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A balanced chemical equation must have the same number of atoms of each element on both the reactant side (LHS) and the product side (RHS). This follows the Law of Conservation of Mass, which states that atoms are neither created nor destroyed in a chemical reaction. If the numbers are different, the equation is unbalanced. So “same on both sides” is correct.
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Chemical equations are balanced using the Law of Conservation of Mass, which states that 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, energy, and motion are not used for balancing equations. So law of conservation of mass is correct.
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Melting ice is a physical change — solid ice changes to liquid water, but no new substance is formed. Cooking food involves chemical changes (proteins denature, carbohydrates break down), rusting iron is oxidation (formation of iron oxide), and burning a candle is combustion (formation of CO₂ and H₂O). So melting ice is NOT a chemical reaction.
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Coefficients are the numbers placed in front of chemical formulas in a balanced equation. They indicate the number of molecules (or moles) of each substance involved in the reaction. For example, in 2H₂ + O₂ → 2H₂O, the coefficient 2 before H₂ means 2 molecules of hydrogen. Coefficients do not show shape, colour, or directly the mass (though they relate to mass via molar mass). So number of molecules is correct.
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In the reaction Mg + O₂ → MgO, MgO (magnesium oxide) is formed as a result of the reaction. It is written on the right side of the arrow, so it is the product. Mg and O₂ are the reactants on the left side. MgO is a compound (not an element) and not a catalyst. So product is correct.
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Burning coal is a chemical reaction because coal (carbon) reacts with oxygen to form carbon dioxide and other products, releasing heat and light. Cutting wood, tearing paper, and freezing water are physical changes — no new substances are formed. So burning coal shows a chemical reaction.
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In option B, the number of atoms on both sides: Left side has H = 4, O = 2. Right side has H = 4, O = 2. Both sides have equal atoms. Option A uses O (oxygen atom, not O₂ — incorrect formula). Option C has H=2, O=2 on left and H=2, O=1 on right (unbalanced). Option D uses H (hydrogen atom, not H₂ — incorrect formula). So only B is balanced.
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In the reaction Zn + HCl → ZnCl₂ + H₂, hydrogen gas (H₂) is formed and appears on the right side of the arrow. Therefore, hydrogen is a product. Zinc (Zn) and HCl are reactants. Hydrogen is not a catalyst (it does not speed up the reaction), not a solvent (it is a gas), and not a reactant (it is formed, not consumed). So product is correct.
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The Law of Conservation of Mass states that in a chemical reaction, mass is neither created nor destroyed. The total mass of reactants equals the total mass of products. Mass cannot be changed into something else (it is conserved), not destroyed, and not created. So neither created nor destroyed is correct.
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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 (left side). Elements and mixtures are types of substances but not the specific term for substances formed after a reaction. So products is correct.
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Reactants are always written on the left side of the arrow in a chemical equation. Products are written on the right side. Below or after the arrow are not correct positions for reactants. For example: Mg + O₂ → MgO — Mg and O₂ are on the left side. So left side is correct.
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The chemical formula of water is H₂O, which means one molecule of water contains two hydrogen atoms and one oxygen atom. H₂ is hydrogen gas, H₂O₂ is hydrogen peroxide, and HO₂ is not a stable compound. So H₂O is correct.
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The state symbol (s) in a chemical equation indicates that the substance is in the solid state. For example, NaCl(s) means solid sodium chloride. (g) is gas, (l) is liquid, (aq) is aqueous (dissolved in water). So solid is correct.
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The symbol Δ (Greek letter delta) is written above or below the arrow in a chemical equation to indicate that heat is supplied to the reaction. The plus sign (+) separates substances, the equals sign (=) is not used in chemical equations, and the arrow (→) means “gives” or “yields”. So Δ is used for heat.
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For a chemical equation to be balanced, the number of atoms of each element must be equal on both sides (reactant and product sides). This is achieved by adjusting coefficients, not by changing formulas (which would change the substances), removing atoms, or adding atoms (which would violate conservation of mass). So equal atoms on both sides is correct.
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The substances that take part in a chemical reaction and are consumed are called reactants. They are written on the left side of the equation. Products are formed after the reaction. Catalysts speed up reactions but are not consumed. Residues are leftover substances. So reactants is correct.
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A chemical reaction always involves the breaking of old bonds between atoms in the reactants and the formation of new bonds to create products. This is what makes it a chemical change. Freezing, melting, and boiling are physical changes that do not involve breaking and forming chemical bonds. So breaking and making bonds is correct.
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A balanced chemical equation follows the Law of Conservation of Mass, meaning the total mass of reactants equals the total mass of products. This is achieved by having equal numbers of atoms of each element on both sides. It does not have unequal atoms, only products, or only reactants. So equal mass on both sides is correct.
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Iron (Fe) and sulphur (S) chemically combine to form iron sulphide (FeS), which is a compound — a substance made of two or more elements chemically bonded together in fixed proportions. It is not a mixture (mixtures can be separated physically), not an element (it contains two elements), and not a solution (solution is a homogeneous mixture in a liquid). So a compound is correct.
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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 is not read as “plus” (that’s +), “equals” (that’s not used), or “minus” (that’s not used). So “gives” is correct.
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A chemical reaction is defined as a process in which new substances (products) are formed from reactants. This involves breaking and making chemical bonds. Energy changes always occur (either absorbed or released), but the defining feature is formation of new substances. Physical changes (like change in shape) do not form new substances. So formation of new substances is correct.
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Rusting of iron is a chemical change because iron reacts with oxygen and moisture to form a new substance — rust (iron oxide). Melting wax, breaking chalk, and dissolving salt in water are physical changes (no new substance is formed). So rusting of iron is a chemical change.
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The state symbol (l) in a chemical equation indicates that the substance is in the liquid state. For example, H₂O(l) means liquid water. (s) is solid, (g) is gas, (aq) is aqueous (dissolved in water). So liquid is correct.
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Coefficients are numbers placed in front of chemical formulas in a chemical equation. For example, in 2H₂ + O₂ → 2H₂O, the coefficient 2 is placed before H₂ and before H₂O. They are not placed below or above the arrow (that’s for conditions like heat or catalyst), and not after formulas. So before formulas is correct.
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A product is the substance that is formed as a result of a chemical reaction. Reactants are starting materials that are consumed. Catalysts speed up reactions but are not consumed and are not products. So substance formed is correct.
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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. (s) is solid, (l) is liquid, (g) is gas. So dissolved in water is correct.
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Products are written on the right side of the arrow in a chemical equation. Reactants are on the left side. For example: Mg + O₂ → MgO — MgO is the product on the right side. Before arrow means left side (reactants). Below arrow is for conditions. So right side is correct.
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A balanced chemical equation always has equal mass on both sides (Law of Conservation of Mass). The number of molecules can be different (e.g., 2H₂ + O₂ → 2H₂O has 3 molecules on left and 2 on right). Volume and temperature are not necessarily equal. So equal mass on both sides is correct.
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Burning magnesium is a chemical reaction — magnesium reacts with oxygen to form magnesium oxide (MgO), a new substance. Cutting paper, boiling water, and freezing milk are physical changes (no new substance is formed). So burning magnesium forms a new substance.
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The state symbol (g) in a chemical equation 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 gas is correct.Close