Chemical Reactions and Equations

When two solutions are mixed and an insoluble solid appears, the solid is called a precipitate. Its formation usually means that ions from the original solutions have rearranged to produce a new, insoluble substance; therefore, a chemical reaction has occurred. A change of state does not create a new substance, a shape change only alters form, and dissolution produces a solution rather than a precipitate. Hence option A is correct.

The correct answer is two. In \,\(2H_2 + O_2 \rightarrow 2H_2O\), the left side has 4 hydrogen atoms and 2 oxygen atoms. Placing 2 before water gives the right side 4 hydrogen atoms and 2 oxygen atoms as well. A coefficient of 1 would not balance the numbers of hydrogen and oxygen atoms.

Magnesium burns with a bright white flame because it reacts with oxygen in the air. The balanced equation is 2Mg + O₂ → 2MgO, showing that oxygen combines with magnesium to form magnesium oxide. Nitrogen is abundant in air but is not the reacting gas in this standard combustion example. Chlorine and carbon dioxide are not the intended reactants. Therefore option B is correct.

The reaction is \(2\mathrm{Mg} + \mathrm{O}_2 \rightarrow 2\mathrm{MgO}\). Here, two reactants, magnesium and oxygen, combine to form a single product, magnesium oxide. Therefore, it is a combination reaction. In a displacement reaction, one element replaces another element from its compound, which does not occur here.

Calcium oxide reacts with water to form calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Heat is released during this reaction, so the container becomes hot. Therefore, it is an exothermic reaction. In contrast, if heat were absorbed as in option A, the container would tend to become cooler.

The correct answer is A: Zn + CuSO₄ → ZnSO₄ + Cu. In a displacement reaction, a more reactive element takes the place of a less reactive element in its compound. Zinc is more reactive than copper, so zinc removes copper from copper sulfate and forms zinc sulfate; copper is released. Option B is a combination reaction because two reactants form one product, calcium hydroxide. Option C is a decomposition reaction because one compound breaks into simpler substances. Option D is a neutralisation and double-displacement reaction in which acid and base exchange ions to form salt and water. The key pattern for A is element + compound → new compound + displaced element.

Silver chloride undergoes photodecomposition in sunlight: \(2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)\). The metallic silver formed is grey, so white silver chloride appears grey. Chlorine gas is also produced, but it does not cause the grey colour.

Silver chloride decomposes in sunlight to form silver and chlorine: \(2\mathrm{AgCl}\xrightarrow{\text{sunlight}}2\mathrm{Ag}+\mathrm{Cl}_2\). A decomposition caused by light energy is called photolytic decomposition. In contrast, thermal decomposition occurs when a substance decomposes on heating.

On heating, lead nitrate undergoes thermal decomposition to form lead oxide, nitrogen dioxide, and oxygen:
\[2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO}+4\mathrm{NO_2}+\mathrm{O_2}\]
\(\mathrm{NO_2}\) is a brown gas, so nitrogen dioxide is correct. Oxygen is also formed, but it is colourless.

On heating, lead nitrate decomposes: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). Since heat is supplied to initiate the reaction, it is called a thermal decomposition reaction. Decomposition caused by electrical energy is electrolysis, which is not involved here.

Electrolysis of water is a decomposition reaction in which electric current breaks water into its constituent elements: \(2H_2O \rightarrow 2H_2 + O_2\). Thus, hydrogen and oxygen gases are produced, so option A is correct. Option D includes hydrogen, but nitrogen is not a constituent of water.

Rusting is a corrosion process in which iron is slowly oxidised in the presence of oxygen and water. Moist air supplies both necessary conditions at the surface of the grill, so rust is most likely to form there. Dry oxygen lacks moisture, and oxygen-free water lacks the oxidising gas needed for ordinary rusting. Carbon dioxide alone does not provide the complete required conditions. Thus option C is correct.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The reaction is: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Therefore, iron sulphate is formed in the solution and copper is deposited on the nail. Zinc sulphate cannot form because zinc is not present in the reaction.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The displaced copper deposits on the iron nail as a reddish-brown coating. Blue is the colour of the copper sulphate solution, not of the coating on the nail. Reaction: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\).

In a metal displacement reaction, a free metal replaces another metal from its compound only if the free metal is more reactive. For example, zinc displaces copper from copper sulphate because zinc is higher in the reactivity series. Lustre, colour, and mass do not determine whether displacement can occur. The essential property is comparative chemical reactivity, so option A is correct.

The correct answer is A: 2Mg + O₂ → 2MgO. A combination reaction occurs when two or more reactants join to form a single product. Magnesium and oxygen combine to produce magnesium oxide, so the reaction follows the pattern A + B → AB. Option B is decomposition because one compound, calcium carbonate, breaks into calcium oxide and carbon dioxide. Option C is displacement because zinc replaces copper in copper sulfate. Option D is double displacement because the ions of two compounds exchange partners and form silver chloride and sodium nitrate. The important test is to count the products: in a combination reaction, there is one main product, not two or more separate products.

An insoluble solid formed when two aqueous solutions are mixed is called a precipitate. Therefore, this is a precipitation reaction. For example, mixing silver nitrate and sodium chloride solutions produces a white precipitate of silver chloride. In a neutralisation reaction, an acid and a base generally form salt and water; formation of an insoluble solid is not its defining feature.

This is a double-displacement reaction: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions form insoluble white \(\mathrm{BaSO_4}\), so barium sulphate is the precipitate. Sodium chloride remains dissolved in water.

In a double displacement reaction, the cations and anions of two compounds exchange partners to form new compounds. Its general form is \(AB + CD \rightarrow AD + CB\). For example, in \(Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl\), ions are exchanged; heat or colour is not exchanged.

On heating, hydrogen removes oxygen from copper(II) oxide to form water, and copper(II) oxide is reduced to copper. The equation is \(\mathrm{CuO + H_2 \xrightarrow{heat} Cu + H_2O}\). Therefore, copper and water are formed. Copper sulphate cannot form because no source of sulfate ions is present in the reaction.

According to the oxygen-based definition of redox reactions, removal of oxygen from a substance is reduction. Thus, when oxygen is removed from copper(II) oxide, CuO is reduced, commonly producing copper in a suitable reaction. Oxidation is the addition of oxygen, neutralisation involves acid and base reacting, and displacement describes replacement of one element by another. Therefore option B is correct.

The reaction is CuO + H₂ → Cu + H₂O. Hydrogen combines with the oxygen removed from copper(II) oxide, so it changes into water. At the same time, CuO loses oxygen and is reduced to copper, while hydrogen gains oxygen and is oxidised. Oxygen itself is not produced, and hydrogen peroxide is not the product under these conditions. Hence option A is correct.

In the classical oxygen-transfer definition, addition of oxygen to a substance is oxidation. Therefore, the substance receiving oxygen is said to be oxidised. For example, magnesium gains oxygen to form magnesium oxide and is consequently oxidised. Reduction means removal of oxygen, neutralisation is an acid-base process, and precipitation is formation of an insoluble solid. Thus option A is correct.

Removal of oxygen from a substance is called reduction in the oxygen-based description of redox reactions. For example, copper(II) oxide can lose oxygen and change into copper, so the oxide is reduced. Oxidation is the opposite oxygen change, namely addition of oxygen. Neutralisation concerns acid-base reactions, while displacement concerns replacement of one element by another. Therefore option B is correct.

Rusting is a chemical corrosion process in which iron reacts gradually with oxygen in the presence of moisture and forms hydrated iron(III) oxide, commonly called rust. Since iron combines with oxygen, the process is classified as oxidation. Freezing and melting are physical changes of state, while filtration separates substances without forming a new compound. Therefore, oxidation is the correct choice, so option A is correct.