Pure Substances vs. Mixtures

Physical and Chemical Changes

Everything that has mass and takes up space is called matter.

Matter that has a fixed composition and definite properties. (chemical and physical)

There are two kinds of pure substances

Elements

Compounds

Elements• Substance that cannot be separated or

broken down into simpler substances by chemical means.

• Ex’s- any box on the periodic table

• Made of only 1 type of atom– The smallest unit of an element that

maintains the properties of that element

• HUH?

The smallest unit of an element that

maintains the properties of that element???

• The element sodium has certain properties– 11 protons, 11 electrons…boiling point, etc

Compounds cont…

• Substance made of atoms of 2 or more different elements that are CHEMICALLY combined.

• Elements are combined in a definite way and this changes their properties

Na- lethal if ingested Cl- lethal if ingested

NaCl- table salt

Molecules

• Two or more atoms bonded together…– Ex- O2, NaCl, etc.

– Not all molecules are compounds, but all compounds are molecules…explain

Compounds• Molecules made by

two or more elements bonded together -always in a definite ratio

Elements• Molecules made of

just one element

Na (sodium)

NaCl (sodium chloride/salt)

• Cannot be separated into simpler substances

by physical methods (physical changes)

• Fixed composition– Properties do not vary

• Can be expressed with a chemical formula– Ex H2O, NaCl, H

•Mixtures are two or more substances that are physically combined.

• (NOT chemically combined like a pure substance)

•May be separated into pure substances by physical methods

Examples of Mixtures

• Components retain their characteristic properties and can be separated by physical means.

• http://videos.howstuffworks.com/hsw/22540-together-but-separate-suspensions-video.htm

This sand and iron filings mixture can be separated using a magnet.

Two types of mixtures

• Homogenous mixtures look the same throughout • Types: solutions, alloys, etc.• Example: salt water, brass

• Have the same composition throughout

• Components are indistinguishable• Will not scatter light• Particle size is small

Can they still be separated by physical means ?

YES!!! It may be more complicated, but it can be done…distillation , centrifugation

• Solutions are …

• homogenous mixtures that do not scatter light.

• separated by physical means (including distillation or evaporation.)

• created when something is completely dissolved in pure water.

Examples: sugar water, salt water

Parts of a Solution

• Solute- substance that dissolves in solvent

ex. Salt (“U dissolve”)

• Solvent- substance that does the dissolving (ex.water)

Well, not really, because you don’t dissolve, but you get the picture

Why don’t you dissolve?

• You are not “Miscible”– Miscible- two or more liquids that can dissolve

into each other

– Immiscible- liquids that do NOT mix • Ex- oil and water

Heterogeneous mixtures are composed of large pieces that are easily separated by physical means (ie. density, polarity, metallic properties).

• Do not have same composition throughout

• Components are distinguishable

• Particle size is medium or large

Examples: fruit salad, vegetable soup, etc.

Tyndall Effect

• Scattering of light due to particles

1. Colloid –medium particles•Particles stay suspended in the mixture and scatter light (Tyndall effect)

•Ex- mayonnaise, milk, fog

1. Suspension- large particles•Tyndall effect•Particles will settle out when the mixture is allowed to stand

•Ex-muddy water, OJ with pulp

Physical

Chemical

• Physical properties are those that we can determine without changing the identity of the substance we are studying.

• Properties we can observe or measure– Ex- color, state of matter, etc

Property Description Example

Hardness Ability of 1 substance to scratch another

Diamond can scratch glass

Viscosity Resistance to flowing

Water= low visc.Honey= high visc.

Solubility Ability to dissolve in water

NaCl dissolves easily in water so it’s soluble

Property Description Example

Superconductor Substance that conducts electricity at lower temperatures

Tin, aluminum, used in trains

Semiconductor Substance that has a resistance b/w a conductor & insulator

Silicon-used in computers or radios

DENSITYMass per unit volume

Calculated by: Density= Mass/Volume

D=M/V

• Hardness, color, melting point and density are all physical properties.

• The physical properties of sodium (NaCl) metal– soft, lustrous– silver-colored metal – relatively low melting point – low density.

• A physical change occurs when the substance changes physically but does not change its chemical composition. • For example: water freezing into ice, cutting a

piece of wood into smaller pieces, etc. • The form or appearance has changed, but the

properties of that substance are the same (i.e. it has the same melting point, boiling point, chemical composition, etc.)

• Temperature– Melting– Boiling – Freezing

• Color

• Conducting electricity• Dissolving

• Chemical properties describe the way a substance can change or react to form other substances.

• These properties, then, must be determined using a process that changes the identity of the substance of interest.

• One of the chemical properties of alkali metals such as sodium and potassium is that they react with water. To determine this, we would have to combine an alkali metal with water and observe what happens.

• In other words, we have to define chemical properties of a substance by the chemical changes it undergoes.

• A chemical change occurs when a substance changes into something new.

• This occurs due to heating, chemical reaction, etc.

• You can tell a chemical change has occurred if the density, melting point or freezing point of the original substance changes. Many common signs of a chemical change can be seen (bubbles forming, mass changed, etc).

– You can’t go back to the original substances after a chemical change (reaction) has occurred!!

Baking soda reacting w/ vinegar

Burning wood

There is no observable change in the quantity of matter during a chemical reaction or a physical change.

In other words, matter cannot be created nor destroyed. It is just converted from one form to another