Science term 2

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Physical and Chemical Changes

In the chemical sciences, changes to materials can be categorised as either physical change or chemical change. Physical change may include change of state (for example, from solid to liquid) and change of form (for instance, forming into crystals or dissolving). Chemical changes are the changes where new substances are formed.

Chemical Reaction

Also called a chemical change

In a chemical reaction, one or two new substances are formed

Signs of a Chemical Reaction

There are four main, observable signs of a chemical reaction

They are:

Colour change

Increase or decrease in temperature

Gas production (bubbles in liquid, not boiling)

Precipitation (tow liquids react to make a solid)

y.

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Reaction number and method

What I Observed

Sign of chemical reaction

1. magnesium + hydrochloric acid à magnesium chloride + hydrogen

The magnesium dissolved completely

It started slowly degrading

2.Copper sulphate + magnesium à magnesium sulphate + copper

Over time the magnesium started to break down and there were black particles floating in the liquid

Bits started falling off

3. copper sulphate + sodium hydroxide à copper hydroxide + sodium sulphate

Changed to a blue gel like substance

Precipitation

4. barium chloride + sulphuric acid à barium sulphate + hydrochloric acid

Changed to a murky white colour

It looks like it has settled into a solid from a liquid

5. calcium carbonate (marble) + hydrochloric acid à calcium chloride + carbon dioxide + water

Marble dissolved/fizzed

The marble chip dissolved in the hyparchic acid

6. water + carbon dioxide à carbonic acid

Went from a blue colour to a yellow/green colour

The bromothymol blue mixed with the water for a colour change.

Reactants and products

You can divide the substances in any chemical reaction into 2 groups:

Reactants

Reactants are the substances you start with in a chemical reaction and they react with each other

Products

Products are the new substances that are produced in a chemical reaction

Chemical/Word equation

There is a shorthand way of describing what happens in a chemical reaction – called a chemical/word equation

They follow a specific format. Reactants are found on the left side and products are found on the right side. An arrow ( -> ) is used to give direction, from reactants to products

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1. breaking a bone – physical

2. burning paper – chemical

3. slicing potatoes for fries – physical

4. mixing sugar with coffee – physical

5. frying chicken – chemical

6. a nail rusting – chemical

7. paper ripping – physical

8. mixing water and food colouring – physical

9. food molding (rotting) – chemical

10. fogging a mirror with your breath – physical

Atoms

An atom is the smallest piece of matter

Atoms are extremely small. They cannot be seen with the naked eye or with normal light microscopes.

Atoms are made up of subatomic particles:

Protons which are positively charged

Neutrons which have no charge

Electrons which are negatively charged

Molecules

Molecules are small groups of atoms held together by strong bonds.

The atoms can be the same or different

Elements

And element is a pure substance that cannot be broken down into other substances.

The smallest piece of an element is an atom, but an element is made up of only one atom type

Compounds

A chemical compound is a pure chemical substance consisting of two or more different chemical elements.

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Periodic table

Groups are considered the most important method of classifying the elements.

In some groups, the elements have very similar properties and exhibit a clear trend in properties down the group.

If you looked at one atom of every element in a group you would see every atom in a group has the same number of electrons in its outermost level.

Valence Electrons

The electrons in an atom's outermost level are called the valence electrons.

The number of valence electrons in an atom affects the way the atom bonds with other atoms.

This is why elements in a groups have similar properties- because they have the same number of valence electrons.

Group - Valence Electrons

1 - 1

2 - 2

13 - 3

14 - 4

15 - 5

16 - 6

17 - 7

18 - 8

Periods

The rows in the periodic table are referred to as periods.

The period number indicates the number of shells the atoms has when drawing shell diagrams.

Hydrogen and Helium both have 1 electron shell.

Flame tests

Each flame is a unique colour, so it can be used to identify metals.

Only a small number of metals have identifiable colours through this method.

Simple flame tests have a high degree of uncertainty.

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Electron Shells

The first, innermost, shell, contains only two electrons at most.

The other shells are larger

The innermost shells are used and filled first.

When a shell is filled, electrons begin to fill the next shell

Electron shell Number of electrons

1 2

2 8

3 8

4 18

Lewis dot diagrams

A lewis structure uses dots to show eectron positions around the atoms

Write the chemical symbol

Find the number of valence electrons (groups)

Draw the valence electrons

Flame test

Sunstance - Colour

Unknown - Red

Distilled water - Nothing

Strontium Nitrate - Orange

Barium Chloirde - Lime green

Copper Sulpahte - Forest green

Potassium Chloride - Yellow/Orange

Calcium Chloride - Red

Sodium Chloride - Orange

Bohr Model - electron configuration

Protons and neutrons are confined to the nucleus of an atom.

Electrons go around in specific regions called shells.

Each shell corresponds to a specific amount of energy held by the electrons.

Electrons use the least amount of energy to go around the nucleus possible.

11p

12n

= Nucleus containing 11protons and 12 neutrons

Shell 1 - max. 2

Shell 2 - max. 8

Shell 3 - max. 8

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Neutral atoms

A neutral atom has the same number of protons and electrons

In chemical reactions it is only the outermost (valence) electrons that are invlolved

Ions

During chemical reactions...

When an atom loses an electron, it becomes positive overall because there are more protons -> CATION (metals)

When an atom gains an electron, it becomes negative overall because there are more electrons -> ANION (non-metals)

Ions are charged particles. They are atoms that have unequal numbers of protons and electrons.

Only electrons are gained or lost (nucleus remains the same)

Cations are positively charged ions -> have more protons than electrons

Anions are negatively charged ions -> have more electrons than protons

Atoms of metals tend to lose electrons

Atoms of non metals tend to form ions by gaining extra electrons

For example, chlorine atoms form negative chloride ions Cl-

Valence vs Valency

The valence electrons are the electrons in the outermost shell of an atom

Valency is the charge of an ion, that is the number of electrons the atom has gained or lost to become an ion

So Ovygen has a valence of 6 (group 16) and a Valency of 2- (because it gains 2 e-)

Group - Valence - Valency

1 - 1 - 1+

2 - 2 - 2+

13 - 3 - 3+

14 (non metal) - 4 - 4-

14 (metal) - 4 - 4+

15 - 5 - 3-

16 - 6 - 2-

17 - 7 - 1-

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Metals give up their outermost electrons, becoming positivley charged cations, to achieve this perfect value of 8.

The electrons that are given up are the ones that are shown in a Lewis Dot Diagram.

Non-metals gain electrons from metals, becoming negativley charged anions, to achive this perfect value.

or

They share electrons with other non-metals to achieve this perfect value

The number of electrons needed to be gained/shared are the number of empty spots in a Lewis Dot Diagram.

Hydrogen is a special case.

Sometimes it reacts like a mtal and sometime like a non-metal.

For year 9 we are going to treat Hydrogen like a metal only.

When it acts like a metal its perfect number is ZERO, so it looses its 1 electron and becomes H+.

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Non-metals gain electrons from metals, bevoming negativly charged anions, to ahieve this perfect value.

Or

They chare electrons with other non-metals to achieve this perfect value

The number of electrons needed to be gained/shared are the number of empty spots in a Lewis Dot Diagram.

Hydrogen is a special case.

Sometimes it reacts like a metal and sometimes like a non-metal

For year 9 we are going to treat it like a metal only.

When it acts like a metal its perfeect number is ZERO, so it loses its 1 electron and becomes H+.

ions are charged particles

ions can be made of a single element (eg F-) or groups of elements (eg SO4 2-).

Ionic bonds are formed by 2 ions being attracted to each other by their opposit charges.

The total charge of the compound made by an ionic bond is ZERO.

When a reaction happens the charges are balanced so they equal zero.