Elements, Compounds, Symbols and Formulae
Learn pure substances, metals, non-metals, atoms, molecules, atomicity, chemical symbols, formulae and Periodic Table basics through complete solutions, answer reveals and an instant quiz.
What you will learn
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Practice section
Exercise I — Objective Type Questions
All pure substances have:
the same physical state
the same colour
the same composition
a definite set of properties
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a definite set of properties
Reason: All pure substances have a definite set of properties. These properties include physical state, colour and composition, but they are not the same for all substances.
Pure substances can exist in different states - solid, liquid or gas. Different pure substances have different colours. Each pure substance has a fixed composition, but it is not the same as that of other substances.
Sugar is a compound which consists of the elements:
carbon and hydrogen
hydrogen and oxygen
carbon, hydrogen and oxygen
hydrogen, carbon and sulphur
✓ Show answer and explanation
carbon, hydrogen and oxygen
Reason: Sugar has the chemical formula C12H22O11, which signifies that it is made up of carbon (C), hydrogen (H) and oxygen (O).
Carbon and hydrogen describe compounds like hydrocarbons, not sugar. Hydrogen and oxygen form water. Sugar doesn't contain sulphur.
Sulphur and carbon are:
metals
non-metals
metalloids
noble gases
✓ Show answer and explanation
non-metals
Reason: Sulphur and carbon are non-metals with dull surfaces that do not shine.
Sulphur and carbon do not show metallic properties like conductivity or malleability. Metalloids have properties of both metals and non-metals (e.g., silicon). Noble gases (like helium, neon) are inert gases, unlike sulphur and carbon.
The most abundant elements in the universe are:
neon and argon
hydrogen and helium
aluminium and copper
oxygen and nitrogen
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hydrogen and helium
Reason: The most abundant elements in the universe are hydrogen and helium.
Neon and argon are noble gases, but not the most abundant in the universe. Aluminium and copper are metals found mainly on Earth, not abundant in the universe. Oxygen and nitrogen are abundant in Earth’s atmosphere, not in the entire universe.
The symbol of which of the following elements is derived from its Latin name ?
Cobalt
Copper
Calcium
Chlorine
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Copper
Reason: Among the following elements the symbol of copper i.e., Cu is derived from its Latin name "Cuprum".
The metal which is not a good conductor of electricity is:
silver
aluminium
copper
tungsten
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tungsten
Reason: Among the given choices, tungsten is a comparatively poorer conductor of electricity than silver, copper and aluminium. It still conducts electricity, which is why it can be used as a filament material, but its conductivity is much lower than that of the other listed metals.
The symbol of sodium metal is:
S
Sn
Na
Ne
✓ Show answer and explanation
Na
Reason: The symbol of sodium metal is Na, derived from its Latin name "Natrium".
S is sulphur.
Sn is tin.
Ne is neon.
From the list given below, select the correct substance which is most suitable to the given statements:
[oxygen, diamond, zinc, graphite, gold]
(a) A metal which is brittle.
(b) A non-metal which is a good conductor of electricity.
(c) The hardest naturally occurring substance.
(d) The most ductile metal.
(e) A gaseous non-metal.
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(a) Zinc
(b) Graphite
(c) Diamond
(d) Gold
(e) Oxygen
Classify the following substances into elements and compounds.
Mercury, sulphur, sugar, water, sand, gold, carbon, oxygen, alcohol, iron, marble, baking soda.
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Elements — Mercury, sulphur, gold, carbon, oxygen, iron
Compounds — Sugar, water, alcohol, marble, baking soda
Important note: Pure silica (silicon dioxide, SiO2) is a compound. Natural sand usually contains silica along with other minerals, so natural sand is generally a mixture.
Give the symbols of: carbon, calcium, copper, chlorine, cobalt, argon, silicon, aluminium.
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Carbon — C
Calcium — Ca
Copper — Cu
Chlorine — Cl
Cobalt — Co
Argon — Ar
Silicon — Si
Aluminium — Al
Give the names of the elements represented by the following symbols:
(a) B
(b) He
(c) Si
(d) Be
(e) Li
(f) P
(g) K
(h) F
✓ Show answer and explanation
(a) B — Boron
(b) He — Helium
(c) Si — Silicon
(d) Be — Beryllium
(e) Li — Lithium
(f) P — Phosphorus
(g) K — Potassium
(h) F — Fluorine
Practice section
Exercise I — Short Answer Type Questions
Define a pure substance.
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A substance which has a definite composition and consistent properties throughout is called a pure substance.
Name the types of pure substances you know.
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Pure substances are of two types,
Elements
Compounds
Define:
(a) Elements
(b) Compounds.
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(a) An element is defined as a pure substance made up of only one kind of atom that cannot be converted into anything simpler than itself by any physical or chemical process.
(b) Compounds are pure substances formed by the chemical combination of two or more elements in a definite proportion by mass.
Give two examples for each of the following:
(a) Non-metals which are solids
(b) Metals which are soft
(c) Non-metals which are lustrous
(d) Elements which are liquids
(e) Inert gases
✓ Show answer and explanation
(a) Non-metals which are solids:
Sulphur (S)
Phosphorus (P)
(b) Metals which are soft:
Sodium (Na)
Potassium (K)
(c) Non-metals which are lustrous:
Iodine (I)
Carbon (C) in the form of diamond
(d) Elements which are liquids:
Mercury (Hg) — Metal
Bromine (Br2) — Non metal
(e) Inert gases:
Helium (He)
Neon (Ne)
Name the elements present in the following compounds.
(a) Sugar
(b) Ammonia
(c) Marble
(d) Washing soda
✓ Show answer and explanation
(a) Sugar — Carbon, hydrogen and oxygen
(b) Ammonia — Nitrogen and hydrogen
(c) Marble — Calcium, carbon and oxygen
(d) Washing soda (Na2CO3·10H2O) — Sodium, carbon, oxygen and hydrogen
Practice section
Exercise I — Long Answer Type Questions
Give three differences between metals and non-metals
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The differences between metals and non-metals are given below:
| Metals | Non-metals |
|---|---|
| Metals are generally lustrous. | Non-metals are generally non-lustrous. |
| Metals generally have high melting and boiling points. | Non-metals generally have low melting and boiling points. |
| Metals are ductile. | Non-metals are generally not ductile. |
| Metals generally have high density. | Non-metals generally have low density. |
Give three differences between elements and compounds.
✓ Show answer and explanation
The differences between elements and compounds are given below:
| Elements | Compounds |
|---|---|
| An element is made up of only one kind of atom. | A compound is made up of atoms of two or more different elements chemically combined in a fixed ratio. |
| An element has its own characteristic properties. | The properties of a compound are different from those of its constituent elements. |
| An element cannot be broken down into simpler substances by ordinary physical or chemical means. | A compound can be broken down into its constituent elements by chemical means. |
Name the elements which form water. State three characteristics of water to justify that it is a compound.
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Hydrogen and Oxygen combine to form water.
Three characteristics of water are :
Water consists of two elements, hydrogen and oxygen.
In the formation of water, hydrogen and oxygen always combine in a fixed proportion i.e., two hydrogen atoms and one oxygen atom.
The properties of water are entirely different from those of its constituent elements. Hydrogen and oxygen are gases but water is a liquid under normal conditions.
State four important characteristics of compounds.
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A compound consists of two or more elements.
The constituting elements of a compound are always same, combined in a definite proportion in every sample of that compound, i.e. from whichever source it is obtained.
A compound is pure and homogeneous and has the same properties throughout its bulk.
The physical and chemical properties of a compound differ from those of its constituent elements.
How is sodium chloride different from its constituent elements, sodium and chlorine in its properties? Justify.
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The compound sodium chloride is made up of sodium and chlorine. Sodium is a metal that is stored in kerosene oil as it reacts very fast with air and water. Chlorine is a reactive greenish yellow gas which is poisonous.
When these two elements combine chemically they form common salt, a non poisonous white solid substance that we use in our food to add taste and to obtain nutrition.
(a) What are the four types of elements ? Name them.
(b) Give two examples of each type of the elements you have mentioned above.
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(a) Four types of elements are :
Metals
Non-metals
Metalloids
Noble (Inert) gases
(b) Examples :
Metals — Zinc, Lead
Non-metals — Carbon, Hydrogen
Metalloids — Boron, Silicon
Noble (Inert) gases — Neon, Argon
Practice section
Exercise II — Objective Type Questions
Atoms of different kinds combine to form molecules of:
an element
a compound
a mixture
all of the above
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a compound
Reason: When atoms of two or more different elements combine, they form a molecule of a compound. Whereas, molecule of element forms when two or more atoms of the same element combine.
Molecules of an element are made of same kind of atoms. In mixtures, substances are not chemically combined.
The compound used as common salt is:
sodium chloride
calcium chloride
sodium oxide
hydrogen chloride
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sodium chloride
Reason: The compound used as common salt is sodium chloride made up of sodium and chlorine.
Sand is a compound of:
silicon and nitrogen
silicon and oxygen
oxygen and sulphur
none of the above
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silicon and oxygen
Reason: The main constituent of most sand is silica, or silicon dioxide (SiO2), which contains silicon and oxygen in an atomic ratio of 1 : 2. Natural sand may also contain other minerals, so it is usually a mixture rather than a single pure compound.
The atomicity of ozone is:
2
3
4
8
✓ Show answer and explanation
3
Reason: The number of atoms in a molecule of an element is called its atomicity. Ozone (O3) is made of three oxygen atoms, hence the atomicity of ozone is three.
The molecular formula of ammonia is:
NO
NH3
NO2
N2O
✓ Show answer and explanation
NH3
Reason: Ammonia contains nitrogen and hydrogen. Nitrogen has valency 3 and hydrogen has valency 1. Using the criss-cross method:
Therefore, the molecular formula of ammonia is NH3.
The combining power of magnesium and oxygen is the same, i.e. 2. What will be the molecular formula of magnesium oxide ?
Mg2O2
MgO2
Mg2O
MgO
✓ Show answer and explanation
MgO
Reason: Magnesium and oxygen both have valency 2. The criss-cross method first gives Mg2O2. Dividing both subscripts by their common factor 2 gives the simplest formula, MgO.
Practice section
Exercise II — Short Answer Type Questions
Define:
(a) Atom
(b) Molecule
(c) Atomicity
(d) Formula
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(a) An atom can be defined as the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have independent existence.
(b) A molecule can be defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has independent existence. They are divisible into atoms.
(c) The number of atoms in a molecule of an element is called its atomicity.
(d) A formula (plural: formulae) is a short way of representing the molecule of an element or a compound. It indicates the types and numbers of atoms present in a molecule.
State the difference between the following:
(a) 2H and H2
(b) H2O and 3H2O?
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(a) 2H represents two atoms of hydrogen while H2 represents one molecule of hydrogen.
(b) H2O indicates that water molecule is formed by the combination of two elements i.e., Hydrogen and Oxygen. The number of hydrogen atom is two and that of oxygen atom is one.
If we write 3H2O it represents three molecules of water. It contains six atoms of hydrogen and three atoms of oxygen.
Write the molecular formulae of compounds calcium oxide, hydrogen sulphide, lead sulphide, zinc oxide and carbon dioxide.
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Calcium oxide — CaO
Hydrogen sulphide — H2S
Lead sulphide — PbS
Zinc oxide — ZnO
Carbon dioxide — CO2
Give two examples each of compounds existing in the following states:
(a) Solid
(b) Liquid
(c) Gaseous
✓ Show answer and explanation
(a) Solid:
Common salt (sodium chloride, NaCl)
Sodium hydroxide (NaOH)
(b) Liquid:
Water (H2O)
Hydrochloric acid solution (HCl in water)
(c) Gaseous:
Carbon dioxide (CO2)
Ammonia (NH3)
State two conditions necessary to write the molecular formula of a compound.
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Two conditions necessary to write the molecular formula of a compound are :
The symbols of elements.
The valency of that element i.e. its combining power.
Practice section
Exercise II — Long Answer Type Questions
State the number of atoms of each element, present in
(a) C6H12O6
(b) H2SO4
(c) HNO3
(d) CaCO3
Also name these compounds.
✓ Show answer and explanation
(a) C6H12O6 — Glucose
Carbon (C): 6 atoms
Hydrogen (H): 12 atoms
Oxygen (O): 6 atoms
(b) H2SO4 — Sulphuric acid
Hydrogen (H): 2 atoms
Sulphur (S): 1 atom
Oxygen (O): 4 atoms
(c) HNO3 — Nitric acid
Hydrogen (H): 1 atom
Nitrogen (N): 1 atom
Oxygen (O): 3 atoms
(d) CaCO3 — Calcium carbonate
Calcium (Ca): 1 atom
Carbon (C): 1 atom
Oxygen (O): 3 atoms
Why are symbols and formulae of substances important?
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The symbols and formulae of substances give information about the elements present in the compound and the number of each kind of atom in one molecule of the compound. They also allow us to represent chemical reactions using equations.
Mention three gaseous elements and write their molecular formulae.
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Three gaseous elements along with their molecular formulae are given below:
Hydrogen – H2
Oxygen – O2
Nitrogen – N2
State the information obtained from the formula of a compound.
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A formula gives us the following information about a compound.
Types of elements present in the compound.
Number of each kind of atoms in one molecule of the compound.
Ratio of different types of atoms present in the molecules.
Practice section
Exercise III — Objective Type Questions
Gold is used to make jewellery because it is:
dull
lustrous and attractive
highly reactive
very cheap
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lustrous and attractive
Reason: Gold is used to make jewellery because it is lustrous and attractive. It looks shiny and does not tarnish easily in air.
Brass and bronze are:
elements
mixtures
compounds
all of the above
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mixtures
Reason: Brass is a mixture of copper and zinc and bronze is a mixture of copper and tin.
Brass and bronze are not single elements; they are made of more than one metal. So, they can't be elements. Compounds have a fixed chemical formula, but alloys do not.
The hardest naturally occurring substance is:
graphite
iron
diamond
coal
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diamond
Reason: Diamond is a form of carbon, it is hardest naturally occuring substance. Hence, it is used to cut glass.
Graphite is soft and slippery (used in pencils). Iron is strong, but not the hardest natural substance. Coal is soft and brittle.
The inert gases commonly used in electric bulbs are:
Helium
Argon
Neon
Both argon and neon
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Both argon and neon
Reason: Noble gases such as argon and neon are chemically inert. They do not readily react with the hot tungsten filament, which helps protect the filament. Argon is the gas most commonly used in ordinary incandescent bulbs.
Which of the following metals is used to make heavy tools and machines?
Iron
Aluminium
Copper
Silver
✓ Show answer and explanation
Iron
Reason: Iron is strong and easily available. Therefore, it is used to make heavy tools and machines.
Aluminium is light and used in aircraft, utensils, and packaging, not heavy machines. Copper is used mainly for electrical wiring due to good conductivity. Silver is expensive and used in jewellery and electronics, not heavy machinery.
Practice section
Exercise III — Assertion Reason Type Questions
Assertion: Elements and compounds are pure substances.
Reason: Elements and compounds are made up of only one kind of atom and molecules respectively.
Both A and R are true and R is the correct explanation of A.
Both A and R are true and R is not the correct explanation of A.
A is true and R is false.
A is false and R is true.
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Both A and R are true and R is the correct explanation of A.
Reason: A is true as pure substances have a fixed composition and definite properties. Both elements and compounds fit this definition.
R is true as an element contains only one type of atom, and a compound contains only one type of molecule.
Therefore, Both A and R are true and R is the correct explanation of A is the correct option.
Assertion: The chemical formula of a compound gives the exact number of atoms of different types present in it.
Reason: Carbon and oxygen combine chemically to form a molecule of carbon dioxide.
Both A and R are true and R is the correct explanation of A.
Both A and R are true and R is not the correct explanation of A.
A is true and R is false.
A is false and R is true.
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Both A and R are true and R is not the correct explanation of A.
Reason: A is true as a chemical formula is like a symbolic representation of a molecule. The formula directly gives the exact number of each type of atom.
R is true as carbon and oxygen elements combine to form a molecule of carbon dioxide.
Therefore, Both A and R are true and R is not the correct explanation of A is the correct option.
Assertion: Graphite is used to make the lead of pencils.
Reason: Graphite can mark paper black.
Both A and R are true and R is the correct explanation of A.
Both A and R are true and R is not the correct explanation of A.
A is true and R is false.
A is false and R is true.
✓ Show answer and explanation
Both A and R are true and R is the correct explanation of A.
Reason: A is true as Pencil “lead” is actually made of graphite, not lead metal.
R is true as Graphite is soft and slippery, so it leaves a black mark on paper when rubbed.
Therefore, Both A and R are true and R is the correct explanation of A is the correct option.
Assertion: Metals have low melting and boiling points.
Reason: Most non-metals are dull in appearance and brittle in nature.
Both A and R are true and R is the correct explanation of A.
Both A and R are true and R is not the correct explanation of A.
A is true and R is false.
A is false and R is true.
✓ Show answer and explanation
A is false and R is true.
Reason: A is false as most metals actually have high melting and boiling points.
R is true as non-metals are generally dull (not shiny) and brittle (break easily).
Therefore, A is false and R is true is the correct option.
Practice section
Exercise III — Fill in the blanks
Fill in the blanks:
(a) ............... refers to the number of atoms in the molecule of an element.
(b) The most abundant element in the Earth's crust is ............... .
(c) A metal which is a liquid at room temperature is ............... .
(d) The most abundant element in the atmosphere is ............... .
(e) A metal which is a poor conductor of electricity is ............... .
(f) A diatomic gaseous element is ............... .
(g) A liquid non-metal is ............... .
(h) Elements that do not react chemically are ............... .
(i) The Latin name of iron is ............... .
(j) The horizontal rows in the Periodic Table are called ............... .
(k) The vertical columns in the Periodic Table are called ............... .
✓ Show answer and explanation
(a) Atomicity refers to the number of atoms in the molecule of an element.
(b) The most abundant element in the Earth's crust is oxygen.
(c) A metal which is a liquid at room temperature is mercury.
(d) The most abundant element in the atmosphere is nitrogen.
(e) A metal which is a poor conductor of electricity is lead.
(f) A diatomic gaseous element is oxygen(O2).
(g) A liquid non-metal is bromine.
(h) Elements that do not react chemically are noble gases.
(i) The Latin name of iron is ferrum.
(j) The horizontal rows in the Periodic Table are called periods.
(k) The vertical columns in the Periodic Table are called groups.
Practice section
Exercise III — State True or False
Indicate whether the following statements are true or false.
(a) A compound is made up of just one kind of atom. ...............
(b) Metals reflect light and are good conductors of electricity. ...............
(c) Metals can be polished. ...............
(d) Elements are made up of compounds. ...............
(e) All elements are artificially prepared. ...............
(f) Molecules can exist independently. ...............
(g) Molecules combine to form atoms. ...............
(h) Noble gases are highly reactive. ...............
(i) Ozone is a triatomic molecule. ...............
✓ Show answer and explanation
(a) False
Correct Statement — A compound is made of two or more different kinds of atoms.
(b) True
(c) True
(d) False
Correct Statement — Compounds are made up of elements.
(e) False
Correct Statement — Most elements occur naturally, only a few are synthetic.
(f) True
(g) False
Correct Statement — Atoms combine to form molecules.
(h) False
Correct Statement — Noble gases are inert, i.e., non-reactive under normal conditions.
(i) True
Practice section
Exercise III — Match the Following
Match the columns:
| Column A | Column B |
|---|---|
| (a) Metals | (i) Non-reactive |
| (b) Molecules | (ii) Brittle |
| (c) Non-metals | (iii) Lustrous |
| (d) Noble gases | (iv) Smallest unit of compound |
✓ Show answer and explanation
| Column A | Column B |
|---|---|
| (a) Metals | (iii) Lustrous |
| (b) Molecules | (iv) Smallest unit of compound |
| (c) Non-metals | (ii) Brittle |
| (d) Noble gases | (i) Non-reactive |
Practice section
Exercise III — Select the Odd One
Circle the odd one out on the basis of the property given in the brackets.
(a) hydrogen, nitrogen, chlorine, phosphorus (state)
(b) sodium, potassium, iron, lithium (hardness)
(c) graphite, diamond, iodine, coal (forms of a same element)
(d) helium, neon, argon, bromine (reactivity)
✓ Show answer and explanation
(a) Phosphorus
Reason: Phosphorus is present in solid state under normal conditions, whereas, hydrogen, nitrogen, and chlorine are gases at room temperature.
(b) Iron
Reason: Iron is hard metal, while, sodium, potassium and lithium are soft metals.
(c) Iodine
Reason: Coal, diamond and graphite are all different forms of carbon but iodine is a different element.
(d) Bromine
Reason: Bromine is a reactive element. while helium, neon and argon are non-reactive noble gases.
Practice section
Exercise III — Short Answer Type Questions
Name:
(a) Three different forms of carbon.
(b) A form of carbon used as a gem.
(c) Two substances used to make electric wires.
(d) Two substances used to make jewellery.
(e) A substance used as an insulator.
✓ Show answer and explanation
(a) Coal, diamond and graphite.
(b) Diamond
(c) Copper and aluminium
(d) Gold and platinum
(e) Wood or Plastic
Give one use of each of the following substances:
(a) Iron
(b) Brass
(c) Coal
✓ Show answer and explanation
(a) Iron is used to make heavy tools and machines.
(b) Brass is used to make utensils, door knobs, handles, etc.
(c) Coal is used as fuel.
Practice section
Exercise III — Long Answer Type Questions
Give reason:
A frying pan is made up of steel but its handle is made up of wood.
✓ Show answer and explanation
A frying pan is made up of steel but its handle is made up of wood or plastic. This is because steel is a good conductor of heat and it allows heat to pass to the food kept in the pan for cooking while wood and plastic being bad conductors or insulators do not get too hot to burn our hand.
Give reason:
Graphite is used to make lead of the pencils.
✓ Show answer and explanation
Graphite can mark the paper black. So, it is used to make the lead of pencils.
Give reason:
Argon is filled in electric bulbs.
✓ Show answer and explanation
Argon is filled in electric bulbs due to its inert nature. It does not react with the tungsten filament of the bulb and prevents it from destruction.
Give reason:
Copper and aluminium are used to make electric wires.
✓ Show answer and explanation
Copper and aluminium are good conductors of heat and electricity. They can be drawn into wires and beaten into sheets. Therefore, they are used to make electric wires.
(a) What do you understand by 'Periodic Table'?
(b) A part of the Periodic Table is given below with the symbols of some elements. Write the symbols of the missing elements.
Group →
Period ↓ IA IIA IIIA IVA VA VIA VIIA Zero
1 H He
2 Li B N F
3 Mg Si S Ar
4 K Ca x x x x x x
✓ Show answer and explanation
(a) Periodic Table is a systematic arrangement of elements in a tabular form arranged into rows ("periods") and columns ("groups"), for our convenient study.
(b)
Group →
Period ↓ IA IIA IIIA IVA VA VIA VIIA Zero
1 H He
2 Li Be B C N O F Ne
3 Na Mg Al Si P S Cl Ar
4 K Ca x x x x x x
What do you understand by the statement — 'Metals are ductile and malleable'? Give one example of each.
✓ Show answer and explanation
Metals are ductile, which means they can be drawn or stretched into thin wires. Metals are malleable, which means they can be beaten into thin sheets.
Example of a ductile metal: Copper is highly ductile and is commonly used in electrical wiring.
Example of a malleable metal: Aluminium is malleable and is used to make foil and packaging materials.
Practice section
Case Study Type Questions
An electrician was repairing an electric wire in John's house. John noticed that the wire was coated with plastic, but a small portion of it was exposed. Since John had studied the properties and uses of elements and compounds in class, he could easily recognize that the wire was made of copper.
(a) What type of element is copper?
(b) Why is copper used for making electrical wires?
(c) Although silver is more ductile and a better conductor of electricity than copper, it is not used to make wires. Explain why.
(d) Why is it necessary to cover metal wires with plastic?
(e) What type of substance is plastic?
✓ Show answer and explanation
(a) Copper is a metal.
(b) Copper is used for making electrical wires because it is a good conductor of electricity and is ductile, so it can be drawn into thin wires.
(c) Silver is a better conductor and is more ductile than copper, but it is not normally used for household wiring because it is much more expensive.
(d) Metal wires are covered with plastic because plastic is a poor conductor of electricity. It acts as an insulator and helps protect us from electric shock.
(e) Plastic is a non-metallic insulating material. Most plastics are made of polymers and may also contain additives.
Interactive revision quiz
Choose one answer for each question, then check your score.
1. Which pair is most abundant in the universe?
2. What is the symbol of sodium?
3. What is the atomicity of ozone?
4. Which formula represents ammonia?
5. Which substance is the hardest naturally occurring material?
6. Which one is a noble gas?
7. How many hydrogen atoms are present in 3H2O?
8. What is the simplest formula of magnesium oxide?
Frequently asked questions
What is a pure substance?
A pure substance has a fixed composition and definite properties throughout. Elements and compounds are pure substances.
What is the difference between an element and a compound?
An element contains only one kind of atom, while a compound contains atoms of two or more different elements chemically combined in a fixed ratio.
What information does a chemical formula provide?
A chemical formula shows the elements present and the number or ratio of atoms of each element in one molecule or formula unit.
What is atomicity?
Atomicity is the number of atoms present in one molecule of an element. Ozone, O3, has an atomicity of three.
Why are copper and aluminium used for electrical wires?
Copper and aluminium conduct electricity well and are ductile, so they can be drawn into thin wires.
Why is argon used in electric bulbs?
Argon is chemically inert and does not readily react with the hot tungsten filament, helping to protect it.
Is natural sand a compound?
Pure silica, SiO2, is a compound, but natural sand usually contains silica and other minerals, so natural sand is generally a mixture.
How is 2H different from H2?
2H represents two separate hydrogen atoms, while H2 represents one hydrogen molecule containing two bonded hydrogen atoms.
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