Class 9 Science · Original EZT practice
Atomic Foundations of Matter (Class 9 Science Chapter 9): summary and practice
Why do elements combine in fixed ratios, and how do we write what they form? This chapter lays chemistry's foundations: the law of conservation of mass, the law of constant proportions, Dalton's atomic theory, covalent and ionic bonding, and the rules for writing chemical formulae. All questions are original EZT tutor practice following the chapter's own section order.
6 original worked examples, 6 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 22 October 2026:
1. Laws of chemical combination
Matter is neither created nor destroyed in a reaction (the law of conservation of mass), and a pure compound always contains the same elements in the same proportion by mass (the law of constant proportions).
2. Dalton's atomic theory
Dalton proposed that matter is made of indivisible atoms, that atoms of an element are identical in mass and properties, and that compounds form when atoms combine in simple whole-number ratios.
3. How atoms combine
Atoms bond in two main ways - by sharing electrons (a covalent bond, as in H2O) or by transferring electrons so the resulting ions attract (an ionic bond, as in NaCl).
4. Writing chemical formulae
Write the symbols with their valencies, criss-cross the valencies as subscripts, drop a valency of 1, and divide the subscripts by any common factor so the formula shows the simplest ratio - as in H2S and MgCl2.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Apply conservation of mass
Choose the method: Total mass in equals total mass out.
EZT practice question
10 g of calcium carbonate gives 5.6 g of calcium oxide and some carbon dioxide on heating. Find the mass of carbon dioxide.
Show the worked solution
- Mass is neither created nor destroyed in a reaction.
- So products together weigh what the reactant weighed: 10 g.
- Carbon dioxide = 10 - 5.6 = 4.4 g.
Answer: 4.4 g of carbon dioxide.
Check: 5.6 g + 4.4 g = 10 g - the books balance, as the law demands.
Watch for this: Do not subtract the other way round or guess - set reactant mass equal to total product mass.
Now try it yourself
3 g of hydrogen combines with 24 g of oxygen to form water. What mass of water forms?
Check your answer
27 g.
3 + 24 = 27 g - mass is conserved.
Identify constant proportions
Choose the method: Same compound, same mass ratio - whatever the source.
EZT practice question
Water from a river and water from a lab both contain hydrogen and oxygen in the ratio 1:8 by mass. Which law does this show?
Show the worked solution
- Both samples are the same pure compound - water.
- Each shows hydrogen and oxygen in the same proportion by mass (1:8), whatever the source.
- That is exactly the law of constant proportions.
Answer: The law of constant proportions.
Check: Conservation of mass is about totals before and after a reaction - this example compares composition, so constant proportions is the right law.
Watch for this: Do not answer conservation of mass - no reaction is being weighed here, only compositions compared.
Now try it yourself
State the law of constant proportions.
Check your answer
A pure compound always contains the same elements combined in the same proportion by mass.
The source of the sample does not matter.
Recall Dalton's theory
Choose the method: Indivisible atoms, identical per element, whole-number ratios.
EZT practice question
State any two postulates of Dalton's atomic theory.
Show the worked solution
- Dalton rebuilt chemistry on atoms.
- Postulate one: matter consists of indivisible atoms.
- Postulate two: atoms of a given element are identical in mass and properties - another acceptable one: atoms combine in simple whole-number ratios to form compounds.
Answer: Matter consists of indivisible atoms; atoms of a given element are identical in mass and properties (also: atoms combine in simple whole-number ratios).
Check: The whole-number ratio postulate is what explains the fixed proportions in compounds like water.
Watch for this: Do not call atoms divisible in Dalton's theory - later science split the atom, but Dalton's model treats them as indivisible.
Now try it yourself
In Dalton's theory, how do compounds form?
Check your answer
Atoms combine in simple whole-number ratios.
That is why compounds show fixed proportions by mass.
Classify the bond
Choose the method: Sharing is covalent; transferring is ionic.
EZT practice question
Classify the bonding in (a) H2O and (b) NaCl.
Show the worked solution
- In water, hydrogen and oxygen hold together by sharing electrons.
- In sodium chloride, sodium transfers an electron to chlorine.
- The resulting positive and negative ions attract - an ionic bond.
Answer: (a) covalent - hydrogen and oxygen share electrons; (b) ionic - sodium transfers an electron to chlorine, and the ions attract.
Check: The quick test: sharing means covalent, transfer plus attraction of ions means ionic.
Watch for this: Do not call NaCl covalent - no electron pair is shared; an electron changes hands completely.
Now try it yourself
Is the bond in Cl2 covalent or ionic? Why?
Check your answer
Covalent - the two identical chlorine atoms share electrons.
Neither atom can take an electron from an identical partner.
Criss-cross a formula
Choose the method: Swap the valencies down as subscripts; drop any 1.
EZT practice question
Write the formula of the compound of hydrogen (valency 1) and sulphur (valency 2).
Show the worked solution
- Write the symbols with their valencies: H with 1, S with 2.
- Criss-cross: hydrogen takes the 2, sulphur takes the 1.
- A subscript of 1 is never written - the formula is H2S.
Answer: H2S.
Check: Two hydrogen atoms (valency 1 each) balance one sulphur (valency 2) - the valencies match.
Watch for this: Do not write HS2 - the criss-cross sends each valency to the other element.
Now try it yourself
Write the formula of the compound of magnesium (valency 2) and chlorine (valency 1).
Check your answer
MgCl2.
Magnesium's 2 goes to chlorine; chlorine's 1 is dropped.
Reduce to the simplest ratio
Choose the method: Divide subscripts by any common factor.
EZT practice question
A criss-cross gives subscripts 2 and 4. What is the correct formula?
Show the worked solution
- A formula shows the simplest whole-number ratio of atoms.
- Subscripts 2 and 4 share a common factor of 2.
- Dividing both by 2 gives 1 and 2 - and the 1 is not written.
Answer: Divide by the common factor 2 to get 1 and 2 - the simplest ratio.
Check: 2:4 and 1:2 describe the same proportion, but convention keeps the smallest numbers.
Watch for this: Do not leave common factors in the subscripts - the formula must show the simplest ratio.
Now try it yourself
A criss-cross gives subscripts 3 and 6. What is the simplest ratio?
Check your answer
1 and 2.
Divide both by the common factor 3.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Law of conservation of mass?
Show answer and reason
Total mass of reactants equals total mass of products.
Mass is neither created nor destroyed in a reaction.
2. Law of constant proportions?
Show answer and reason
A pure compound has the same elements in the same proportion by mass.
The source of the sample does not matter.
3. Covalent vs ionic?
Show answer and reason
Covalent shares electrons; ionic transfers them.
H2O shares, NaCl transfers.
4. Formula from valencies?
Show answer and reason
Criss-cross valencies as subscripts, drop 1, reduce.
Gives H2S and MgCl2.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- 10 g of calcium carbonate gives 5.6 g of calcium oxide and some carbon dioxide. Find the mass of carbon dioxide.
- Is the bond in Cl2 covalent or ionic? Why?
- Write the formula of the compound of magnesium (valency 2) and chlorine (valency 1).
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Questions about this practice page
- What is the law of conservation of mass?
- Mass is neither created nor destroyed in a chemical reaction - total mass of reactants equals total mass of products.
- What is the law of constant proportions?
- A pure compound always contains the same elements combined in the same proportion by mass, whatever its source.
- Covalent vs ionic bond?
- Covalent bonds form by sharing electrons (H2O, Cl2); ionic bonds form by transferring electrons, giving ions that attract (NaCl).
- How do you write a chemical formula from valencies?
- Write the symbols with their valencies, criss-cross the numbers as subscripts, omit any 1, and reduce subscripts to the simplest ratio - like H2S and MgCl2.
- Are these the NCERT exercise questions for Class 9 Science Chapter 9?
- No. EZT pages are original practice following the chapter's section order - use them to prepare for the textbook exercises, not as a substitute for your book.