Chemistry year 8 basic ppt

7/28/2019 Chemistry year 8 basic ppt

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The atom is built from three types of basic

Subatomic particles: the proton (p+) and the neutron (n)

found in nucleus

the electron (e): Electrons are very light. They are only about

1/2000th the mass of a proton and neutron

Electrons spin around the nucleus at very

high speeds.


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All atoms are electrically neutral So, in any atom: the numberof protons in an atom = the number of electrons in an atom

Atomic number- The number of protons in an atomis called its atomic number. The atomic number gives theatom its place in the Periodic Table.

Atomic number = number of protons = number of electrons

in an atom

Mass number- The mass number of an atom isthe total number of

particles in its nucleus. Mass number = number of protons +number of neutrons


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To find out how many protons, neutrons and electronsthere are in an atom:

protons = atomic number = place in Periodic Table

neutrons = mass number atomic number

electrons = protons = atomic number = place inPeriodic Table.


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An elementis an absolutely puresubstance that cannot be broken down

into other substances.

A list of all the known elements is called the

Periodic Table

A compoundconsists of two or more elementschemically combined. Compounds can be molecules or alattice structure.


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The properties of compounds (colour, texture, smell, density) areusually very different when compared with the elements theycontain.

For example, the compound water (H2O) is a liquid at roomtemperature, but the elements that make it up, hydrogen andoxygen, are colourless gases.

Sodium is an explosive metal and chlorine is a poisonous gas, butthese elements combine to form salt, sodium chloride (NaCl), a solidthat we safely sprinkle on food.

Compound formulas The best way of describing which atoms are

bonded in a molecule or lattice is to write its chemical formula usingelement symbols. Small subscripts show how many atoms of eachelement there are in the molecule.


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Mixtures A mixture contains two or more substances(elements or compounds) simply mixed together.

These substances are not bonded together and nonew substance is formed when they are combined.This means that a mixture can easily be separatedinto its ingredients using simple techniques such asfiltration or evaporation.

Soft drink is an example of a mixture. It contains thecompounds sugar, water, flavouring,

colouring and carbon dioxide gas.

Muddy water, sea water, shampoo, air and paint areall mixtures, too.


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A physical change occurs when a substance changes, but nonew substance is formed.

Physical changes are happening when a substance:

changes shape; for example, an aluminium can is crushed or anelastic band is stretched

is crushed or broken into smaller pieces; for example, a glasswindow is shattered or an aspirin is crushed to make it easier toswallow

is dissolved; for example, sugar is dissolved in hot coffee orturpentine removes a paint stain

is simply mixed with other substances; for example,concentrated cordial is diluted by mixing it with water

changes its state by melting, evaporating, freezing orcondensing; for example, an ice cube melts.


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A chemical change or chemical reaction hasoccurred whenever a new substance has formed. Somechemical changes are obvious: the substance formed looks oracts very differently to what was there before.

Sometimes, however, the change may be difficult to detect. Theonly indication may be a change in colour, a change intemperature or the production of light or bubbles.

Some chemical changes that you meet almost every day

include: cooking; such as cooking a cake or grilling a sausage burning; such as lighting a match or burning your toast explosions; such as petrol exploding in the cylinders of a car


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In a chemical reaction, the substances you start withare called the reactants.

The substances you end up with are called its products.All chemical reactions can be represented by a

chemical equation. The symbol indicates that achemical reaction has taken place.

All chemical reactions can be shown as: reactants products

Sometimes subscript symbols (s), (l) and (g) are addedto give extra information about whether the chemical isa solid, liquid or gas. The symbol (aq) indicates that thesubstance is dissolved in water as an aqueous solution


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In combination reactions, reactants combine to form a single newsubstance. Rusting of iron is an example of

A combination reaction where iron (Fe) reacts with the oxygen gas (O2)found in air or water to form rust (the compound iron oxide Fe2O3).

This could be written as a word equation:

iron + oxygen gas iron oxide

or it could be written as a formula equation using element symbols andchemical formulas:

Fe + O2 Fe2O3


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Breaking-down reactions In breaking-down reactions onesubstance breaks up into a number of smaller ones.

For example, some photographic film is coated with silverchloride (AgCl), a substance that breaks down when lightfalls on it, leaving a dark image, or negative, on the film.

The reaction could be written:

2AgCl 2Ag + Cl2 showing the states of matter: 2AgCl(s) 2Ag(s) + Cl2 (g)

Breaking-down reactions are also known as decompositionreactions.


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Sometimes a solid forms when two solutionsare mixed together. The solid formed is called

a precipitate.

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3 (aq) It starts as a fine powder that then settle out into a layer of sediment

at the bottom of a clear solution.

Precipitates are insoluble, meaning they do not dissolve in water.Some precipitates are very colourful and are often used as paint

pigments.


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Combustion reactions happenwhenever you burn explode

something. A substance reacts withoxygen, usually from the air around it.

New (often gaseous) substances formand heat and light may be released,sometimes as a flame or explosive

flash.


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A Bunsen burner works because methane gas fromthe gas supply reacts with oxygen drawn through itsair hole.

Carbon dioxide, water vapour and a flame (heat andlight energy) are produced:

CH4 (g) + O2(g) CO2 (g) + H2O(g)

When magnesium ribbon burns, it combines withoxygen in the air to produce a white powder,magnesium oxide: magnesium

Mg(s) + O2(g) MgO(s)


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Rate of Reaction is how fast the products areproduced per second.

The Rate of Reaction can be increased by:

1.Increasing concentration of reactants

2.Increasing surface area by decreasing particle size3.Increasing temperature

4.Using a catalyst

Chemistry year 8 basic ppt

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