Class 10 Science · Original EZT practice
Chemical Reactions and Equations (Class 10): original worked practice
Counting atoms and interpreting an observation are different skills. Try each paper-based question, explain your choice, and then use the atom count or a counterexample to check it.
8 original worked examples, 8 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 17 October 2026:
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.
Separate an observation from a conclusion
Choose the method: A gas, colour change, precipitate or temperature change can be evidence of a reaction. No single visible signal proves a chemical change in every situation.
EZT practice question
A learner sees bubbles in two labelled diagrams: warmed water and an acid reacting with a carbonate. They claim both diagrams prove a new gas substance formed. What is missing?
Show the worked solution
- In warmed water, bubbles may be escaping dissolved gas or water vapour: neither alone proves a new substance.
- In the labelled carbonate reaction, the proposed chemical account predicts carbon dioxide.
- Ask for the starting materials and a suitable teacher-led identification of the products, not just the appearance of bubbles.
Answer: Bubbles are an observation. A chemical-change conclusion needs evidence that new substances formed.
Check: The same observation can have different explanations, so the conclusion must fit the starting materials and products.
Watch for this: Do not taste, heat or mix unknown substances to test a guess; these are paper-based comparisons.
Now try it yourself
A blue solution becomes paler after water is added. Does that alone prove a chemical reaction?
Check your answer
No. Dilution can make the same dissolved substance look paler.
A colour change is a clue, not proof without context or product evidence.
Balance by counting whole formula units
Choose the method: Keep every chemical formula fixed. Change coefficients, then count each element on both sides. This question is symbolic, not a procedure for burning a chemical.
EZT practice question
Balance CH₃OH + O₂ → CO₂ + H₂O using the smallest whole-number coefficients.
Show the worked solution
- Start with 1 CH₃OH: its 1 carbon needs 1 CO₂ and its 4 hydrogens need 2 H₂O.
- Those products contain 4 oxygen atoms. CH₃OH already supplies 1; the other 3 would require 3/2 O₂.
- Multiply all coefficients by 2 to remove the fraction.
Answer: 2 CH₃OH + 3 O₂ → 2 CO₂ + 4 H₂O
Check: Left: C 2, H 8, O 2 + 6 = 8. Right: C 2, H 8, O 4 + 4 = 8.
Watch for this: Changing CH₃OH into another formula would change the substance, not balance this equation.
Now try it yourself
Balance C₃H₈ + O₂ → CO₂ + H₂O on paper.
Check your answer
C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
Carbon: 3 each; hydrogen: 8 each; oxygen: 10 each. This equation does not instruct you to handle or ignite a fuel.
Use the least common atom count
Choose the method: When oxygen appears as O₂ and as an oxide with an odd oxygen count, choose an even multiple of the oxide first.
EZT practice question
Balance Fe + O₂ → Fe₂O₃. Explain why Fe + O₂ → Fe₂O₂ is not an acceptable shortcut.
Show the worked solution
- Oxygen counts of 2 and 3 meet at 6, so use 3 O₂ and 2 Fe₂O₃.
- The products now contain 4 iron atoms, so put 4 before Fe.
- Fe₂O₂ is not the stated product Fe₂O₃; altering the subscript abandons the question.
Answer: 4 Fe + 3 O₂ → 2 Fe₂O₃
Check: Fe: 4 = 4; O: 6 = 6. Coefficients 4, 3 and 2 share no common factor greater than 1.
Watch for this: A balanced count alone is not enough if a formula was changed.
Now try it yourself
Balance N₂ + H₂ → NH₃ using whole-number coefficients.
Check your answer
N₂ + 3 H₂ → 2 NH₃
Both sides contain 2 nitrogen and 6 hydrogen atoms. The coefficient multiplies the whole NH₃ formula.
A reaction can carry more than one label
Choose the method: Combination and decomposition describe the pattern of reactants and products; exothermic describes an energy transfer. The labels answer different questions.
EZT practice question
Two paper equations are supplied: A, 2 H₂ + O₂ → 2 H₂O with energy released; B, 2 H₂O → 2 H₂ + O₂ with energy supplied. Classify the patterns and the energy description.
Show the worked solution
- A combines two substances to make one kind of product: combination. The stated release makes it exothermic.
- B splits one substance into simpler substances: decomposition. It needs energy under the stated conditions.
- The coefficient 2 counts units, not two different kinds of substance.
Answer: A: combination and exothermic. B: decomposition, with energy supplied.
Check: Count substance kinds: A has 2 reactant kinds and 1 product kind; B has 1 reactant kind and 2 product kinds.
Watch for this: Do not use the number of molecules as the number of different products. No gas preparation or ignition is part of this page.
Now try it yourself
An equation shows one compound forming two different substances. Is it automatically a displacement reaction?
Check your answer
No; that pattern is decomposition.
A displacement pattern needs an element replacing another element in a compound.
Read the reactivity condition
Choose the method: In a metal displacement model, a more reactive metal can replace a less reactive metal in a suitable salt solution. More reactive does not mean physically stronger.
EZT practice question
A paper record says magnesium is more reactive than zinc and gives Mg + ZnSO₄ → MgSO₄ + Zn. Identify the displaced element and explain whether the reverse equation is predicted by that ordering.
Show the worked solution
- Magnesium enters the sulphate compound while zinc appears as the free metal.
- The displaced element is zinc.
- With the given ordering, zinc is not predicted to displace magnesium from magnesium sulphate.
Answer: Zinc is displaced; the reverse displacement is not predicted by the stated reactivity order.
Check: Each side has Mg 1, Zn 1, S 1 and O 4, so the supplied equation is balanced.
Watch for this: A reactivity ordering supports the chemical prediction; it is not a statement about hardness or density.
Now try it yourself
Suppose metal P is less reactive than metal Q. Is P + Q-salt → P-salt + Q predicted by this model?
Check your answer
No.
The proposed incoming metal P is less reactive, so the stated ordering does not support that displacement.
Track the insoluble product
Choose the method: A precipitation reaction forms an insoluble solid from dissolved reactants. For these supplied aqueous salts, use the stated insolubility and count the spectator ions too.
EZT practice question
For a paper model, calcium carbonate is insoluble. Complete and balance CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + NaCl(aq), and identify the precipitate.
Show the worked solution
- Calcium and carbonate form the solid CaCO₃.
- The reactants contain 2 sodium and 2 chlorine atoms, so put 2 before NaCl.
Answer: CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2 NaCl(aq); the precipitate is calcium carbonate.
Check: Ca 1, C 1, O 3, Na 2 and Cl 2 occur on each side.
Watch for this: The solid state label (s) identifies the precipitate; the dissolved sodium chloride is not the precipitate.
Now try it yourself
In a supplied reaction, two clear solutions form a cloudy mixture with a new insoluble solid. What observation supports precipitation?
Check your answer
The appearance of the new insoluble solid.
Cloudiness can be caused by suspended solid particles; the product identification supplies the chemical context.
Name oxidation and reduction together
Choose the method: In the oxygen-transfer view used here, gaining oxygen is oxidation and losing oxygen is reduction. Electron transfer is a wider definition, so not every redox equation must show oxygen.
EZT practice question
In the supplied equation Fe₂O₃ + 3 H₂ → 2 Fe + 3 H₂O, identify what is oxidised and what is reduced.
Show the worked solution
- Iron(III) oxide loses its oxygen and forms iron: the oxide is reduced.
- Hydrogen gains oxygen and forms water: hydrogen is oxidised.
- Both changes belong to the same reaction; the oxygen moves from the oxide to hydrogen.
Answer: Hydrogen is oxidised; iron(III) oxide is reduced.
Check: Fe 2, O 3 and H 6 balance on each side. The oxygen lost by the oxide appears in the water.
Watch for this: Do not reverse the labels because the word oxide sounds like oxidation. This is a symbolic question, not a hot-gas experiment.
Now try it yourself
A learner says a reactant that gains oxygen has been reduced. Correct the label in this view.
Check your answer
It has been oxidised.
Reduction is loss of oxygen in the restricted oxygen-transfer description.
Avoid a misleading combination label
Choose the method: A summary of glucose respiration is an exothermic redox change. A combination pattern has one kind of product, which is not the pattern of this summary.
EZT practice question
A learner labels C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O a simple combination reaction because oxygen is a reactant. Evaluate the label.
Show the worked solution
- The products are carbon dioxide and water: two different substance kinds.
- The overall process releases usable energy and is commonly described as exothermic.
- Redox is the relevant chemical description; oxygen being present does not make every reaction a combination.
Answer: The simple combination label is unsuitable. The overall respiration summary is exothermic and redox.
Check: Count C 6, H 12 and O 18 on each side. Energy release and product pattern are separate properties.
Watch for this: This equation is an overall summary, not a claim that cellular respiration happens in one step.
Now try it yourself
A reaction gives out heat but forms several products. Must it be a combination reaction?
Check your answer
No.
Exothermic describes heat transfer, not how many kinds of product form.
Try a mixed set
Choose a method or explanation before opening each answer.
1. A beaker becomes colder when a solid dissolves. Does that observation alone establish a chemical reaction?
Show answer and reason
No.
Temperature change can accompany dissolving as well. Identify the substances and evidence of new products before deciding.
2. In 3 CaCl₂, how many calcium and chlorine atoms are represented?
Show answer and reason
3 calcium and 6 chlorine atoms.
The coefficient multiplies every atom count in the formula.
3. An equation is balanced. Does that alone prove the proposed reaction occurs under ordinary conditions?
Show answer and reason
No.
Atom conservation is necessary, but it does not establish feasibility, rate or suitable conditions.
4. Why is rust not best treated here as one pure compound with a single fixed formula?
Show answer and reason
Ordinary rust is a mixture of hydrated iron oxides and related products.
The school description hydrated iron(III) oxide is an approximation, not a guarantee of fixed water content.
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Questions about this practice page
- Do bubbles or a colour change prove a reaction?
- Not alone. They are clues. Physical processes such as boiling and dilution can give similar observations; use the starting materials and evidence of new products.
- Why are subscripts fixed while balancing?
- A subscript is part of the substance formula. Change only coefficients to conserve the atom count without replacing the substances in the stated equation.
- Can a reaction be both combination and exothermic?
- Yes. Combination describes the substance pattern; exothermic describes energy release. The two labels do not compete.
- Is respiration a simple combination reaction?
- Its glucose summary has carbon dioxide and water as different products, so a simple combination label is unsuitable. It is an overall exothermic redox process, not a one-step cellular mechanism.
- Are these practical experiment instructions or NCERT exercise answers?
- Neither. They are original EZT paper-based practice prompts and explanations. Do not use them to handle chemicals; follow teacher-supervised safety procedures for any school practical.