chapter 3: matter and energy · 2014-10-10 · distinguish between physical and chemical...

Chapter 3: Matter and Energy

Distinguish between physical and chemical properties.

Distinguish between physical and chemical changes.

Apply the law of conservation of mass.

Physical property: a property that a substance displays without changing its composition.

Chemical property: a property that a substance displays only through changing its composition.

melting point state of matter

mass color

flammability

reacts with oxygen

reacts with an acid

Gasoline:

Physical property: its characteristic odor

▪ gasoline does not change its composition when it exhibits its odor

Chemical property: its flammability

▪ gasoline does change its composition when it burns

The atomic or molecular composition of a substance does not change when the substance displays its physical properties.

A chemical or physical property can also be described as either intensive or extensive.

Intensive Properties: Properties that do not depend on the amount of substance present

Examples: color, melting point, density, luster

Extensive Properties: Properties that do depend on the amount of substance present.

Examples: mass, volume, amount of energy within a substance

Physical property: the boiling point of water

How is the water changing once it boils?

When water boils, it turns into a gas, but the water molecules are the same in both the liquid water and the gaseous steam.

Physical change: matter changes its appearance but not its composition

When ice melts, it looks different but its molecular composition is the same. Solid ice and liquid water are both composed of water molecules, so melting is a physical change.

Chemical change: matter does change its composition

For example, copper turns green upon continued exposure to air because it reacts with gases in air to form new compounds. This is a chemical change.

State changes: transformations from one state of matter to another, such as from solid to liquid

These are always physical changes

Condensation

Gas to liquid

Freezing

Liquid to solid

Melting

Solid to liquid

Evaporating

Liquid to gas

Sublimation

Solid to gas

Deposition

Gas to solid

cutting breaking tearing boiling

color change

Four good indications of a chemical change...

gas released (often with an

odor)

energy change (light, heat, …) precipitate

formed

A chemical property: The susceptibility of iron to rusting.

Is rusting a physical or chemical change?

When iron rusts, it turns from iron to iron oxide.

Classify each property as physical or chemical.

a) the tendency of copper to turn green when exposed to air

b) the tendency of automobile paint to dull over time

c) the tendency of gasoline to evaporate quickly when spilled

d) the low mass (for a given volume) of aluminum relative to other metals

Matter undergoes a chemical change when it undergoes a chemical reaction.

In a chemical reaction, the substances present before the chemical change are called reactants. The substances present after the change are called products.

The difference between chemical and physical changes is seen at the molecular and atomic level.

In physical changes, the atoms that compose the matter do not change their fundamental associations, even though the matter may change its appearance.

A physical change results in a different form of the same substance.

In chemical changes, atoms do change their fundamental associations, resulting in matter with a new identity.

A chemical change results in a completely new substance.

Classify each change as physical or chemical.

a) the rusting of iron

b) the evaporation of fingernail-polish remover (acetone) from the skin

c) the burning of coal

d) the fading of a carpet upon repeated exposure to sunlight

In this figure liquid water is vaporizing into steam.

Which diagram best represents the molecules in the steam?

Compounds are pure substances

Compounds can only be separated by chemical reactions.

Mixtures can never consist of only a single substance

Mixtures can be separated by physical changes

Filtration

Separates liquid from a solid

The mixture is poured over a mesh-like filter paper and the liquid is allows to pass through, leaving behind the solid

Density

Liquids of different densities will separate into layers.

The lowest density liquid will rest on top.

Centrifuge

A centrifuge is a machine that separates solids from liquids by spinning very quickly.

This causes solids to fall to the bottom of the test tube, separating them from the liquids in the mixture.

Decant

Separates solid from liquid by pouring the liquid off the top and leaving the solid behind

It can be difficult to ensure that all of the liquid is removed by this technique

Magnetism

Can be used to remove magnetic solids from a mixture

Magnetic elements include iron, nickel, and cobalt

Distillation

To separate a mixture of liquids, the liquid can be heated to force components, which have different boiling points, into the gas phase. The gas is then condensed back into liquid form and collected.

Can be used to separate several different substances

Chromatography

Techniques which use a stationary phase (e.g. paper) and a mobile phase (e.g. some sort of solvent like water or alcohol) to physically separate substances.

A sample of the mixture is placed on the paper (stationary phase). The solvent will then dissolve the components of the mixture.

The components of the mixture will move with the solvent (mobile phase) up the paper.

Based on the properties of each component (e.g., polarity) the components will travel at different speeds with the solvent.

Many complex variations, but basic principles stay the same

Law of Conservation of Mass: matter is neither created nor destroyed in a chemical reaction.

In a nuclear reaction, significant changes in mass can occur.

During physical and chemical changes, the total amount of matter remains constant.

Suppose that we burn 58 g of butane in a lighter. It will react with 208 g of oxygen to form 176 g of carbon dioxide and 90 g of water.

A chemist forms 16.6 g of potassium iodide by combining 3.9 g of potassium with 12.7 g of iodine. Show that these results are consistent with the law of conservation of mass.

Suppose 12 g of natural gas combines with 48 g of oxygen in a flame. The chemical change produces 33 g of carbon dioxide. How many grams of water form?

Classify each property as physical or chemical.

(a) the explosiveness of hydrogen gas

(b) the bronze color of copper

(c) the shiny appearance of silver

(d) the ability of dry ice to sublime (change from solid directly to vapor)

Classify each change as physical or chemical.

(a) copper metal forming a blue solution when it is dropped into colorless nitric acid

(b) a train flattening a penny placed on a railroad track

(c) ice melting into liquid water

(d) a match igniting a firework

Consider a drop of water that is put into a flask, sealed with a cap, and heated until the droplet vaporizes. Is the mass of the container and water different after heating?