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Q1. What is a chemical equation?
A representation of a chemical reactionA chemical equation uses chemical formulas and symbols to show what happens during a chemical reaction. It identifies the reactants (starting substances) on the left and the products (substances formed) on the right, providing a concise way to represent the transformation of matter.
Q2. The substances that take part in a chemical reaction are called:
ReactantsReactants are the initial substances that undergo change in a chemical reaction. They are written on the left side of a chemical equation and are consumed as the reaction proceeds to form new products.
Q3. The substances formed after a chemical reaction are called:
ProductsProducts are the new substances that are created as a result of a chemical reaction. They appear on the right side of a chemical equation and have different chemical properties from the reactants.
Q4. Which symbol separates reactants and products in a chemical equation?
→The arrow (→) is read as “yields,” “produces,” or “forms.” It separates the reactants (left side) from the products (right side) and indicates the direction in which the reaction proceeds.
Q5. What does the plus sign (+) indicate in a chemical equation?
AndThe plus sign (+) is used to separate two or more reactants or two or more products. It is read as “and” or “reacts with,” indicating that multiple substances are combined or produced together.
Q6. Which of the following is a chemical change?
Rusting ironRusting is a chemical change because iron (Fe) reacts with oxygen (O₂) and moisture to form a new substance, hydrated iron oxide (rust). The other options are physical changes where no new substance is formed.
Q7. A chemical equation written in words is called:
Word equationA word equation uses the names of reactants and products instead of chemical symbols. For example, “Hydrogen + Oxygen → Water” is a word equation, which is helpful for describing a reaction in simple language before writing symbolic formulas.
Q8. Which law is used to balance chemical equations?
Law of conservation of massThe law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. Balancing an equation ensures that the number of atoms of each element is the same on both sides, thus obeying this law.
Q9. In a balanced chemical equation, the number of atoms on both sides is:
SameA balanced chemical equation has an equal number of atoms for each element on the reactant and product sides. This equality reflects that atoms are merely rearranged during a reaction, not lost or gained.
Q10. Which of the following is a word equation?
Hydrogen + Oxygen → WaterA word equation spells out the names of the reactants and products. Options A, C, and D use chemical symbols and formulas, making them symbolic or skeleton equations.
Q11. What does the symbol (s) indicate?
SolidThe symbol (s) is a state symbol written after a chemical formula in an equation. It indicates that the substance is in the solid state at the reaction temperature.
Q12. The symbol (l) represents:
LiquidThe symbol (l) stands for the liquid state. It is used to show that a substance is in liquid form, such as water written as H₂O(l) in an equation.
Q13. Which symbol shows gaseous state?
(g)The symbol (g) denotes a gas. For example, oxygen gas is written as O₂(g) in a chemical equation to indicate its physical state.
Q14. The symbol (aq) means:
Dissolved in water(aq) stands for “aqueous,” meaning the substance is dissolved in water. For instance, NaCl(aq) represents a solution of table salt in water.
Q15. A skeletal equation is:
Unbalanced equationA skeletal equation uses chemical formulas but does not have correct coefficients to balance the number of atoms. For example, H₂ + O₂ → H₂O is skeletal because oxygen atoms are not equal on both sides.
Q16. Which of the following must be true for a balanced equation?
Same mass on both sidesDue to the law of conservation of mass, the total mass of reactants must equal the total mass of products. This is achieved when the equation is balanced, though the number of molecules may differ.
Q17. Which side of a chemical equation contains reactants?
Left sideBy convention, reactants are always written on the left side of the arrow in a chemical equation. The arrow points toward the products on the right side.
Q18. Products are written on which side of the equation?
Right sideProducts are written on the right side of the arrow in a chemical equation. The arrow points from reactants (left) to products (right).
Q19. What does the arrow (→) mean?
GivesThe arrow is read as “gives,” “yields,” “forms,” or “produces.” It indicates the direction of the chemical change from reactants to products.
Q20. Which of the following shows a chemical reaction?
Burning coalBurning coal (carbon) is a chemical reaction because carbon combines with oxygen to produce carbon dioxide and heat, forming new substances. The other options are physical changes.
Q21. Which of the following is NOT shown by a chemical equation?
Speed of reactionA chemical equation tells you what reacts and what forms, and if balanced, it shows mass conservation. However, it does not provide any information about how fast the reaction occurs (reaction rate).
Q22. Why are coefficients used in chemical equations?
To balance equationsCoefficients are numbers placed in front of chemical formulas. They multiply the entire formula to increase the number of atoms or molecules, allowing the equation to have equal atom counts on both sides without changing the substances’ identities.
Q23. In H₂ + O₂ → H₂O, H₂ and O₂ are:
ReactantsH₂ (hydrogen gas) and O₂ (oxygen gas) appear on the left side of the arrow, so they are the starting substances or reactants in this unbalanced equation.
Q24. In the reaction Mg + O₂ → MgO, MgO is a:
ProductMgO (magnesium oxide) appears on the right side of the arrow. It is the new substance formed when magnesium reacts with oxygen, hence it is a product.
Q25. What type of change is represented by a chemical equation?
Chemical changeA chemical equation represents a chemical change, meaning one or more substances are transformed into new substances with different chemical properties. Physical changes (like melting) are not represented by chemical equations.
Q26. Which of the following obeys the law of conservation of mass?
Balanced equationOnly a balanced chemical equation has the same number of atoms of each element on both sides, ensuring that total mass is conserved. Skeletal and unbalanced equations do not satisfy this law.
Q27. What happens to atoms during a chemical reaction?
They are rearrangedAtoms are neither created nor destroyed in a chemical reaction. Instead, bonds between atoms break, and new bonds form, rearranging the atoms into different molecules or compounds.
Q28. Which symbol is used to show heat?
ΔThe Greek letter delta (Δ) written above or below the arrow indicates that heat is supplied to the reaction. For example, CaCO₃ → CaO + CO₂ (with Δ above arrow) shows thermal decomposition.
Q29. A chemical equation is balanced by changing:
CoefficientsTo balance an equation, you adjust coefficients (numbers in front of formulas). You never change the subscripts inside a formula, as that would alter the identity of the substance.
Q30. Which of the following is a balanced equation?
2H₂ + O₂ → 2H₂OIn option B, there are 4 H atoms and 2 O atoms on the left, and 4 H atoms and 2 O atoms on the right. The other options have unequal numbers of atoms.
Q31. Chemical equations are useful because they:
Show reactions clearlyChemical equations provide a concise, standardized way to communicate exactly what happens in a reaction — which substances react, which are produced, and in what proportions — without lengthy descriptions.
Q32. Which of the following is a reactant?
Starting substanceA reactant is a starting substance that is consumed during a chemical reaction. It is present before the reaction begins and changes into products.
Q33. Which one is a chemical formula?
H₂OA chemical formula uses element symbols and subscripts to show the exact composition of a substance. H₂O indicates two hydrogen atoms and one oxygen atom in a water molecule.
Q34. What is the formula of carbon dioxide?
CO₂Carbon dioxide consists of one carbon atom covalently bonded to two oxygen atoms, hence the formula CO₂. CO is carbon monoxide, a different compound.
Q35. Which change forms a new substance?
Rusting ironRusting forms iron oxide, a new substance with different properties from iron. Boiling, melting, and freezing are physical changes that do not produce new substances.
Q36. What is always conserved in a chemical reaction?
MassAccording to the law of conservation of mass, the total mass of reactants always equals the total mass of products. Volume, colour, and shape may change.
Q37. Which part of the equation shows physical states?
State symbolsState symbols such as (s), (l), (g), and (aq) are written after chemical formulas to indicate whether a substance is solid, liquid, gas, or aqueous.
Q38. A chemical equation does NOT show:
Colour changeWhile a chemical equation may sometimes indicate energy change (e.g., Δ for heat), it never shows observable changes like colour, odor, or physical appearance of the substances.
Q39. Which of the following is an element?
OxygenOxygen (O₂) is an element because it consists of only one type of atom. Water, salt (NaCl), and sugar (C₁₂H₂₂O₁₁) are compounds made of multiple elements.
Q40. What does a balanced equation indicate?
Same massA balanced equation indicates that the total mass of reactants equals the total mass of products, satisfying the law of conservation of mass. It does not imply equal temperature, volume, or colour.
Q41. Which symbol is used for solid sodium chloride?
NaCl(s)Sodium chloride at room temperature is a crystalline solid, so its state symbol is (s). (aq) would mean dissolved in water, (l) means liquid, and (g) means gas.
Q42. Chemical equations are written to save:
Time and spaceInstead of writing a long sentence describing a reaction, a chemical equation conveys the same information quickly and compactly, saving time and space, especially in scientific communication.
Q43. What is formed when magnesium burns in oxygen?
Magnesium oxideMagnesium reacts with oxygen during combustion to form magnesium oxide (MgO), a white powder. The balanced equation is: 2Mg + O₂ → 2MgO.
Q44. In a chemical reaction, old bonds are:
BrokenFor a chemical reaction to occur, bonds between atoms in the reactant molecules must break. New bonds then form to create the product molecules.
Q45. Which is always written first in a chemical equation?
ReactantsBy convention, the reactants are always written on the left side of the arrow, and products on the right. This format is universally followed in chemistry.
Q46. What does the word “balanced” mean?
Equal atomsA balanced chemical equation has the same number of atoms of each element on both the reactant and product sides. This ensures mass conservation.
Q47. Which of the following is NOT a chemical reaction?
Melting iceMelting ice is a physical change — solid water (ice) becomes liquid water, but no new substance is formed. Burning, rusting, and cooking all produce new chemical substances.
Q48. A chemical equation shows:
Complete reactionA properly written chemical equation shows the complete reaction, including all reactants, all products, their relative amounts (if balanced), and sometimes physical states and conditions.
Q49. What is formed when iron reacts with sulphur?
Iron sulphideWhen iron and sulphur are heated together, they chemically combine to form iron sulphide (FeS), a compound with properties different from the original elements.
Q50. Why should chemical equations be balanced?
To follow law of conservation of massBalancing equations ensures that the number of atoms of each element is the same on both sides, which is required because mass is neither created nor destroyed in a chemical reaction. This makes the equation scientifically accurate.