Chem I: Ch. 1 Matter on the Atomic Scale

Classify each of the following particulate level illustrations as a representation of either an element or a compound.

compound - A compound is a pure substance that is composed of two or more elements. All of the molecules in a pure substance are the same. The sample may be a solid, a liquid, a gas, or a mixture of states. element - An element is a pure substance that is composed of only one kind of atom. The atoms may be combined to form molecules. As long as all of the atoms are the same, the substance is an element. The sample may be a solid, a liquid, a gas, or a mixture of states.

Is the salt-water mixture heterogeneous or homogeneous?

homogeneous The salt is fully dissolved in the water, so the composition of the sample is consistent everywhere.

Now examine tin, Sn. Does it appear to have atoms bonded to just a few atoms (like sulfur), or is each atom surrounded by many other atoms (like magnesium)?

many Tin has a structure like most metallic elements, where each atom is surrounded by and forms bonds to many other atoms (often up to 12 atoms).

element

matter that is composed of only one kind of atom

chemical compound

matter that is composed of two or more kinds of atoms, chemically combined in definite proportions

Now consider a sample where salt (NaCl) has been dissolved in the water. It is completely dissolved. Is this a pure substance or a mixture?

mixture This sample now contains two different compounds: H2O and NaCl. It is therefore a mixture.

When methanol evaporates, liquid CH3OH molecules separate to form gas-phase CH3OH molecules. Is this a chemical change or a physical change?

physical change Because the CH3OH molecules do not change their composition (that is, they are still CH3OH molecules, not molecules with some other formula) this is a physical change.

macroscopic scale

processes and properties on a scale large enough to be observed directly

physical properties

properties of a substance that can be observed and measured without changing the composition of the substance

Consider a sample of water with nothing dissolved in it. Is water a pure substance or a mixture?

pure substance Although water contains more than one element (H and O), the entire sample is composed of only water molecules. It is therefore a pure substance.

freezing point

the temperature at which the solid and liquid phases are in equilibrium at 1 atm

This is an animation of the movement of molecules in a liquid. Are the molecules attracted to one another?

yes The particles stay near one another as though they feel a force of attraction.

Now examine magnesium. Does it appear to have atoms bonded to just one or two other atoms, like we saw in sulfur?

no Magnesium, like most metals, is composed of atoms packed together like marbles. This is typical of metallic elements.

Finally, this is an animation of the movement of particles in the gaseous state. Do gas molecules show significant attractions to one another?

no Molecules in the gas phase show little interaction. Gases therefore flow and fill the container in which they are held.

Do the ions in the solid ever change position with one another?

no This feature of atomic or molecular motion is what leads to a solid keeping its shape.

This is a photo of water being poured from one beaker to another. Does liquid water keep its shape?

no Water, like all liquids, flows.

Finally, examine silicon, Si. Simply from its molecular structure would you expect silicon to be a metallic element like Mg or an element like S, which is not a metal?

not a metal Silicon has a molecular structure like most nonmetal elements, where each atom forms bonds to a few other atoms.

When the methanol is ignited, what other reactant is involved in the reaction?

oxygen Most combustion processes involve reactions with oxygen (O2) in the air.

Watch the movie that shows some of the properties of methanol. When the movie begins, is the methanol stable or is it reactive?

stable The methanol is not changing. However, although you can't detect it in the movie, it is slowly evaporating (changing from liquid methanol to gaseous methanol).

solid

the phase of matter in which a substance has both definite shape and definite volume

gas

the phase of matter in which a substance has no definite shape and a volume defined only by the size of its container

liquid

the phase of matter in which a substance has no definite shape but a definite volume

chemical change

the properties of a substance that involve a chemical change in the material and often involve a substance interacting with other chemicals

atom

the smallest particle of an element that retains the characteristic chemical properties of that element

molecules

the smallest unit of a compound that retains the composition and properties of that compound

chemistry

the study of matter, its transformations, and how it behaves

melting point

the temperature at which the crystal lattice of a solid collapses and solid is converted to liquid

boiling point

the temperature at which the vapor pressure of a liquid is equal to the external pressure on the liquid

states of matter

the three physical states: 1. solid 2. liquid 3. gas

Which of the following are correct? - x^(a/b)=(x^a)^(1/b) - (x^a)^b=x^(ab)

x^(a/b)=(x^a)^(1/b) When raising a variable to a power, multiply exponents. When raising a variable to a power, multiply exponents.

Do you think it would be possible to form rings with different numbers of atoms?

yes A large number of different sulfur ring sizes have been observed, but the 8-member ring is the most stable.

This is an animation showing how atoms within a solid move over time. Do atoms in a solid move at all?

yes The atoms clearly vibrate.

Classify each of the following particulate level illustrations as a representation of either an element or a compound.

element - An element is a pure substance that is composed of only one kind of atom. The atoms may be combined to form molecules. As long as all of the atoms are the same, the substance is an element. The sample may be a solid, a liquid, a gas, or a mixture of states. compound - A compound is a pure substance that is composed of two or more elements. All of the molecules in a pure substance are the same. The sample may be a solid, a liquid, a gas, or a mixture of states.

Now imagine adding some hair clippings to the water instead of salt. Is this mixture homogeneous or heterogeneous?

heterogeneous One way to determine if a sample is heterogeneous is to ask if you could separate its components with tweezers or with a syringe. That is, is the mixture such that you could grab one component but not another? If so, it is hetergeneous.

Which of the following best describes this substance? Consider only the contents of the glass, not the glass or the container. I glass of soda water with bubbles

a heterogeneous mixture The soda water in the glass is a heterogeneous mixture because the solution (the liquid) and the carbon dioxide bubbles are in different phases. One part of the mixture is not like all the other parts, so this is a heterogeneous mixture.

Which of the following best describes this substance? potassium dichromate K2Cr2O7 dissolved in water H2O

a homogeneous mixture This sample of potassium dichromate is dissolved in water. It is a mixture of the two compounds, water and potassium dichromate. Because potassium dichromate is dissolved evenly in the water, any portion of the sample is identical to any other. Therefore, this is a homogeneous mixture.

homogeneous mixture

a mixture in which the properties are the same throughout, regardless of the optical resolution used to examine it

Which of the following best describes this substance? a pile of potassium dichromate K2Cr2O7

a pure substance This sample of the chemical compound potassium dichromate is not mixed with any other compounds, so it is a pure substance.

Which of the following best describes this substance? Consider only the contents of the glass, not the glass or the container. Assume that the water has nothing dissolved in it. A bottle of water and a cup filled with water

a pure substance Water is composed of O and H atoms, but they are always combined in the same compound. This means it is a pure substance. This assumes that the water does not have anything dissolved in it.

This image shows a crystal of potassium chloride alongside a model of the crystal showing the individual potassium and chloride ions. Does the crystal have a definite shape?

yes The forces holding the ions in place are strong enough that they cannot move past one another and therefore the crystal keeps its shape.

Do the particles in a liquid ever move past one another?

yes Unlike a solid, the atoms or molecules in a liquid can move relative to one another. This is the feature of molecular motion that allows a liquid to flow.

pure substance

a substance that contains only one type of element or compound and has fixed chemical composition

matter

anything that has mass and occupies space

What are the products of the reaction? Enter compound names, not formulas.

carbon dioxide water The combustion of most organic compounds produces carbon dioxide and water.

Which of the following best describes this substance? crystals of two different minerals iron pyrite in quartz

a heterogeneous mixture This sample of a mixture of different solids. Because the sample is different in different places, it is a heterogeneous mixture.

heterogeneous mixture

a mixture in which the properties in one region or sample are different from those in another region or sample

Which of the following best describes this substance? a diamond

a pure substance Diamond is made of a single element with a single, consistent structure. It is a pure substance.

atomic scale

a scale of measurement used to describe individual atoms or molecules

In this process, CH3OH and O2 react to form CO2 and H2O. Is this a chemical change or a physical change?

chemical change The process involves creation of new molecules and is therefore a chemical process.

Is water an element or a compound?

compound Water is composed of molecules that contain two different elements. It is therefore a compound.

Examining the figure, how many orders of magnitude larger is a person than a bacterial cell?

6 orders of magnitude This is a million times larger. People are roughly 1.5 meters tall. A bacterial cell is about 10^-6 m wide.

Sulfur exists as rings. How many atoms make up a ring?

8

How many times taller are most people than a piece of paper (measured on its long side)? Enter an integer.

People are 6 times taller than a piece of paper. Most people are about as tall as of 5-6 sheets of paper.

Classify each of the following pure substances as either an element or a compound: (a) CO2 (b) Rn (c) O3

(a) CO2 - compound (b) Rn - element (c) O3 - element Elements consist of only one type of atom, so the chemical formula has the symbol of only one element. Compounds consist of two or more types of atoms, so the chemical formula has two or more different element symbols. (a) CO2 is a compound. It consists of more than one type of atom. (b) Rn is an element. It consists of only one type of atom. (c) O3 is an element. It consists of only one type of atom. While O3 is a molecular substance, it consists of only a single type of atom and is therefore an element.

Classify each of the following pure substances as either an element or a compound: (a) Cu (b) C12H22O11 (c) NaOH

(a) Cu - element (b) C12H22O11 - compound (c) NaOH - compound An element cannot be separated into other stable pure substances by ordinary chemical means. In a sample of an element, all of the particles (atoms) are of the same element. Each element is represented by a one- or two-letter symbol. The first letter of the symbol for an element is always upper case, and, if there is a second letter, it is always lower case. Some elements exist as molecules that contain two or more atoms, but all of the atoms are of the same element. An example is the oxygen molecule, O2, that contains two oxygen atoms. A compound is made of two or more elements combined in a definite way. A compound may be separated into its elements by ordinary chemical means. A compound is represented symbolically by a formula that contains the symbols of all of the elements that are present in the compound. An example is water, H2O, that contains hydrogen atoms and oxygen atoms in a 2:1 ratio.

Classify each of the following as macroscopic, microscopic or atomic scale: (a) a copper atom (b) a neon sign (c) a bacterium

(a) a copper atom - atomic (b) a neon sign - macroscopic (c) a bacterium - microscopic Macroscopic objects can be seen with the naked eye. Objects that you see in everyday life are macroscopic. Macroscopic scale objects are comprised of microscopic and/or atomic scale objects. Microscopic objects are smaller than macroscopic objects and cannot be seen with the naked eye. Microscopic objects are commonly viewed with an optical microscope. Atomic scale objects are too small to be seen even through the most powerful optical microscope. Atoms and molecules are atomic scale (sometimes called nanoscale or molecular scale) objects.

Classify each of the following as macroscopic, microscopic or atomic scale: (a) a gold atom (b) a wire (c) a chair

(a) a gold atom - atomic (b) a wire - macroscopic (c) a chair - macroscopic Macroscopic objects can be seen with the naked eye. Objects that you see in everyday life are macroscopic. Macroscopic scale objects are comprised of microscopic and/or atomic scale objects. Microscopic objects are smaller than macroscopic objects and cannot be seen with the naked eye. Microscopic objects are commonly viewed with an optical microscope. Atomic scale objects are too small to be seen even through the most powerful optical microscope. Atoms and molecules are atomic scale (sometimes called nanoscale or molecular scale) objects.

Classify each of the following as either macroscopic-scale objects or atomic-scale objects. (a) a lead atom (b) a sample of liquid water (c) baking powder

(a) a lead atom - atomic (b) a sample of liquid water - macroscopic (c) baking powder - macroscopic Macroscopic-scale objects can be seen with the naked eye. Atomic-scale objects are too small to be seen with the naked eye even through the most powerful optical microscope. (a) A lead atom: atomic scale - cannot be seen with the naked eye or with an optical microscope. (b) A sample of liquid water: macroscopic scale - can be seen with the naked eye. (c) Baking powder: macroscopic scale - can be seen with the naked eye. Is your answer reasonable? Objects that you can see around you in everyday life are macroscopic-scale objects. Atoms and molecules are atomic-scale objects.

Classify each of the following as macroscopic, microscopic or atomic scale: (a) a mercury atom (b) beach sand (c) a carbon dioxide molecule

(a) a mercury atom - atomic (b) beach sand - macroscopic (c) a carbon dioxide molecule - atomic Macroscopic objects can be seen with the naked eye. Objects that you see in everyday life are macroscopic. Macroscopic scale objects are comprised of microscopic and/or atomic scale objects. Microscopic objects are smaller than macroscopic objects and cannot be seen with the naked eye. Microscopic objects are commonly viewed with an optical microscope. Atomic scale objects are too small to be seen even through the most powerful optical microscope. Atoms and molecules are atomic scale (sometimes called nanoscale or molecular scale) objects.

Classify each of the following pure substances as either an element or a compound: (a) carbon tetrachloride (b) mercury (c) platinum

(a) carbon tetrachloride - compound (b) mercury - element (c) platinum - element An element cannot be separated into simpler substances by ordinary chemical means. In a sample of an element, all of the particles (atoms) are the same element. Each element is represented by a one- or two-letter symbol and has a one-word name. A compound is made of two or more elements combined in a definite way. A compound is represented symbolically by a formula that shows all of the elements that are present in the compound. A compound may be separated into its elements by ordinary chemical means. The chemical name of a compound often consists of more than one word, but many compounds, like water, have common one-word names.

A person is about 6 orders of magnitude larger than a cell. The cell is about 3 orders of magnitude larger than a molecule. By how many orders of magnitude is a person larger than a molecule?

9 orders of magnitude Molecules are about a billion (a thousand million) times smaller than a person. Molecules are really, really small.

Identify physical and chemical properties and physical and chemical changes. (a) When aluminum foil is placed into liquid bromine a white solid forms. Is the underlined a chemical or physical property of aluminum? (b) Silver is a shiny metal that has a density of 10.5 g/mL. Is the underlined a chemical or physical property of silver? (c) Classify each of the following changes as chemical or physical. (i) removing nail polish (ii) scorching butter

(a) chemical (b) physical (ci) physical (cii) chemical Map: The keys to the questions are the following definitions: Chemical changes (chemical reactions) are transformations in which old substances are destroyed and new substances are formed. Chemical properties define the ways in which substances enter into chemical reactions. Physical changes alter the physical form of substances without changing their chemical identities. Physical properties may be observed without changing the chemical identities of the substances. (a) chemical property - Describes conditions for transformation to other substances. (b) physical property - Is observed without changing the identity of the substance. (ci) physical change - Takes place without changing the identity of the substance. (cii) chemical change - The original substance is transformed into other substances. Chemical reactions are often accompanied by the evolution of heat or light, by the evolution of a gas, or by changes in color, taste or smell. A physical change may involve changing the state of a substance, such as melting ice (changing solid water to liquid water), or changing the shape or the size of the pieces in a sample. Other physical changes involve the formation of mixtures in which all of the components retain their chemical identities, such as dissolving sugar in water.

Identify pure substances and mixtures. Classify each of the following as either a pure substance, a homogeneous mixture, or a heterogeneous mixture: (a) chunky peanut butter (b) apple juice (c) iron fillings

(a) chunky peanut butter - heterogeneous mixture (b) apple juice - homogeneous mixture (c) iron fillings - pure substance The keys to the questions are in the following definitions: Pure substance: Cannot be separated into other substances by physical means. Elements and compounds are pure substances. Mixture: Consists or two or more pure substances. Can be separated by physical means. A homogeneous mixture has a uniform appearance and composition throughout. A heterogeneous mixture does not have a uniform composition throughout and usually does not have a uniform appearance at the macroscopic level. (a) chunky peanut butter is a heterogeneous mixture. (b) apple juice is a homogeneous mixture. (c) iron filings is a pure substance. This is an example of an element. If you recognize something to be an element or a compound, it is a pure substance. Liquid mixtures that are transparent are usually solutions, and solutions are homogeneous mixtures. If different components of a mixture are visible to the naked eye, the substance is a heterogeneous mixture.

Classify each of the following pure substances as either an element or a compound: (a) titanium (b) water (c) silver

(a) titanium - element (b) water - compound (c) silver - element An element cannot be separated into simpler substances by ordinary chemical means. In a sample of an element, all of the particles (atoms) are the same element. Each element is represented by a one- or two-letter symbol and has a one-word name. A compound is made of two or more elements combined in a definite way. A compound is represented symbolically by a formula that shows all of the elements that are present in the compound. A compound may be separated into its elements by ordinary chemical means. The chemical name of a compound often consists of more than one word, but many compounds, like water, have common one-word names.

Click on sulfur. This element exists as rings. How many atoms is each sulfur atom bonded to?

2 This is typical for a nonmetallic element: each atom is bonded to only a few other atoms.

When one thing is ten times as large as another, it is said to be "an order of magnitude" larger. A hundred times larger is two orders of magnitude. A thousand is three orders of magnitude. By how many orders of magnitude is the Empire State Building taller than a chair seat? Enter an integer.

3 orders of magnitude A chair seat sits about 18 inches off the ground - a foot and a half. The building is about 1500 feet high. That's a thousand times taller - three orders of magnitude.

Again using the figure, how many orders of magnitude larger is a bacterial cell than an aspirin molecule?

3 orders of magnitude The bacterial cell is about a thousand times larger than the molecule. The bacterial cell is about 10^-6 m wide; the molecule is about 10^-9 m wide.

physical change

a change that involves only physical properties

chemical change

a change that involves the transformation of one or more substances into one or more different substances

properties

a collection of characteristics that describe matter

mixture

a combination of two or more substances in which each substance retains its identity

Which of the following best describes this substance? A chocolate chip cookie

a heterogeneous mixture The cookie clearly has different composition in different areas. This means it is a heterogeneous mixture.