Ch 3.2 Elements Mixtures and Compounds

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Chapter 3.2 Elements, Compounds and Mixtures Name: ______________________( ) Class: ______ Date: ____________

CHAPTER MAP & OVERVIEW

M. Heyworth Rex, & J G R Briggs. (2013). All About Chemistry 'O' Level. Malaysia: Pearson

Education South Asia Pte Ltd., pages 48 to 62

Learning Outcomes: Pupils are expected to: (a) recognise that materials can be classified as elements, compounds and mixtures (b) distinguish among elements, compounds and mixtures in terms of composition, properties, appearance

and ease of separation (c) differentiate atoms and molecules (d) identify examples of metals and non-metals from the periodic table For Elements: (a) identify an element as the basic building block of matter (b) recognise that elements are classified according to their properties (c) classify elements as metals and non-metals based on their characteristic properties (d) distinguish between metals and non-metals using physical properties such as electrical conductivity,

sonority, melting and boiling points, malleability, ductility and appearance (e) recall the symbol for first 20 elements (f) give examples of uses of common metals (Al, Zn, Fe, Cu, Mg) and non-metals (Cl, S, C, O, H, N, He,

Ne, I)

CHAPTER 3.2 ELEMENTS, COMPOUNDS AND MIXTURES

Molecules

Group together

Atoms Is the simplest component of

Elements

Mixture

Chapter 3.2.4 Compounds

Chemically Combined Not

Chemically Combined

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For Compounds: (a) describe compounds as substances consisting of two or more chemically combined elements (b) recognise that compounds have particular properties, different from the elements from which they are

produced. For Mixtures: (a) describe mixtures as two or more elements and/or compounds that are not chemically combined.

Elements Sodium chloride (table salt) is a white solid. If you melt it at a high temperature and then pass an electric current through the molten liquid, it breaks down (decomposes) into a silvery liquid and a yellow-green gas. The silvery liquid is molten sodium and the gas is chlorine. This decomposition reaction is summarised below: sodium chloride sodium + chlorine (silvery liquid) (yellow-green gas) No matter how hard you try, it is impossible to break down sodium or chlorine into simpler substances. Sodium and chlorine are examples of elements. An element is a substance that cannot be broken down into simpler substances by any known chemical methods. There are about 110 known elements. All substances are made up of these 110 elements. For example, water is made up of two elements called hydrogen and oxygen. Sugar is made up from the elements hydrogen, oxygen and carbon. The human body is made up of about 10 main elements and small amounts of many others. The element calcium is found in our bones and the element iron in our blood.

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Classifying Elements • Scientists such as Mendeleev have classified elements into a chart called the Periodic

Table. • The Periodic Table has vertical columns called groups and horizontal rows called periods. • Elements are grouped according to their properties. • Each element is represented by a unique symbol. • The symbol is based on either the element's English name or its name in another

language. o E.g. the symbol for the element iron is Fe - which comes from the Latin name for iron,

ferrum • Each element is also given a number (just like your index number) called the atomic

number. • Some examples of common elements and their symbols are given in the table below. Let’s think… Referring to a Periodic Table, what do you notice as we move across the period from left to right? We see a gradual change in properties from metallic to non-metallic. Elements are classified as metals or non-metals, depending on how well they conduct electricity. Metals are good conductors of electricity; non-metals are usually very poor electrical conductors. Most elements are metals; only 22 elements are non-metals. Metals can easily be recognised by their physical properties.

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Properties and Uses of Metallic Elements

Metals and their Symbols

Appearance Properties Uses

Copper (Cu)

Reddish brown solid

- Good conductor of electricity

- Ductile - Corrosion resistant - Strong

- Copper wires - Pipes

Zinc (Zn)

Grey solid - Good conductor of electricity

- Corrosion resistant - Strong

- Zinc is used inside batteries to produce electricity

- Zinc plating for coins

Aluminum (Al)

Silvery shiny solid

- Low density - Corrosion resistant - Strong and light - Malleable

- Cooking foils - Drink cans

Mercury (Hg)

Silvery liquid - Does not react easily with other chemicals

- Mercury vapour is fluorescent (gives out light)

- Good conductor of heat - Expands evenly on heating

- Used in thermometers - Used to measure blood

pressure

Magnesium (Mg)

Grey solid - Burns with a dazzling white light

- Forms alkali to neutralize excess acid in stomach

- Used to make milk of magnesia which is used to relieve acid indigestion

Iron (Fe)

Grey solid - Good conductor of heat and electricity

- Ductile - Strong - Magnetic

- Used to make cutlery.

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Properties and Uses of Non-Metallic Elements Non-Metals and their symbols

Appearance Properties Uses

Iodine (I)

Black Crystals - Poisonous - Antiseptic - (prevents wound

infection)

- Used as antiseptic in medicine

Chlorine (Cl)

Greenish yellow gas

- Poisonous - Bleaches dyes

- Used for bleaches for clothes

Sulphur (S)

Yellow powdery solid

- Poisonous - Soluble in organic

solvent - (e.g. alcohol) but not in

water

- Used to make rubber tyres harder through a process called “vulcanization”

Diamond (Carbon, C)

Hardest substance known to man

- Can be polished to form a shiny, reflecting, transparent solid

- Used as diamond drills to cut through very hard metal

Graphite (Carbon, C)

Black solid - Smooth - Lightweight - Good conductor of

electricity

- Used to make rackets

Hydrogen (H)

Colourless gas - Gas with the lowest density

- Explosive

- Used to fill weather balloons and blimps

Nitrogen (N) Colourless gas - Non-reactive, does not burn or support combustion

- Low boiling point

- Used to make fertiliser for farm

Oxygen (O)

Colourless gas - Essential for life - Does not burn but

support combustion

- Used in the flames for welding

Neon (Ne)

Colourless and odorless

- Exist as a single atom - Noble gas

Used in making neon advertising signs

Helium (He)

Colourless and odorless

- Exist as a single atom - Noble gas

- airships and balloons can be filled with the gas to gain lift. - Helium is used to condense hydrogen and oxygen to make rocket fuel.

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Atoms We know that matter is made up of tiny particles. These particles are called atoms. An atom is the smallest particle of any element Atoms are very small. Each atom has a diameter of about 0.1 nanometre or 0.1 X 10 -9 metre. It would take about 1 000 000 000 000 000 or 1015 atoms to completely cover the head of a pin! We cannot see an atom with our naked eye. But if we look through an electron microscope, an instrument that can magnify objects millions of times, we might be able to see a crude picture of atoms as shown.

The bumps are individual atoms on the surface of a crystal of silicon. This is the best picture of atoms taken so far. A special electron microscope was use.

Each element consists of a particular type of atom. The atoms of each element are different from the atoms of any other element. For example, all hydrogen atoms are the same (although there can be slight differences between them – e.g. isotopes) and are different from atoms of carbon or iron. Molecules

A molecule is a group of two or more atoms held together by chemical bonds. A molecule may consist of atoms of a single element, as with oxygen (O2), or of different elements, as with water (H2O).

Model of an oxygen (O2) molecule. This molecule consists of 2 atoms of a single element (2 oxygen atoms)

Model of a water (H2O) molecule. This molecule consists of 3 atoms of 2 different elements (1 oxygen atom and 2 hydrogen atoms)

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Physical and Chemical Changes In a physical change, no new substance is formed. These changes are easily reversed. An example is the dissolving of salt in water. The solution still consists of salt and water but does not contain any new substance. The change can "easily be reversed by evaporating the water to crystallise the salt. Other examples of physical changes include melting, boiling, mixing two solids or two liquids, and dissolving solids in solvents such as petrol and alcohol. In a chemical change, a new substance is formed. The new substance has different properties such as different melting points and different chemical reactions. You can often recognise a chemical change from two observations:

1. The new substance has a different appearance, such as a different colour or a different physical state. For example, the rusting of iron is a chemical change. The iron is a shiny grey colour but the rust produced is a brown colour.

2. A lot of heat is often given out in a chemical change. For example, the burning of a candle or cylinder gas in air is a chemical change. These chemical changes produce heat. The heat from such changes is used to provide us with energy.

Chemists call chemical changes chemical reactions. Compounds A compound is a substance containing two or more elements joined together by a chemical reaction. A compound has very different properties from its elements. Magnesium oxide is a compound made of two elements: magnesium and oxygen. It is made by burning magnesium in air.

The heat and light show that a chemical reaction took place. The elements are a silvery solid and a colourless gas. The compound has different properties: it is a white solid. Water is a compound made up of two elements: hydrogen and oxygen. It can be made by applying a lighted splint to a mixture of hydrogen and oxygen.

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The heat and light produced by the lighted splint show that a chemical reaction took place. The element hydrogen is a flammable gas. The compound water has very different properties: it is a liquid and does not burn. The figure below shows the changes in the particles when water is formed from its elements. Each `circle' represents one atom. So the particles of hydrogen, for example, consist of two atoms joined together. In the mixture of hydrogen and oxygen, before reaction, the hydrogen and oxygen atoms are not joined together. When the reaction takes place, the hydrogen and oxygen atoms join together to form the particles of water. Each particle of water consists of two hydrogen atoms and one oxygen atom chemically joined together.

The atoms of oxygen and hydrogen are tightly joined together in water

A particular compound always contains the same elements. These elements are always present in a fixed proportion in the compound. For example, the percentage composition by mass of hydrogen and oxygen in water is always: 11% hydrogen, 89% oxygen. Water can be obtained from rain, rivers, the sea or a tap. It can be in the solid, liquid or gaseous state. In every case, it has the same percentage composition by mass of hydrogen and oxygen.

When the elements combine together to form a compound, this compound cannot be separated again into its elements by physical means - such as filtering, distillation or dissolving in water. The atoms are firmly joined together. The atoms of different elements in a compound can only be broken apart in chemical reactions.

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Mixtures In a mixture of elements, the atoms of the elements are not chemically combined The percentage of each element in a mixture is not always the same. A mixture does not have a fixed composition. A mixture of oxygen and hydrogen, for example, can have any composition of each element. Brass is a mixture of two metals, copper and zinc. The percentage of zinc in most brass samples varies between 10% and 60%. Each element in a mixture retains its properties. A mixture can easily be separated by physical means such as filtration, crystallisation or distillation.

A mixture can also be made up of different compounds, like the black and white rock crystals in granite. A `picture' of a mixture of two compounds is shown on the left. A good way of distinguishing a single compound from a mixture of compounds, is from its melting point and its boiling point. A single compound has a fixed melting point and (at sea level) a fixed boiling point. For example, pure water (a single compound) melts at 0°C and boils at 100°C. Butter (a mixture of compounds) melts over the range 30°C to 50°C. This means butter starts melting at 30°C and is completely melted at 50°C. Kerosene fuel used for jet aircraft

(also a mixture of compounds) boils over the range of 170°C to 250°C. Many common substances are mixtures - including air, sea water, concrete and cooking oil. Question: Why do you think that most metals are used as mixtures? Because they are stronger than pure metals. That is why brass is used for the pins of electric plugs instead of pure copper. Such mixtures of metals are called alloys.

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Read this handout and summarise the characteristics of compounds and mixtures in the table below: