Decomposition reactions involve a single reactant breaking down into two or more simpler products, usually requiring energy. Combination reactions involve two or more reactants combining to form a single product, often releasing energy. Thus, their definitions and processes are reversed.
Example Decomposition: 2H2O(l) → 2H2(g) + O2(g) (electrolysis)
Example Combination: 2H2(g) + O2(g) → 2H2O(l)
More Questions on Chemical Reactions and Equations
A solution of potassium bromide is mixed with a solution of lead nitrate. What will be the observation, and what type of reaction is it?
→ 2Which type of reaction involves the breakdown of a single compound into two or more simpler substances?
→ 3How do you test for the presence of hydrogen gas?
→ 4What is the balanced equation for the reaction of hydrogen gas with chlorine gas to form hydrogen chloride gas?
→ 5If a student heats a blue crystalline solid, it turns white, and water droplets are observed on the cooler parts of the test tube. What is the blue crystalline solid?
→ 6In the reaction: FeSO4(aq) + Cu(s) → No reaction. Why does this happen?
→ 7Which gas is released when dilute hydrochloric acid reacts with calcium carbonate?
→ 8Explain why an iron gate needs to be painted regularly.
→ 9What are the reactants required for the process of photosynthesis?
→ 10Which of the following is an example of a precipitate forming reaction?
→ 11Consider the reaction: 2SO2(g) + O2(g) → 2SO3(g). If this reaction is carried out at high temperature and pressure, how would it affect the rate of reaction?
→ 12What is the common term for a reaction that involves both oxidation and reduction simultaneously?
→ 13Identify the reaction where electricity is used to break down a compound.
→ 14Why are antioxidants added to food items containing fats and oils?
→ 15A white solid on heating produces a gas that turns limewater milky and a residue that is yellow when hot but white when cold. Identify the white solid.
→ 16Which type of reaction is the thermite reaction (Fe2O3 + 2Al → Al2O3 + 2Fe)?
→ 17Assertion (A): Rusting of iron is a fast process. Reason (R): Rusting requires specific conditions of air and moisture to occur.
→ 18Balance the equation: C6H12O6 + O2 → CO2 + H2O. What is the coefficient of O2?
→ 19Why is a black coating formed on the surface of some utensils exposed to a gas flame for a long time?
→ 20What is the general name for a reaction that releases energy, often in the form of heat or light?
→ 21In the reaction: Fe2O3 + 2Al → Al2O3 + 2Fe. Which element is acting as the reducing agent?
→ 22Give a reason why balanced chemical equations are important for industrial processes.
→ 23What type of chemical reaction is involved in the digestion of food?
→ 24When a student observed that a certain metal tarnished over time, forming a green layer, what could be the metal?
→ 25Which of the following reactions is an example of electrolytic decomposition?
→ 26Why does food cook faster in a pressure cooker? Relate it to chemical reactions.
→ 27Assertion (A): White silver chloride turns grey in sunlight. Reason (R): Decomposition of silver chloride in the presence of sunlight forms silver metal and chlorine gas.
→ 28What is the purpose of adding iron filings to a solution of copper sulphate in a laboratory experiment?
→ 29A reaction is represented as A + B → C + D. If A and B are reactants, and C and D are products, what must be true about the total mass of A and B compared to C and D?
→ 30What is the term for a substance that slows down the rate of rancidity?
→ 31When a student mixes solutions of copper sulphate and iron filings, what change would be observed over time?
→ 32Consider the reaction: Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen. Is this a redox reaction? Justify your answer.
→ 33Why is it dangerous to add water to concentrated acid rapidly?
→ 34Balance the following chemical equation: Fe + H2O → Fe3O4 + H2. What is the sum of the coefficients of the reactants?
→ 35Which of the following substances will show a color change when heated strongly, and what is the color change?
→ 36Why is solid calcium oxide stored in airtight containers?
→ 37What type of reaction occurs when an acid reacts with a base?
→ 38Identify the reducing agent in the following reaction: CuO + H2 → Cu + H2O.
→ 39A student performs an experiment where he adds dilute sulphuric acid to barium chloride solution. What visual change will he observe?
→ 40How is the phenomenon of 'corrosion' different from 'rancidity'?
→ 41Balance the equation: Al + H2SO4 → Al2(SO4)3 + H2. What is the coefficient of H2SO4?
→ 42Which of the following indicates that a chemical reaction has taken place?
→ 43Give an example of a reaction where a gas is evolved and a change in temperature is observed.
→ 44Why are decomposition reactions generally endothermic?
→ 45A student observes bubbles forming when a dilute acid is added to a metal sample. What gas is most likely being produced?
→ 46Assertion (A): Photosynthesis is considered an endothermic reaction. Reason (R): Energy is absorbed from sunlight during photosynthesis.
→ 47What is the general term for a reaction in which a more reactive metal displaces a less reactive metal from its salt solution?
→ 48When solid lead nitrate is heated, it decomposes. Name the gas(es) evolved that have a reddish-brown colour.
→ 49Identify the reaction type: A + Oxygen → AO (e.g., C + O2 → CO2).
→ 50What is meant by rancidity and how can it be prevented?
→ 51Write the balanced chemical equation for the reaction of sodium with water.
→ 52Why does the color of copper sulphate solution change when an aluminium foil is dipped in it?
→ 53Which type of reaction is characterized by the exchange of ions between two compounds?
→ 54Give one example of a combination reaction that is also exothermic.
→ 55What is indicated by the symbol (aq) next to a chemical formula in an equation?
→ 56A solution of sodium carbonate is added to a solution of barium chloride. Which of the following observations is true?
→ 57Which of the following metals would displace hydrogen from dilute acids?
→ 58What is the role of a catalyst in a chemical reaction?
→ 59Assertion (A): Oxidation involves the loss of electrons. Reason (R): Reduction involves the gain of electrons.
→ 60***New Question Draft (replacing the problematic one):*** When 10g of calcium carbonate is heated, it decomposes to calcium oxide and carbon dioxide. If 5.6g of calcium oxide is formed, what mass of carbon dioxide is produced?
→ 61Consider the reaction: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq). If 10g of AgNO3 reacts completely with NaCl, and 8.5g of NaNO3 is formed along with AgCl. What is the mass of AgCl formed?
→ 62What are the conditions necessary for rusting to occur?
→ 63A solution turns blue litmus red. It reacts with zinc metal to evolve a gas that burns with a 'pop' sound. What type of substance is the solution?
→ 64Why is it important to balance chemical equations?
→ 65When magnesium ribbon is burnt in air, a white powder is formed. What is the chemical name of this powder?
→ 66Consider the reaction: 2Pb(NO3)2(s) --(Heat)--> 2PbO(s) + 4NO2(g) + O2(g). What type of reaction is this?
→ 67Balance the following chemical equation: (NH4)2Cr2O7 → N2 + Cr2O3 + H2O. What is the sum of the stoichiometric coefficients of the products?
→ 68What observation helps to distinguish between a physical change and a chemical change in a reaction?
→ 69Identify the type of reaction when an aqueous solution of iron(III) chloride is mixed with an aqueous solution of ammonium hydroxide.
→ 70During the electrolysis of water, if 20 mL of hydrogen gas is collected at one electrode, what volume of oxygen gas will be collected at the other electrode under the same conditions?
→ 71A solution of substance X is used for whitewashing. The substance X is formed by the reaction of water with what?
→ 72Which of the following reactions is used for the preparation of oxygen in the laboratory?
→ 73Why is respiration considered an exothermic reaction?
→ 74What is the significance of the arrow pointing downwards (↓) in a chemical equation?
→ 75In the reaction represented by the equation: MnO2 + 4HCl → MnCl2 + 2H2O + Cl2. Identify the oxidizing agent.
→ 76A student dropped a few pieces of marble in dilute hydrochloric acid contained in a test tube. The gas evolved was passed through limewater. What changes would be observed in the limewater?
→ 77Which of the following is an example of a redox reaction?
→ 78What happens when an iron nail is dipped in a copper sulphate solution?
→ 79Assertion (A): Calcium carbonate decomposes into calcium oxide and carbon dioxide when heated. Reason (R): The decomposition reaction requires energy in the form of heat.
→ 80When hydrogen gas is passed over heated copper(II) oxide, a black substance, it turns reddish-brown. Write the balanced chemical equation for this reaction and identify the substance reduced.
→ 81A shiny brown element X on heating in air becomes black. Name the element X and the black-coloured compound formed.
→ 82Which of the following processes involves a chemical reaction?
→ 83Why are silver articles found to turn black when exposed to air for a prolonged period?
→ 84Consider the following reaction: A + B → C + Heat. What can be inferred about this reaction?
→ 85A student places a piece of copper wire in a test tube containing silver nitrate solution. After some time, the solution turns bluish, and a shiny gray solid deposits on the copper wire. i) Write the balanced chemical equation for the reaction. ii) Identify the oxidizing agent and the reducing agent in this reaction.
→ 86How does painting prevent corrosion?
→ 87Consider the reaction: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l). i) Is this reaction exothermic or endothermic? Justify your answer. ii) What type of reaction is this?
→ 88What are the two main types of corrosion? Give an example of each.
→ 89Which of the following metals would react most vigorously with dilute hydrochloric acid?
→ 90Explain why reduction and oxidation always occur simultaneously in a chemical reaction.
→ 91A white solid, when strongly heated, melts and then decomposes to produce a brownish gas and a residue that is yellow when hot and white when cold. Identify the white solid.
→ 92Why is the reaction between hydrogen gas and chlorine gas to form hydrogen chloride gas considered a combination reaction?
→ 93During a laboratory experiment, a student mixes two colorless solutions, X and Y. Immediately, a blue precipitate forms. What could be X and Y?
→ 94What is the significance of the state symbols (s), (l), (g), and (aq) in a balanced chemical equation?
→ 95Consider the reaction: 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g). If this reaction is carried out in a sealed container, what observation might violate the Law of Conservation of Mass if the container was not sealed?
→ 96Why are decomposition reactions typically endothermic?
→ 97When a few drops of phenolphthalein indicator are added to a solution of sodium carbonate, the solution turns pink. What will happen if dilute hydrochloric acid is added drop by drop to this solution?
→ 98Which of the following is an example of a single displacement reaction?
→ 99A student performs an experiment where he mixes an aqueous solution of Barium Chloride with an aqueous solution of Sodium Sulphate. i) What is the immediate observation? ii) Write the balanced chemical equation for the reaction. iii) What type of reaction is this?
→ 100Distinguish between an oxidizing agent and a reducing agent in terms of electron transfer.
→ 101During the electrolysis of water, if the volume of hydrogen collected is 10 mL, what will be the volume of oxygen collected at the same conditions?
→ 102A chemist tests two unknown white powders. Powder A, when heated, produces a gas that rekindles a glowing splint. Powder B, when heated, produces a gas that turns limewater milky. Identify Powder A and Powder B.
→ 103When a student burns a piece of magnesium ribbon in a jar of oxygen, a white powder is formed. If this white powder is then dissolved in water and a drop of red litmus paper is added, what color change will be observed and why?
→ 104State one difference between displacement and double displacement reactions.
→ 105Which of the following compounds is a strong oxidizing agent often used in redox titrations?
→ 106How can you demonstrate experimentally that the reaction of hydrogen and oxygen to form water is a combination reaction?
→ 107Describe the role of oxygen in both corrosion and rancidity.
→ 108What is the color change observed when ferrous sulphate crystals are heated?
→ 109Why is it necessary to balance a chemical equation? Explain with reference to a fundamental law.
→ 110Predict the product(s) of the following reaction and identify the type of reaction: Al(s) + HCl(aq) →
→ 111Which of the following reagents can be used to distinguish between magnesium oxide and calcium oxide?
→ 112A student observed that when dilute hydrochloric acid was added to a piece of magnesium ribbon, the temperature of the test tube increased. What type of reaction is this?
→ 113Why are certain types of chemical reactions, like photosynthesis and thermal decomposition, considered endothermic?
→ 114An aqueous solution of metal nitrate P reacts with sodium bromide solution to form a yellow precipitate of compound Q which is used in photography. Q on exposure to sunlight undergoes decomposition to form a metal R and a reddish-brown gas. Identify P, Q, and R.
→ 115What is the role of manganese dioxide (MnO2) in the decomposition of potassium chlorate (KClO3)?
→ 116Give an example of a reaction that is both a combination reaction and an exothermic reaction.
→ 117A student wanted to store fresh potato chips for a long time. What method should they use to prevent them from becoming rancid?
→ 118Which of the following observations indicates that a chemical reaction has taken place?
→ 119When silver chloride is exposed to sunlight, it decomposes to form silver metal and chlorine gas. i) Write the balanced chemical equation for this reaction. ii) What type of reaction is this? iii) What is the commercial application of this reaction?
→ 120Consider the reaction: Fe2O3(s) + 2Al(s) → Al2O3(s) + 2Fe(s). Which of the following statements about this reaction is incorrect?
→ 121A student observed that on mixing solutions of two salts, a precipitate was formed. What are the common characteristics of such reactions?
→ 122Which of the following is a balanced equation?
→ 123Why is photosynthesis considered an endothermic reaction?
→ 124In the decomposition of lead nitrate powder, what is the color of the gas evolved and the residue left?
→ 125A student noted that a strip of aluminum metal was placed in a beaker containing a solution of copper chloride. After some time, the blue color of the solution faded, and a reddish-brown deposit was observed on the aluminum strip. What is the reason for this observation?
→ 126Which of the following reactions is used for the preparation of oxygen?
→ 127Define a redox reaction. Give one example.
→ 128Why are chips manufacturers flush bags of chips with nitrogen gas?
→ 129Consider the reaction: CuO(s) + H2(g) → Cu(s) + H2O(l). Identify the substance undergoing reduction.
→ 130When a zinc granule is added to a test tube containing dilute sulphuric acid, a gas is evolved. What is the gas, and how would you test for it?
→ 131Which of the following is not a characteristic of a chemical reaction?
→ 132Balance the following chemical equation: KMnO4 + HCl → KCl + MnCl2 + H2O + Cl2
→ 133In an experiment, when a student added a few drops of universal indicator to a solution of sodium hydroxide, the color changed to blue. What kind of reaction would occur if hydrochloric acid was then slowly added to this solution?
→ 134Which of the following statements about the given reaction are correct? 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) (i) Iron metal is rusting. (ii) Water is getting reduced. (iii) Water is acting as a reducing agent. (iv) Iron is getting oxidized.
→ 135A solution of a substance 'X' is used for whitewashing. i) Name the substance 'X' and write its formula. ii) Write the reaction of the substance 'X' with water.
→ 136Identify the substance oxidized in the following reaction: ZnO + C → Zn + CO
→ 137Why does a brown layer form on iron articles when left exposed to air and moisture for some time?
→ 138A student dropped a small piece of marble into a test tube containing dilute hydrochloric acid. A gas evolved which turned lime water milky. What type of reaction occurred?
→ 139Which of the following is an exothermic process?
→ 140Explain why iron articles are often painted.
→ 141When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is formed and sulphuric acid is formed. The reaction is an example of:
→ 142Which of the following processes involves a chemical change?
→ 143A shiny brown element X on heating in air becomes black. Name the element X and the black-colored compound formed.
→ 144When a student mixes solution A (containing lead(II) nitrate) with solution B (containing potassium iodide), a yellow precipitate forms. What type of chemical reaction is this, and write the balanced chemical equation?
→ 145What do you mean by a precipitation reaction? Explain by giving examples.
→ 146In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
→ 147What is the difference between displacement and double displacement reactions? Write equations for these reactions.
→ 148Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
→ 149Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions
→ 150Why is respiration considered an exothermic reaction? Explain
→ 151What does one mean by exothermic and endothermic reactions? Give examples.
→ 152What is a balanced chemical equation? Why should chemical equations be balanced?
→ 153What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer. (a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced
→ 154Fe₂O₃ + 2Al → Al₂O₃ + 2Fe The above reaction is an example of a (a) combination reaction. (b) double displacement reaction. (c) decomposition reaction. (d) displacement reaction.
→ 155Identify the substances that are oxidised and the substances that are reduced in the following reactions. (i) 4Na(s) + O2(g) → 2Na2O(s) (ii) CuO(s) + H2(g) → Cu(s) + H2O(l)
→ 156Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
→ 157A solution of a substance ‘X’ is used for whitewashing. (i) Name the substance ‘X’ and write its formula. (ii) Write the reaction of the substance ‘X’ named in (i) above with water.
→ 158Write a balanced chemical equation with state symbols for the following reactions. (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride. (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water
→ 159Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine → Hydrogen chloride (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (iii) Sodium + Water → Sodium hydroxide + Hydrogen
→ 160Why should a magnesium ribbon be cleaned before burning in air?
→ 161A solution of substance ‘X’ is used for white washing. What is the substance ‘X’? State the chemical reaction of ‘X’ with water.
→ 162Hydrogen being a highly inflammable gas and oxygen being a supporter of combustion, yet water which is a compound made up of hydrogen and oxygen is used to extinguish fire. Why?
→ 163What is meant by a chemical reaction?
→ 1641 g of copper powder was taken in a China dish and heated. What change takes place on heating? When hydrogen gas is passed over this heated substance, a visible change is seen in it. Give the chemical equations of reactions, the name and the colour of the products formed in each case.
→ 165Oil and fat containing food items are flushed with nitrogen. Why?
→ 166A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
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