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Q1. What helps us determine that a chemical reaction has taken place?
A chemical reaction can be detected by various observable changes. These include a change in state (e.g., solid to liquid), change in colour (e.g., green to brown in rusting), change in temperature (heat released or absorbed), or evolution of gas (bubbles). A single change alone (like only colour or only gas) is not sufficient because physical changes can also show some of these. The combination of any of these indicates a chemical reaction. So change in state, colour, temperature, or evolution of gas is correct.
Q2. What is the chemical change described when a magnesium ribbon is burnt in oxygen?
When a magnesium ribbon is burnt in oxygen, it undergoes a chemical reaction. Magnesium (Mg) reacts with oxygen (O₂) to form a new substance called magnesium oxide (MgO). The reaction is: 2Mg + O₂ → 2MgO. Magnesium oxide is not converted to magnesium (that would be the reverse reaction), oxygen does not become magnesium, and a new substance is definitely formed. So magnesium gets converted to magnesium oxide is correct.
Q3. What is the simplest way to write a description of a chemical reaction?
The simplest way to describe a chemical reaction is using a word-equation, which uses the names of the substances involved. For example: “Magnesium + Oxygen → Magnesium oxide”. A paragraph or essay is too long and detailed. A single symbol is insufficient because it doesn’t show reactants and products. So a word-equation is the simplest and most concise description.
Q4. In the word-equation: Magnesium + Oxygen → Magnesium oxide, what are ‘Magnesium’ and ‘Oxygen’ called?
In a word-equation, the substances written on the left side of the arrow are the reactants — the substances that take part in the reaction. Here, Magnesium and Oxygen are on the left side, so they are the reactants. Magnesium oxide is the product on the right side. Results is not a standard term, and mixtures are combinations of substances. So reactants is correct.
Q5. In a word-equation, what does the arrow (→) show?
The arrow (→) in a word-equation shows the direction of the reaction — it indicates that reactants are converted into products. It is read as “gives” or “yields”. It does not mean that reactants and products are equal (mass is equal in a balanced equation, but the arrow itself doesn’t show that), it is not a plus sign, and it does not indicate temperature. So the direction of the reaction is correct.
Q6. In a word-equation, where are the reactants written?
Reactants are always written on the left side of the arrow in a word-equation. If there are multiple reactants, they are separated by a plus sign (+). For example: “Magnesium + Oxygen → Magnesium oxide” — Magnesium and Oxygen are on the LHS. Products are on the RHS. Above or below the arrow are for conditions. So LHS with a plus sign is correct.
Q7. In a word-equation, where are the products written?
Products are written on the right side of the arrow in a word-equation. If there are multiple products, they are separated by a plus sign (+). For example: “Magnesium + Oxygen → Magnesium oxide” — Magnesium oxide is on the RHS. Reactants are on the LHS. Products are not on both sides or nowhere. So RHS with a plus sign is correct.
Q8. What is a more concise way to represent a chemical equation than a word-equation?
A word-equation uses the full names of substances. A more concise representation uses chemical formulae (symbols) instead of words. For example, instead of “Magnesium + Oxygen → Magnesium oxide”, we write “Mg + O₂ → MgO”. This saves time and space. Longer descriptions are not concise, pictures are not standard, and only numbers would not identify the substances. So using chemical formulae instead of words is correct.
Q9. What is a skeletal chemical equation?
A skeletal chemical equation is an unbalanced equation — it uses correct chemical formulae for reactants and products but does not have the same number of atoms on both sides. For example, Mg + O₂ → MgO is a skeletal equation because oxygen is not balanced (2 on left, 1 on right). A balanced equation conserves mass. Word-equations use words, not formulae. State symbols are not required for a skeletal equation. So an unbalanced equation where mass is not conserved is correct.
Q10. Which law states that mass can neither be created nor destroyed in a chemical reaction?
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. Law of constant proportions (or definite proportions) states that a compound always contains the same elements in the same proportion by mass. Law of multiple proportions deals with compounds of the same elements. Newton’s law is about motion. So law of conservation of mass is correct.
Q11. According to the law of conservation of mass, what remains the same before and after a chemical reaction?
According to the Law of Conservation of Mass, atoms are neither created nor destroyed in a chemical reaction. Therefore, the number of atoms of each element remains the same before and after the reaction. The number of molecules may change (e.g., 2H₂ + O₂ → 2H₂O has 3 molecules on left and 2 on right). Physical states and colour can change. So the number of atoms of each element is correct.
Q12. Why do we need to balance a chemical equation?
A chemical equation must be balanced to obey the Law of Conservation of Mass, which states that mass cannot be created or destroyed. Balancing ensures that the number of atoms of each element is the same on both sides, so mass is conserved. It is not to make it look longer, change reactants, or include temperature (temperature is a condition, not the reason for balancing). So to obey the law of conservation of mass is correct.
Q13. In the balanced equation Zn + H₂SO₄ → ZnSO₄ + H₂, how many hydrogen atoms are on each side?
In the equation Zn + H₂SO₄ → ZnSO₄ + H₂, the reactant side has H₂SO₄ which contains 2 hydrogen atoms. The product side has H₂ which also contains 2 hydrogen atoms. So there are 2 hydrogen atoms on each side. This equation is already balanced. So 2 is correct.
Q14. What is the first step suggested for balancing a chemical equation?
The first step in balancing a chemical equation is to draw boxes around each formula. This is done to remind us that we cannot change the formulae inside the boxes while balancing. We can only change the coefficients (numbers outside the boxes). Changing formulae would change the substances. Writing physical states comes later, and balancing oxygen first is not always the rule. So drawing boxes around each formula is correct.
Q15. When starting to balance an equation, it is often convenient to start with the compound that has what?
When balancing a chemical equation, it is often convenient to start with the compound that has the maximum number of atoms (or the most complex molecule). This makes balancing easier because adjusting its coefficient affects multiple atoms at once. Then balance the remaining elements. The minimum number of atoms is not the best starting point. Colour change and gaseous state are not relevant to balancing order. So the maximum number of atoms is correct.
Q16. What does the symbol (s) represent in a chemical equation?
The state symbol (s) in a chemical equation indicates that the substance is in the solid state. For example, Fe(s) means solid iron. (l) is liquid, (g) is gas, (aq) is aqueous (dissolved in water). So solid state is correct.
Q17. What does the symbol (l) represent in a chemical equation?
The state symbol (l) indicates that the substance is in the liquid state. For example, H₂O(l) means liquid water. It does not represent light, large quantity, or “less than”. So liquid state is correct.
Q18. What does the symbol (g) represent in a chemical equation?
The state symbol (g) indicates that the substance is in the gaseous state. For example, O₂(g) means oxygen gas. It does not represent gravitational force, green colour, or grams (that’s a unit of mass). So gas state is correct.
Q19. What does the symbol (aq) represent in a chemical equation?
The state symbol (aq) stands for aqueous, meaning the substance is dissolved in water. For example, NaCl(aq) means sodium chloride dissolved in water. It does not mean a quantity, aquatic (which refers to water life), or air quality. So aqueous (solution in water) is correct.
Q20. In the equation 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g), what does (g) with H₂O indicate?
In the equation, H₂O(g) means water is in the gaseous state. Gaseous water is called steam or water vapour. It is not ice (solid), not liquid (that would be H₂O(l)), and not dissolved (that would be aq). So water is in the form of steam is correct.
Q21. Are physical states always included in a chemical equation?
Physical states (s, l, g, aq) are not always included in chemical equations. They are added when it is necessary to specify the state of the reactants or products to make the equation more informative. For example, in the reaction of iron with steam, the state of water as gas (steam) is important. They are not always included, not always excluded, and not only for solids. So usually they are not included unless necessary is correct.
Q22. Where are reaction conditions like temperature or catalyst indicated in a chemical equation?
Reaction conditions such as temperature, pressure, or the presence of a catalyst are written above or below the arrow (→) in a chemical equation. For example: N₂ + 3H₂ →(Fe, 200 atm, 450°C)→ 2NH₃. They are not written before reactants, after products, or only in word-equations. So above and/or below the arrow is correct.
Q23. In the reaction for the formation of methanol, what condition is written below the arrow?
In the formation of methanol from carbon monoxide and hydrogen, the reaction condition of high pressure (340 atm) is written below the arrow. Sunlight and chlorophyll are conditions for photosynthesis. (s) is a state symbol. So 340 atm is correct.
Q24. In the photosynthesis equation, what are the conditions mentioned above the arrow?
In the photosynthesis equation (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂), the conditions written above the arrow are “Sunlight” and “Chlorophyll”. These are required for photosynthesis to occur in plants. 340 atm is for methanol formation, high temperature is not specifically for photosynthesis, and (aq) is a state symbol. So Sunlight / Chlorophyll is correct.
Q25. What is the product when hydrogen reacts with chlorine according to the word-equation?
When hydrogen reacts with chlorine, they form hydrogen chloride. The word-equation is: Hydrogen + Chlorine → Hydrogen chloride. The chemical equation is: H₂ + Cl₂ → 2HCl. Hydrogen peroxide is H₂O₂, chlorine water is chlorine dissolved in water, and hydrogen gas is H₂ (a reactant, not a product). So hydrogen chloride is correct.
Q26. In a chemical reaction, do atoms of one element change into atoms of another element?
In a chemical reaction, atoms of one element do not change into atoms of another element. Chemical reactions involve the rearrangement of atoms, but the identity of atoms (their element) remains the same. Atoms of one element cannot be converted into atoms of another element in a chemical reaction — that would require a nuclear reaction. So no, they never do is correct.
Q27. What do chemical reactions actually involve at the atomic level?
At the atomic level, a chemical reaction involves the breaking of old bonds between atoms in the reactants and the formation of new bonds to create the products. Atoms do not disappear or appear (they are rearranged), and it is not just mixing (mixing does not form new substances). So breaking and making of bonds between atoms is correct.
Q28. What is the new substance formed in the reaction of zinc and sulphuric acid?
When zinc reacts with sulphuric acid (H₂SO₄), it forms zinc sulphate (ZnSO₄) and hydrogen gas (H₂). The reaction is: Zn + H₂SO₄ → ZnSO₄ + H₂. Zinc and hydrogen are products (zinc is consumed, so not formed). Sulphuric acid is a reactant, not formed. Zinc oxide is not formed in this reaction. So zinc sulphate and hydrogen is correct.
Q29. If the number of atoms of each element is the same on both sides of an equation, what is it called?
When the number of atoms of each element is the same on both sides of a chemical equation, it is called a balanced chemical equation. A skeletal equation is unbalanced. A word-equation uses words, not formulae. An unbalanced equation has different numbers of atoms on each side. So a balanced chemical equation is correct.
Q30. What should you NOT change while balancing a chemical equation?
While balancing a chemical equation, you should not change the chemical formulae inside the boxes (the subscripts). Changing the formula would change the substance itself. You can change coefficients (numbers in front of formulae) to balance. You can also change the order of reactants (though not necessary) and add physical states without affecting balance. So anything inside the boxes drawn around formulae is correct.
Q31. What does the ‘+’ sign between reactants in a word-equation represent?
The plus sign (+) between reactants (and also between products) in a word-equation is read as “and”. It indicates that the substances are combined together in the reaction. It does not mean they are multiplied (that would be coefficients), not that they are in different states (state symbols show that), and not that they are products (reactants are on the left side). So “added together” (meaning “and”) is correct.
Q32. From the atom count for Fe + H₂O → Fe₃O₄ + H₂, how many iron atoms are on the product side?
In the product Fe₃O₄, the subscript 3 after Fe means there are 3 iron atoms. So on the product side, there are 3 iron atoms. This is a skeletal equation and needs balancing. So 3 is correct.
Q33. From the atom count for Fe + H₂O → Fe₃O₄ + H₂, how many oxygen atoms are on the reactant side?
In the reactant H₂O, there is 1 oxygen atom (subscript 1 after O, which is not written). So on the reactant side, there is 1 oxygen atom. On the product side, Fe₃O₄ has 4 oxygen atoms. This shows the equation is unbalanced. So 1 is correct.
Q34. What information does a balanced chemical equation without state symbols NOT give?
A balanced chemical equation shows the identities of reactants and products (through their formulae), the ratio of molecules/atoms (through coefficients), and the direction of the reaction (through the arrow). However, without state symbols, it does not give the physical states (solid, liquid, gas, aqueous) of the reactants and products. So the physical states of reactants and products is correct.
Q35. To make a chemical equation more informative, what can be added along with the chemical formulae?
A chemical equation can be made more informative by adding physical states (s, l, g, aq) of the reactants and products. This tells us whether a substance is solid, liquid, gas, or dissolved in water. The price of chemicals, name of scientist, or date are not relevant to the chemical equation. So the physical states of reactants and products is correct.
Q36. In the final balanced form of the iron and steam reaction, what is the coefficient of H₂?
The balanced equation for the reaction of iron with steam is: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. In this equation, the coefficient of H₂ is 4. The hydrogen atoms are balanced: 4H₂O gives 8 H atoms on the left, and 4H₂ gives 8 H atoms on the right. So the coefficient of H₂ is 4.
Q37. What does a chemical equation represent?
A chemical equation represents the identities (names/formulae) of reactants and products, and their proportions (through coefficients in a balanced equation). It does not represent the speed of the reaction (that’s rate of reaction), colour changes, or only gases. So the identities and proportions of reactants and products is correct.
Q38. What is the first step in going from a word description to a chemical equation?
The first step in going from a word description of a reaction to a chemical equation is to write the word-equation — using the names of reactants and products. Then you write the skeletal formula equation (using chemical symbols), then balance it, and finally add state symbols if needed. So writing the word-equation is the first step.
Q39. What must be true for a chemical equation to obey the law of conservation of mass?
For a chemical equation to obey the Law of Conservation of Mass, it must be balanced — meaning the number of atoms of each element is the same on both sides. A word-equation alone does not show mass balance. State symbols and an arrow are useful but do not ensure conservation of mass. So it must be balanced is correct.
Q40. In the photosynthesis equation 6CO₂(aq) + 6H₂O → C₆H₁₂O₆(aq) + 6O₂(g), what is the physical state of glucose?
In the photosynthesis equation, glucose is written as C₆H₁₂O₆(aq). The state symbol (aq) means aqueous — dissolved in water. This indicates that glucose is in solution within the plant cells. It is not solid (s), gas (g), or liquid (l). So (aq) is correct.
Q41. What is the primary purpose of the arrow in a chemical equation?
The primary purpose of the arrow (→) in a chemical equation is to show that the reactants yield (or give) products. It indicates the direction of the reaction. While it does separate reactants from products, the main meaning is “yields”. It does not indicate heat (that’s Δ) and does not show reversibility (that would be a double arrow ⇌). So to show the reaction yields products from reactants is correct.
Q42. According to the text, what does NOT happen to atoms during a chemical reaction?
During a chemical reaction, atoms do not change into atoms of another element. They are rearranged, bonds break and form, but the identity of each atom remains the same. Atoms do not disappear or appear from elsewhere (they are conserved). So they do not change into atoms of another element is correct.
Q43. What is the reactant in the burning of magnesium?
In the burning of magnesium, the reactants are magnesium (Mg) and oxygen (O₂). They react to form magnesium oxide (MgO). Magnesium oxide is the product, not a reactant. Oxygen alone is not the only reactant — magnesium also reacts. Heat is a condition, not a reactant. So Magnesium and Oxygen is correct.
Q44. In the skeleton equation Mg + O₂ → MgO, why is it unbalanced?
In the skeleton equation Mg + O₂ → MgO, the left side has 2 oxygen atoms (from O₂), but the right side has only 1 oxygen atom (in MgO). The number of magnesium atoms is also not balanced (1 on left, 1 on right — that part is balanced). The formula MgO is correct for magnesium oxide. The plus sign and arrow are present. So the number of oxygen atoms is not equal on both sides is correct.
Q45. Which step in balancing involves listing the number of atoms of each element?
In the balancing process, after writing the skeletal equation (Step I), Step II involves listing the number of atoms of each element present on both sides of the equation. This helps identify which elements are unbalanced. Step III is balancing the equation, and Step VII is checking. So Step II is correct.
Q46. What is the main characteristic of a skeletal chemical equation?
The main characteristic of a skeletal chemical equation is that it is unbalanced — the number of atoms of each element is not the same on both sides. It shows the correct formulae of reactants and products but does not have the correct coefficients. It does not necessarily have state symbols or reaction conditions, and it is definitely not balanced. So it is unbalanced is correct.
Q47. What is the correct word-equation for the reaction of sodium and water?
When sodium reacts with water, it forms sodium hydroxide (NaOH) and hydrogen gas (H₂). The correct word-equation is: Sodium + Water → Sodium hydroxide + Hydrogen. Sodium oxide is not formed. The reverse reaction (sodium hydroxide → sodium + water) is not the reaction of sodium with water. Sodium + Hydrogen does not give sodium hydroxide. So Sodium + Water → Sodium hydroxide + Hydrogen is correct.
Q48. What are the products in the reaction: Barium chloride + Aluminium sulphate → ?
When barium chloride (BaCl₂) reacts with aluminium sulphate (Al₂(SO₄)₃), a double displacement reaction occurs. The products are barium sulphate (BaSO₄) and aluminium chloride (AlCl₃). This is a precipitation reaction where BaSO₄ is insoluble and forms a white precipitate. So barium sulphate + aluminium chloride is correct.
Q49. In the final balanced equation, what is the physical state of the product iron oxide (Fe₃O₄)?
In the reaction of iron with steam, the product iron oxide is Fe₃O₄, which is a solid. In the balanced equation 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g), the state symbol for Fe₃O₄ is (s) — solid. It is not gas (g), aqueous (aq), or liquid (l). So (s) is correct.
Q50. What is the ultimate goal of writing and balancing a chemical equation?
The ultimate goal of writing and balancing a chemical equation is to represent a chemical change accurately and informatively. A well-written balanced equation shows the reactants, products, their proportions, and sometimes physical states and conditions. It is not to make it look complex, use many symbols, or hide details. So to represent a chemical change accurately and informatively is correct.