oxygen atoms in a linear structure

Answer explanation

The correct representation of CO₂ is its chemical formula CO₂, indicating one carbon atom bonded to two oxygen atoms. The linear structure reflects the actual arrangement of atoms in the molecule.

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What does the subscript in a chemical formula indicate?

• The weight of the element

• The number of atoms of each element in a molecule

Answer explanation

The subscript in a chemical formula indicates the number of atoms of each element in a molecule. For example, in H2O, the '2' indicates there are two hydrogen atoms for every one oxygen atom.

How can elementary substances be identified in chemical formulas?

• By using a single type of atom, such as N₂ for nitrogen gas

• By using colors to represent different elements

Answer explanation

Elementary substances are identified by using a single type of atom in chemical formulas, like N₂ for nitrogen gas, which clearly indicates the element and its molecular form.

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Give an example of classifying a compound using chemical formulas.

• Cl is classified as a compound because it consists of chlorine atoms

• H₂O is classified as a compound because it consists of hydrogen and oxygen atoms chemically bonded

Answer explanation

H₂O is the correct choice as it is a compound formed by the chemical bonding of hydrogen and oxygen atoms. In contrast, O₂, Na, and Cl are not compounds; they are either diatomic molecules or elements.

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How does a molecular model depict the atomic composition of methane (CH₄)?

• A model shows two carbon atoms bonded to two hydrogen atoms

• A model shows one central carbon atom bonded to four surrounding hydrogen atoms in a tetrahedral shape

Answer explanation

The correct model depicts one carbon atom at the center, bonded to four hydrogen atoms, forming a tetrahedral shape. This accurately represents methane's molecular structure, unlike the other options.

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NGSS.MS-PS1-1