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Q1. What is a chemical equation?
A chemical equation is a symbolic representation of a chemical reaction. It shows the reactants (starting substances) on the left side and the products (substances formed) on the right side, with an arrow (→) in between. It is not a statement of physical change (that would be physical change description), not a mathematical formula (though it uses symbols), and not a physical law. So a representation of a chemical reaction is correct.
Q2. The substances that take part in a chemical reaction are called:
In a chemical reaction, the substances that react together are called reactants. They are written on the left side of the chemical equation. Products are formed after the reaction (right side). Elements and compounds are types of substances, but the specific term for substances that take part in the reaction is reactants.
Q3. The substances formed after a chemical reaction are called:
The new substances formed as a result of a chemical reaction are called products. They are written on the right side of the chemical equation. Reactants are the starting substances, residues are leftover substances, and mixtures are combinations of substances that are not chemically bonded. So products is correct.
Q4. Which symbol separates reactants and products in a chemical equation?
In a chemical equation, the arrow (→) separates the reactants from the products. It is read as “gives” or “yields”. The plus sign (+) is used to separate multiple reactants or multiple products. The multiplication sign (×) is not used. The equals sign (=) is not used in chemical equations (though sometimes used for ionic equations in some contexts). So → is correct.
Q5. What does the plus sign (+) indicate in a chemical equation?
The plus sign (+) in a chemical equation is read as “and”. It is used to separate two or more reactants (e.g., H₂ + O₂ means hydrogen and oxygen) or two or more products (e.g., CO₂ + H₂O means carbon dioxide and water). It does not mean “gives” (that’s the arrow), “equals” (that’s not used), or “heat” (that’s Δ). So “and” is correct.
Q6. Which of the following is a chemical change?
A chemical change is one in which new substances are formed. Rusting of iron is a chemical change because iron reacts with oxygen and moisture to form a new substance — rust (iron oxide). Melting ice, boiling water, and cutting paper are physical changes — no new substance is formed, only state or size changes. So rusting iron is a chemical change.
Q7. A chemical equation written in words is called:
A word equation uses the names of substances instead of chemical symbols. For example: “Hydrogen + Oxygen → Water” is a word equation. A balanced equation uses symbols and has equal atoms on both sides. A skeletal equation is an unbalanced symbolic equation. A symbolic equation uses chemical formulas. So word equation is correct.
Q8. Which law is used to balance chemical equations?
Chemical equations are balanced using the Law of Conservation of Mass, which states that matter (mass) can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of reactants must equal the total mass of products, which means the number of atoms of each element must be the same on both sides. Law of gravity, motion, and energy are not used for balancing equations.
Q9. In a balanced chemical equation, the number of atoms on both sides is:
In a balanced chemical equation, the number of atoms of each element is exactly the same on the reactant side and the product side. This follows the Law of Conservation of Mass. If the numbers are different, the equation is unbalanced. So “same” is correct.
Q10. Which of the following is a word equation?
A word equation uses the names of substances. Option B uses “Hydrogen”, “Oxygen”, and “Water” — these are words. Options A, C, and D use chemical symbols (H₂, O₂, H₂O) and coefficients, so they are symbolic equations. So Hydrogen + Oxygen → Water is a word equation.
Q11. What does the symbol (s) indicate?
In a chemical equation, the state symbol (s) indicates that the substance is in the solid state. For example, NaCl(s) means solid sodium chloride. (aq) means aqueous (dissolved in water), (g) means gas, and (l) means liquid. So (s) is for solid.
Q12. The symbol (l) represents:
The state symbol (l) in a chemical equation indicates that the substance is in the liquid state. For example, H₂O(l) means liquid water. It does not represent light, layer, or lime. So liquid is correct.
Q13. Which symbol shows gaseous state?
The state symbol (g) indicates that the substance is in the gaseous state. For example, O₂(g) means oxygen gas. (s) is solid, (l) is liquid, (aq) is aqueous (dissolved in water). So (g) is correct.
Q14. The symbol (aq) means:
The state symbol (aq) stands for aqueous, which means the substance is dissolved in water. For example, NaCl(aq) means sodium chloride dissolved in water. It does not mean solid, liquid, or gas — those are (s), (l), and (g) respectively. So dissolved in water is correct.
Q15. A skeletal equation is:
A skeletal equation is an unbalanced chemical equation — it shows the correct formulas of reactants and products but does not have the same number of atoms on both sides. For example: H₂ + O₂ → H₂O is a skeletal equation because oxygen atoms are not balanced (2 on left, 1 on right). It is not balanced, not a word equation, and not a nuclear equation. So unbalanced equation is correct.
Q16. Which of the following must be true for a balanced equation?
A balanced chemical equation follows the Law of Conservation of Mass, which means the total mass of reactants must equal the total mass of products. This is achieved by having the same number of atoms of each element on both sides. The number of molecules may differ (e.g., 2H₂ + O₂ → 2H₂O has 3 molecules on left, 2 on right). Volume and colour are not relevant. So same mass on both sides is correct.
Q17. Which side of a chemical equation contains reactants?
By convention, reactants are always written on the left side of the arrow in a chemical equation. Products are written on the right side. For example: Mg + O₂ → MgO — Mg and O₂ are reactants on the left. Below or above the arrow are used for conditions (like heat or catalyst), not reactants. So left side is correct.
Q18. Products are written on which side of the equation?
Products are always written on the right side of the arrow in a chemical equation. Reactants are on the left. For example: Mg + O₂ → MgO — MgO is the product on the right side. Below or above the arrow are for reaction conditions. So right side is correct.
Q19. What does the arrow (→) mean?
The arrow (→) in a chemical equation is read as “gives”, “yields”, or “forms”. It indicates the direction of the reaction from reactants to products. It does not mean “equals” (though it implies equality of mass in a balanced equation), “plus” (that’s +), or “heat” (that’s Δ). So “gives” is correct.
Q20. Which of the following shows a chemical reaction?
Burning coal is a chemical reaction because coal (carbon) reacts with oxygen to produce carbon dioxide and other products, releasing heat and light. Breaking glass, melting wax, and freezing water are physical changes — no new substances are formed. So burning coal shows a chemical reaction.
Q21. Which of the following is NOT shown by a chemical equation?
A chemical equation shows the reactants, products, and (in a balanced equation) the mass balance (number of atoms). However, it does not show the speed (rate) of the reaction — how fast or slow the reaction occurs. Rate of reaction depends on factors like temperature, concentration, catalyst, etc., which are not indicated in the equation itself. So speed of reaction is not shown.
Q22. Why are coefficients used in chemical equations?
Coefficients (numbers placed before formulas) are used to balance chemical equations. They indicate the number of molecules or moles of each substance so that the number of atoms of each element is the same on both sides. Coefficients do not change chemical formulas (you cannot change subscripts), they do not add new substances, and they do not remove atoms. So to balance equations is correct.
Q23. In H₂ + O₂ → H₂O, H₂ and O₂ are:
In the equation H₂ + O₂ → H₂O, H₂ (hydrogen) and O₂ (oxygen) are written on the left side of the arrow, so they are the reactants. H₂O (water) is the product on the right side. Catalysts speed up reactions but are not consumed. Residues are leftover substances. So reactants is correct.
Q24. In the reaction Mg + O₂ → MgO, MgO is a:
In the reaction Mg + O₂ → MgO, MgO (magnesium oxide) is written on the right side of the arrow, so it is the product formed. Mg and O₂ are the reactants on the left. MgO is a compound (not an element), not a mixture. So product is correct.
Q25. What type of change is represented by a chemical equation?
A chemical equation represents a chemical change — a reaction in which new substances (products) are formed from reactants. Physical changes (melting, boiling) do not form new substances. Temporary and reversible changes are not the correct terms for what a chemical equation represents. So chemical change is correct.
Q26. Which of the following obeys the law of conservation of mass?
The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. Only a balanced chemical equation obeys this law because it has the same number of atoms (and therefore the same mass) on both sides. Word equations and skeletal/unbalanced equations do not necessarily show mass balance. So balanced equation is correct.
Q27. What happens to atoms during a chemical reaction?
During a chemical reaction, atoms are neither created nor destroyed. Instead, they are rearranged to form new substances. Bonds between atoms break and new bonds form. This is the basis of the Law of Conservation of Mass. Atoms are not destroyed, created, or made to disappear — they are just rearranged. So rearranged is correct.
Q28. Which symbol is used to show heat?
The symbol Δ (Greek letter delta) is used above or below the arrow in a chemical equation to indicate that heat is supplied to the reaction. For example: CaCO₃ →Δ→ CaO + CO₂. The plus sign (+) separates substances, the arrow (→) means gives, and (g) indicates gas. So Δ is used for heat.
Q29. A chemical equation is balanced by changing:
A chemical equation is balanced by changing the coefficients (numbers placed in front of formulas). For example, H₂ + O₂ → H₂O is unbalanced; adding coefficient 2 gives 2H₂ + O₂ → 2H₂O, which is balanced. Chemical formulas cannot be changed (that would create different substances), symbols cannot be changed, and the number of atoms is the result of balancing, not the method. So coefficients is correct.
Q30. Which of the following is a balanced equation?
In option B, the number of atoms on both sides: Left: H = 4, O = 2. Right: H = 4, O = 2. Both sides have equal atoms. Option A has H=2, O=2 on left and H=2, O=1 on right (unbalanced). Option C uses O (oxygen atom, not O₂ — incorrect formula). Option D uses H (hydrogen atom, not H₂ — incorrect formula). So only B is balanced.
Q31. Chemical equations are useful because they:
Chemical equations are useful because they clearly and concisely show what happens in a chemical reaction — the reactants, products, their formulas, and (if balanced) the proportions. They do not increase weight, change colour, or slow reactions. So show reactions clearly is correct.
Q32. Which of the following is a reactant?
A reactant is a starting substance that takes part in a chemical reaction. It is consumed during the reaction to form products. Substance formed is a product, final product is also product, residue is leftover. So starting substance is correct.
Q33. Which one is a chemical formula?
A chemical formula is a representation of a compound using chemical symbols. H₂O is the chemical formula for water. “Water” is the name (not formula), “liquid” is a state, and “solid” is a state. So H₂O is the chemical formula.
Q34. What is the formula of carbon dioxide?
Carbon dioxide is composed of one carbon atom and two oxygen atoms. Its chemical formula is CO₂. CO is carbon monoxide (one oxygen), C₂O is not a common compound, CO₃ is carbonate ion (not carbon dioxide). So CO₂ is correct.
Q35. Which change forms a new substance?
Rusting of iron forms a new substance — rust (iron oxide). Boiling water, melting ice, and freezing milk are physical changes; no new substance is formed (milk freezing may separate components but no chemical change). So rusting iron forms a new substance.
Q36. What is always conserved in a chemical reaction?
According to the Law of Conservation of Mass, mass is always conserved in a chemical reaction. The total mass of reactants equals the total mass of products. Volume, colour, and shape can change during a reaction. So mass is conserved.
Q37. Which part of the equation shows physical states?
State symbols (s), (l), (g), and (aq) are used in chemical equations to show the physical state of each substance. Coefficients show the number of molecules, symbols are chemical symbols, and the arrow shows the direction. So state symbols is correct.
Q38. A chemical equation does NOT show:
A chemical equation shows reactants, products, and sometimes energy change (if written with ΔH or heat terms). However, it does not show colour change — colour is an observable property that may change during a reaction, but it is not indicated in the equation. So colour change is not shown.
Q39. Which of the following is an element?
Oxygen (O₂) is an element because it consists of only one type of atom. Water (H₂O) is a compound of hydrogen and oxygen. Salt (NaCl) is a compound. Sugar (C₁₂H₂₂O₁₁) is a compound. So oxygen is an element.
Q40. What does a balanced equation indicate?
A balanced chemical equation indicates that the mass of reactants equals the mass of products (Law of Conservation of Mass). It does not indicate same temperature, volume, or colour. So same mass is correct.
Q41. Which symbol is used for solid sodium chloride?
Solid sodium chloride is represented as NaCl(s). (g) is gas, (l) is liquid, (aq) is aqueous (dissolved in water). So NaCl(s) is correct for solid sodium chloride.
Q42. Chemical equations are written to save:
Chemical equations are written using symbols and formulas instead of lengthy descriptions of reactions in words. This saves time and space. For example, writing “Mg + O₂ → MgO” is much shorter than writing “Magnesium reacts with oxygen to form magnesium oxide.” It does not save energy, money, or heat directly. So time and space is correct.
Q43. What is formed when magnesium burns in oxygen?
When magnesium burns in oxygen, it forms magnesium oxide (MgO). The reaction is: 2Mg + O₂ → 2MgO. A bright white flame is produced. Magnesium chloride would require chlorine, magnesium sulphate requires sulphur and oxygen, magnesium nitrate requires nitrogen and oxygen. So magnesium oxide is correct.
Q44. In a chemical reaction, old bonds are:
In a chemical reaction, old bonds between atoms in the reactants are broken. New bonds are then formed between atoms to create the products. Bonds are not strengthened (they break), not coloured, not frozen. So broken is correct.
Q45. Which is always written first in a chemical equation?
In a chemical equation, reactants are always written first, on the left side of the arrow. Products are written on the right side. Catalysts are written above or below the arrow, not first. Residues are not part of the main equation. So reactants is correct.
Q46. What does the word “balanced” mean?
A balanced chemical equation has an equal number of atoms of each element on both sides of the equation. This ensures that mass is conserved. It does not require equal temperature, volume, or colour. So equal atoms is correct.
Q47. Which of the following is NOT a chemical reaction?
Melting ice is a physical change — solid ice changes to liquid water, but no new substance is formed. Burning paper (combustion), rusting iron (oxidation), and cooking food (chemical changes in proteins, carbohydrates) are all chemical reactions. So melting ice is NOT a chemical reaction.
Q48. A chemical equation shows:
A chemical equation shows a complete reaction by displaying both reactants and products, along with their formulas and (if balanced) proportions. It does not show only reactants or only products — it shows both. It is not “only symbols” because it also includes coefficients and state symbols. So complete reaction is the best answer.
Q49. What is formed when iron reacts with sulphur?
When iron (Fe) reacts with sulphur (S), they combine chemically to form iron sulphide (FeS). This is a compound, not a mixture (mixtures can be separated physically), not a solution, and not an element (it contains two different elements). So iron sulphide is correct.
Q50. Why should chemical equations be balanced?
Chemical equations must be balanced because the Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction. Balancing ensures that the number of atoms (and therefore mass) is the same on both sides. It is not for neatness, speed, or changing products. So to follow the law of conservation of mass is correct.