Elements, Compounds, Mixtures

Classification of Matter

Elements, Compounds, Mixtures

Lesson Objective• Physical and Chemical Changes.• Elements• Compounds• Mixtures

Physical and Chemical Changes

• All reaction in Chemistry can be divided into two types of changes: Physical or Chemical.

• Physical Change:

• NO new substance is formed.• These changes are easily reversible or

temporary.• An example is dissolving salt in water.• Other examples of physical change include:

melting, boiling, mixing 2 solids or liquids.

• Chemical Change:

• A NEW substance is formed.• These changes are irreversible or permanent.• The new substance formed will have different

properties compared to its original elements. (E.g. different melting and different chemical reactions from the original substance.)


• One can recognize a chemical change from 2 observations:

• The new substance has a different appearance, such as different color or different physical state.

• An example is the electrolysis of water.• Reason: Electrolysis of water produces two new

substances, Hydrogen gas and oxygen gas. Hydrogen and oxygen are both gases but water is a liquid.

• A lot of heat is given out in a chemical changes.• Chemical changes are called chemical reactions.


Illustration of chemical changes

An Overview

Elements, Compounds and Mixtures

“ single, attached, play play…”

Introducing Little Miss “Element”

Hi! I am Little Miss “Element”I am PURE SUBSTANCE

I cannot be broken down into any simpler substance by

means of a chemical reaction* or electricity**!

*Chemical process refers to chemical reaction or heat

**Electricity refers to electrolysis

ElementsDefinition of an element:

An element is a pure substance which cannot be split up into two or more

simpler substances by chemical means.

Sugar is not an element as it can be broken down into carbon and water.

Note that an element:

• Consists of only one kind of atom,

• Cannot be broken down into a simpler type of matter by either physical or chemical means

• Can exist as either atoms (e.g. argon) or molecules (e.g., nitrogen).

Elements

Examples of elementsElements are made up of tiny particles

Elements can be further classified into two groups:

Is the smallest particle of an element and has

the same chemical propertiesof the element

Is made up of two or moreatoms that are chemically

bonded together

(note: these atoms are of the SAME element!!)

Hi, I am from the “Noble gas” family and I work alone

We same

same!!

These are elements!

Atoms of same element

Cu

copper element

Na

sodium element

He

helium element

Molecules of same element

HHydrogen gas element

O

H

OO

ozone

AtomsAn element is made of tiny particles called atoms.

The atoms of an element is different from that of another element.

Elements- Atoms

Microscopic view of the atoms of the element argon (gas phase).

Consists of only one kind

of atom

What is an atom?Examples of models of atoms:

H Na Cl

Hydrogen atom

Sodium atom

Chlorine atom

MoleculesVery few elements exists as

atoms besides elements such

as helium and neon.

Most elements exist as

molecules.

For example, hydrogen

is H2.

Ozone is O3.

Hydrogen (H2) is a

diatomic molecule.

Ozone (O3) is a

triatomic molecule.

Molecules consisting of a

few atoms are called

polyatomic molecules.

Elements-Molecules

Microscopic view of the molecules of the element nitrogen (gas phase).

Consists of only one kind

of atom

Examples of molecules (elements)

H H

N N

O

O

O

O O S

SS

SS

S

SS

Allotropes of Carbon molecule (element)

• Carbon is an element but made up of a very large molecule (also known as MACROMOLECULE)

• There are 3 types of carbon molecular structures– C60

– Diamond– Graphite

Chemical Symbols of Elements

Chemists use symbols to represent elements.

For example, O represents oxygen while Fe represents iron.

Element Symbol Element Symbol

Calcium Ca Mercury Hg

Carbon C Neon Ne

Hydrogen H Silicon Si

Iron Fe Sodium Na

Classification of Elements – Metals and Non-metals

There are two major groups of elements – metals and non-metals.

Iron is a metal. Oxygen is a non-metal.

There are some elements called metalloids which behave like both metals and non-metals.

Metals and non-metals are grouped separately on the Periodic Table.

What is an element?An element is a substance that cannot be broken down into two or more simpler substances by any chemical means.

- They are arranged in the Periodic Table, classified as metals and non-metals.- They may consist of atoms of same element or molecules of the same element. Metals (excluding hydrogen)

Non-metals (including hydrogen)

Physical Properties metals and non-metals

Metals Non-metals

Shiny appearance Dull appearance

Solids at r.t.p (except mercury)

Gases, liquids or solids at r.t.p (room temp & pressure)

Malleable, Ductile, Sonorous Brittle if solid (general)

High melting and boiling points

Low melting and boiling points

Good conductors of heat Poor conductors of heat

Good conductors of electricity

Poor conductors of electricity

(expect carbon and graphite)

Summarising

Elements

MetalsNon

Metals

Atoms Atoms Molecules

Introducing Little Miss “Compounds”

Harlow! Hehee!!!*giggles*

I am also a PURE SUBSTANCEI am made up of a fixed number

of two or more elements chemically combined.

CompoundsNote that a compound:

• can be broken down into a simpler type of matter (elements) by chemical means (but not by physical means),

• has properties that are different from its component elements, and

• always contains the same ratio of its component atoms.

A compound is a substance which is made up of two or more elements chemically combined together.

What is a compound?

- Chemical reactions taking place.

Qn: Is this a compound?

It only contains one type of element.


It only contains one type of element.

H

O

HO

O


It is not chemically combined.

So, what is a compound then?

H

Water Ammonia gas

Consists of two or more elements

And

They are chemically combined together!

Compounds

Microscopic view of the molecules of the compound water (gas phase). Oxygen atoms are red and hydrogen atoms are white.

ELEMENTSCOMPOUNDhydrogen

(colourless gas)

oxygen(colourless

gas)

lighted splint

heat, light and

explosion

water(colourless

liquid)

Making compounds from their elementsExample: Making water (picture)

Water molecule

Oxygen molecule

Hydrogen molecule

mixture of

hydrogen and

oxygen

water

Example: Making water (models)

heat, light and

explosion

Making compounds from their elements

Example: Making iron sulphide compound

Iron + sulphur iron sulphide

elementscompound

+

yellow grey

heat

heat

black

Little Miss “Compound” Property #1

I can be represented as a “chemical

formula”

The different elements present in

a compound are combined in a fixed

ratio

General Rules:- Only write the name of the metallic element first

- Write the number of atoms of any element in the compound in subscript (applies to only 2 or more atoms)

- “ide” will always be written at the end of the non metal element

Special Rules:• Does not apply to organic compounds

(chapter 18 onwards)


I am formed by atoms of different elements.

However, I do not have the properties

as them.

Examples

• Na + Cl2 NaCl (s) (g) (s)

• H2 + O2 H2O (g) (g) (l)


Because of my chemical bonds, I cannot be broken down by

physical means. I can only be broken down by chemical

reaction or electricity.

Electrolysis (breaking down of compound by electricity)

2MgCl(s) Mg(s) +Cl2(g)

Thermal Decomposition(breaking down of compound by heat)

HgO(s) Hg(s) + O2(g)

How do we name compounds? Rule 1

A compound made up of two elements has a name that ends in -ide.

• Sodium chloride — made up of the elements sodium and chlorine • Zinc oxide — made up of the elements zinc and oxygen • Carbon dioxide — made up of the elements carbon and oxygen


A compound that contains hydroxide ions , OH–

(a negatively charged ion made up of oxygen

and hydrogen) is named a hydroxide.

• Potassium hydroxide — contains potassium ions and hydroxide ions


A compound that contains a negatively

charged polyatomic ion containing oxygen

usually has a name ending in –ate.

• Copper(II) sulphate — contains oxygen atoms in the sulphate ion

• Sodium nitrate — contains oxygen atoms in the nitrate ion

Fixed Composition of Compounds

For example, water (H2O) is a compound made

only by joining together two atoms of hydrogen

to one atom of oxygen.

That is, the ratio of hydrogen atoms to oxygen

atoms in water is always 2 : 1.

A compound is made up of different elements

chemically combined in a fixed ratio.

Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.

Chemical Formula of a Compound

A compound can be represented by a chemical formula.

The chemical formula states • the types of atoms (i.e. elements) in the compound, • the ratio of the different atoms in the compound.

Types of atoms: hydrogen, oxygen

Ratio of H to O = 2:1

How do we write chemical formula?Rule 1

For many compounds that contain both metallic and non-metallic elements, the symbol of the metallic element is written first.

• calcium oxide (CaO) • sodium chloride (NaCl) • magnesium carbonate (MgCO3)


The number of atoms is written as a subscript, to the right of the atom’s symbol.

• water (H2O, not H2O or 2HO)

• magnesium carbonate (MgCO3, not MgCO3 or MgC3O)


It is not necessary to write the subscript ‘1’.

• water (H2O, not H2O1) • calcium oxide (CaO, not Ca1O1)


The oxygen atom is usually written at the end of the formula.

• water (H2O, not OH2) • carbon dioxide (CO2, not O2C) • nitric acid (HNO3, not O3NH)

How do we calculate the number of atoms in a formula?

Pb(NO3)2

Number of nitrogen (N) atoms

= 1 x 2 = 2

Number of lead (Pb) atoms =

1

Number of oxygen (O) atoms

= 3 x 2 = 6

Heat can be used to break down compounds into elements or simpler compounds. Such a chemical reaction is called thermal decomposition.

Compounds can be Decomposed

Mercury(II) oxide

Oxygen

Summarizing• A compound is made up of two or more

elements chemically joined together• A compound has a fixed composition• Every compound has a unique chemical

formula• A compound has a completely different

properties from its elements• A chemical reaction (decomposition or

electrolysis) is needed to separate the elements in the compound

Molecules- Elements or compounds?

• A molecule consists of two or more atoms of the same element, or different elements, that are chemically bound together.

• It can be a molecule of an element

• E.g. N2, O2, S8

• It can be molecule of a compound.

• H2O, CO2, NH3

N N O OS

SSS

S

S

SS

OH H

O OC N

H

HH

Think about this…ROJAK!!

Introducing Little Mr. “Mixture”

Burp!!!! I am messy!

I am formed when two or more substances joined together physically (without chemical

bonds)

I have the same properties as all the substances

AIR

I am his best friend!

MixturesDefinition of a mixture:

A mixture is not a pure substance as it contains a mixture of atoms of

molecules which are not chemically combined together.

Mixtures• Note that a mixture: • consists of two or more different elements

and/or compounds NOT chemically combined.• Can be homogeneous or non-homogeneous • can be separated into its components by

physical means, and • often retains many of the properties of its

components.

Microscopic view of a gaseous mixture containing two elements (argon and nitrogen) and a compound (water).

Mixtures

Mixtures

• Examples of mixtures include muddy water and air.

Air is made up of gases such as nitrogen and oxygen mixed together.

Little Mr “Mixture”I do not have a fixed composition of the substances.

A Mixture can be:• element + element

• element + compound• compound + compound

(Refer to page 59, Fig 4.7)

He is a “boo-boo!”

He can be separated to its substances by physical

methods

A mixture of 2 elements

A mixture of two elements, e.g. neon (Ne) and hydrogen (H2)

A mixture of 1 element and

1 compoundA mixture of one element and one compound, e.g. hydrogen (H2) and ammonia (NH3)

A mixture of 2 compounds

A mixture of two compounds, e.g. water vapour (H2O) and carbon dioxide (CO2)

Mr Alloy

Alloy: What is it??• It is a mixture of metals with

other elements

• Alloy metals are generally stronger and better in physical properties

• Examples– Steel (Iron + Carbon)– Stainless Steel (Nickel + Iron +

Chromium + carbon)– Brass (Copper + Zinc)

Yup!That’s

ME!

Can you differentiate?

Compound Mixture of element & compound

Mixture of 2 elements-Alloy

Molecules of an element

Mixture of 2 elements

ConclusionAn element is a pure substance which cannot be

split up into two or more simpler substances by chemical means.

A compound consist of a fixed number of different kinds of atoms chemically combined together.

A mixture is not a pure substance as it contains a mixture of atoms of molecules which are not chemically combined together.

Difference between mixtures and compounds.

Compounds

E.g.

Water H2O

Mixtures

E.g.

Salt solution

Separation Cannot be separated into its constituents by physical methods. Separation can only be done by chemical methods or by electricity.

e.g. Water can only be separated into Hydrogen and Oxygen by Electrolysis.

Can be easily separated into its constituents by physical

methods.

e.g. Salt can be obtained from its solution by

evaporation. Water can be obtained by simple

distillation.

Difference between mixtures and compounds.Compounds

E.g.

Water H2O

Mixtures

E.g.

Salt solution

Composition The composition is fixed. The constituents (elements) in the compound are combined in definite amount.

e.g. To form one molecule of water, two atoms of Hydrogen and one atom of Oxygen is needed.

Has no fixed composition. The amount of constituents in a mixture can vary.

e.g. The amount of salt added into the water to make salt solution can vary and vice versa.

Difference between mixtures and compounds.Compounds

E.g.

Water H2O

Mixtures

E.g.

Salt solution

Properties The physical and chemical properties of the compound is different from the properties of its constituents.

e.g. Hydrogen and Oxygen are gases while water is a liquid at room temperature and pressure.

e.g. Water has a fixed melting and boiling point.

The mixture does not have its own properties. It has the average physical and chemical properties of the constituents that made it up. (Each constituents still retain its own properties)

e.g. The salt solution still retains its salt taste.

e.g. Salt solution does not have a fixed melting or boiling point, but

rather over a range of temperature.

Difference between mixtures and compounds.

Compounds

E.g.

Water H2O

Mixtures

E.g.

Salt solution

Energy Change

Energy is usually given out or occasionally taken in when a compound is formed.

Normally little or no energy is given out or

taken in when a mixture is formed.

Differences between mixtures and compounds

*giggles*

HeE stupid…

Laugh what? I know I am a

little “bushy and hairy” than

you…

Little Miss CompoundMr Messy

Comparison between mixtures and compounds

FixedVariableMp / Bp

Has a fixed compositionDoes not have a fixed composition

Composition

A chemical reaction takes place when a compound

is formed

No chemical change takes place when a mixture in formed

Energy Change

Properties are unique and different from its elements

Same properties as its components

Properties

(chemical / physical)

Elements in a compound Cannot be separated by

physical methods

Components can be separated by physical

methods

Separation

/breakdown

CompoundMixture

Challenge Time • Is mineral water an element, mixture or

compound?

Challenge Time

• Are YOU an element, mixture or compound?

EXTRA!• Human body is made up of 28 elements?• About 99% our mass is made up of the 6

main elements– Oxygen (65%)– Carbon (18%)– Hydrogen (10%)– Nitrogen (3%)– Calcium (1.5%)– Phosphorus (1.5%)

The End……

Elements, Compounds, Mixtures

Education

Transcript of Elements, Compounds, Mixtures