Class 10 Science Chapter 1 Previous Year Questions - Chemical Reactions and Equations

Ans: (c)
A decomposition reaction is when a compound breaks down into simpler substances. Option (c) 2AgBr → 2Ag + Br₂ is the correct answer because it requires light energy to break down silver bromide (AgBr) into silver (Ag) and bromine (Br₂). The other options do not use light for decomposition.

Ans: (b)
When sodium hydroxide (NaOH) reacts with zinc (Zn), it produces sodium zincate and hydrogen gas. The balanced equation 2NaOH + Zn → Na₂ZnO₂ + H₂ shows that two molecules of sodium hydroxide react with one piece of zinc to form sodium zincate and hydrogen gas. This is the correct representation of the reaction, making option (b) the right answer.

Ans: (b)
Burning of coal involves the reaction of carbon (coal) with oxygen to form carbon dioxide: $C\; (s)\; +\; O$C (s) + O₂(g) → CO₂(g). This is a combination reaction as two substances (carbon and oxygen) combine to form a single product (carbon dioxide).

Ans: (d)
In these cases, we are looking at reactions between metals and metal sulfates. New products will form when a more reactive metal displaces a less reactive metal from its compound. In (B) and (D), calcium and magnesium are more reactive than copper and zinc, so they will successfully displace them, resulting in new products.

Ans: (c)
In the reaction, hydrogen sulfide (H₂S) donates electrons, which means it acts as a reducing agent, while sulfur dioxide (SO₂) accepts electrons and acts as an oxidizing agent. Therefore, the correct statement is (c) because SO₂ is the oxidizing agent and H₂S is the reducing agent in this reaction.

Ans: (a)
To balance the chemical equation Al₂O₃ + HCl → AlCl₃ + H₂O , we need to ensure that the number of each type of atom is the same on both sides. The correct coefficients are 1 for Al₂O₃, 6 for HCl, 2 for AlCl₃, and 3 for H₂O, giving option (A) as the right choice. This means we have balanced aluminum, chlorine, hydrogen, and oxygen atoms in the reaction.

Ans: (b)
In the reactions listed, options (a), (c), and (d) involve double displacement or exchange of ions between reactants, where compounds are formed by swapping partners. However, option (b) is a combination reaction, where calcium oxide (CaO) reacts with water (H₂O) to form calcium hydroxide (Ca(OH)₂) without exchanging ions. This makes (b) different from the others.

Ans: (b)
In the given reaction, zinc (Zn) replaces hydrogen in acetic acid (CH₃COOH) to form zinc acetate (Zn(CH₃COO)₂) and hydrogen gas (H₂). This type of reaction, where one element displaces another from a compound, is called a displacement reaction. Therefore, option (b) is correct.

Ans: (c)
To balance the equation, we first ensure the number of zinc atoms matches on both sides, leading us to set x = y. Then, balancing nitrogen gives z = 2x since there are twice as many nitrogen atoms in NO₂ compared to zinc. Finally, oxygen atoms are balanced by setting 6x = y + 2z + 2, solving which confirms x = 2,  y = 2, and z = 4. This makes the chemical equation balanced with coefficients 2, 2, 4, respectively.

Ans: (d)
A redox reaction involves both oxidation and reduction, where electrons are transferred between substances. In option (d), iron(III) oxide (Fe₂O₃) is reduced to iron (Fe), while carbon monoxide (CO) is oxidized to carbon dioxide (CO₂). Unlike the other options, which are combination reactions, this one shows the reduction and oxidation of different substances, making it a redox reaction but not a combination reaction.

Ans: Combination reaction – Single product is formed (or any other)

Ans: (i) Change in colour: The solution will become green in colour.

(ii) Change in temperature: The temperature will increase.

(iii) Formation of precipitate: Yellow precipitate of PbI2 is formed.

Ans: 2 Mg + O₂ → 2MgO
Magnesium oxide
Type – Combination reaction
Reason: Two or more substances combine to form a single product.

Ans: Fe(s) + CuSO₄ (aq) → FeSO₄(aq) + Cu(s)
Displacement reaction: A reaction in which a more reactive metal displaces a less reactive metal from its salt solution.
Zinc, Aluminium, Calcium, Magnesium

Ans: (a)
In this scenario, both the Assertion (A) and the Reason (R) are true. When zinc reacts with nitric acid, hydrogen gas is not produced because the nitric acid oxidizes any hydrogen gas formed into water while itself getting reduced. Thus, the reason correctly explains why hydrogen gas is not evolved, making option (a) the right choice.

Ans: A chemical reaction involves the breaking and making of bonds between atoms to produce new substances. when reactant changes to products.
(i) Add lead nitrate solution to potassium iodide solution taken in a test tube. The colour changes from colourless solution to yellow ppt.

(ii) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

Ans: (i) A reactant breaks down to give two or more products. A reaction which requires energy to split a compound or reactant in two or more simple substances.
(I) Water splits into hydrogen gas and oxygen gas.
Electrical energy
(II) Silver bromide decomposes into silver and bromine
Light energy
(ii) (I) Formation of calcium oxide:
It is an endothermic reaction/decomposition reaction.(II) Formation of calcium hydroxide:
It is exothermic/combination reaction

Ans: (a) Copper Oxide
Black
OR(b)

Ans: (A) 

Sunlight is essential for the above reaction to take place.  This is a decomposition reaction. Such reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Silver chloride turns grey after its decomposition into silver and chlorine by sunlight.  This reaction is used in black and white photography.

(B) 

Ans: (b)
When potassium iodide (KI) and lead nitrate (Pb(NO₃)₂) are mixed, they react to form lead iodide (PbI₂), which is an insoluble substance that precipitates out of the solution, and potassium nitrate (KNO₃). The correct balanced equation for this reaction is 2KI+ Pb(NO₃)₂ → Pbl₂ + 2KNO₃. This shows that two potassium iodide molecules are needed to react with one lead nitrate molecule to produce one lead iodide and two potassium nitrate molecules, confirming option (b) as the correct answer.

Ans: (c)
The balanced chemical equation for the reaction between quick lime (calcium oxide, CaO) and water CaO + H₂O → Ca(OH)₂ + Heat. In this reaction, quick lime reacts with water to form calcium hydroxide (Ca(OH)₂), which is also known as slaked lime, and releases heat. This makes option (c) the correct answer, as it accurately represents the products and the heat released during the reaction.

Ans: (a)
Sol: The reaction in which oxygen is added or hydrogen is removed or loss of electrons takes place is called an oxidation reaction. In the reaction,
(i) Carbon is getting oxidised to carbon monoxide.
(ii) Zinc oxide is getting reduced to zinc.
Carbon is a reducing agent that reduces ZnO to Zn.

Ans: (a)
Sol: Reaction of quick lime (CaO) with water is an exothermic reaction because CaO reacts vigorously with water releasing a large amount of heat.
CaO_(s) + H₂O_(I) → Ca(OH)_2(ag) + Heat

Ans: 

(i) Acid is added to water before electrolysis to increase its conductivity. This allows the current to pass through the solution easily. Hydrogen gas is liberated at the cathode, while oxygen gas is liberated at the anode. The volume of gas collected at the anode is twice the volume of gas collected at the cathode. 2H₂O(l) → 2H₂(g) + O₂(g)
(ii) When silver chloride is exposed to sunlight, it decomposes to form silver metal and chlorine gas. 2AgCl(s) → 2Ag(s) + Cl₂(g) This is a photochemical decomposition reaction.

Ans: 

(a) The chemical reaction in which two reactants exchange ions to form two new compounds is called a double displacement reaction.
(b) (i) When an aqueous solution of an acid reacts with a base (alkali) by exchanging their ions/radicals to form salt and water as the only products, the reaction which takes place is called neutralisation reaction.

(ii) When the aqueous solutions of two ionic compounds react by exchanging their ions/radicals, to form two or more new compounds such that one of the products formed is an insoluble salt, and hence forms precipitate, the double displacement reaction is said to be precipitation reaction. When lead nitrate solution is mixed with potassium iodide solution, a yellow precipitate is formed. This reaction is a precipitation reaction and can be expressed as follows:

Ans: (a)
When lead nitrate is heated, it undergoes thermal decomposition, resulting in the formation of lead oxide (a yellow solid), nitrogen dioxide (a brown gas), and oxygen. The brown fumes observed in this setup are due to the release of nitrogen dioxide gas.
The reaction is as follows:

Thus, the correct answer is (a) thermal decomposition of lead nitrate which produces brown fumes of nitrogen dioxide.

Ans: (b)
When carbon dioxide is passed through aqueous calcium hydroxide (lime water), it initially reacts to form calcium carbonate, which is insoluble in water and causes the solution to turn milky:However, when excess carbon dioxide is passed through the solution, the calcium carbonate further reacts with the carbon dioxide and water to form calcium hydrogen carbonate, which is soluble in water, causing the solution to become clear again:

Thus, the correct answer is (b).

Ans: (a)
Assertion (A): The color of the aqueous solution of copper sulfate turns colorless when a piece of lead is added to it. This is true. Copper sulfate solution is blue due to the presence of copper ions. When lead is added, it reacts with copper sulfate, displacing copper and forming lead sulfate, which is colorless in solution.
Reason (R): Lead is more reactive than copper and displaces copper from its salt solution. This is also true. Lead, being more reactive, displaces copper ions from copper sulfate, resulting in the precipitation of copper and the formation of colorless lead sulfate in the solution.
Since the reason correctly explains the assertion, the correct answer is (a)

Ans: (A) (i) NH₃ is the reducing agent because it gets oxidised to NO by the removal of hydrogen and addition of oxygen. O₂ has been reduced to H₂O by the addition of hydrogen. 
(ii) H₂O is the reducing agent. Here, F₂ gets reduced to HF (addition of hydrogen) and H₂O gets oxidised to HOF (removal of hydrogen).
(iii) CO is the reducing agent. Here, CO has been oxidised to CO₂ by the addition of oxygen. Fe₂O₃ has been reduced to Fe by the removal of oxygen.
(iv) H₂ is the reducing agent as it gets oxidised to H₂O by the addition of oxygen. O₂ has been reduced to H₂O by the addition of hydrogen. 
(B) The reaction in which one element gets oxidised or addition of oxygen occurs and other element gets reduced or removed of oxygen occurs in other element is called redox reaction. Example:

Ans: (b)
In the reaction between sodium (Na) and water (H₂O), two sodium atoms react with two water molecules to produce two sodium hydroxide (NaOH) molecules and one hydrogen gas (H₂) molecule. The balanced equation shows that two sodium atoms are needed for every two water molecules, which is why option (b) is correct. It accurately represents the conservation of mass in the reaction.

Ans: (c)
Sol: A balanced equation follows law of conservation of mass that means the total mass of reactants is equal to the total mass of products but the chemical composition of reactants does not remain same before and after the reaction.

_{Ans: (c)
Sol: In the process of respiration, glucose combines with oxygen in cells of our body and provides energy. Thus, respiration is an exothermic process.}
C₆H₁₂O_6(aq) + 6O_2(g) → 6CO_2(g) + 6H₂O_(I) + Energy

Ans: (c)
In the reaction, hydrochloric acid (HCl) loses electrons and is converted to chlorine gas (Cl₂), so statement (i) is correct. Manganese dioxide (MnO₂) gains electrons and is transformed into manganese chloride (MnCl₂), making statement (ii) correct as well. However, MnCl₂ does not act as an oxidizing agent; it is the reduced form of manganese. Therefore, the correct statements are (i) and (ii), which is why option (c) is the right choice.

Ans: (d)
In this case, the Assertion (A) is false because the burning of natural gas (which is primarily methane, CH₄) is actually an exothermic process, meaning it releases heat. The Reason (R) is true because methane does combine with oxygen to produce carbon dioxide (CO₂) and water (H₂O) during combustion. Thus, option (d) is the correct choice, stating that the Assertion is false while the Reason is true.

Ans: (d)
Sol: During sublimation of dry ice, heat is absorbed, so, it is an endothermic process. Dissolution of NH₄CI in water is also an endothermic process.

Ans: (a)

When lead nitrate (Pb(NO₃)₂) is heated, it decomposes and produces brown fumes of nitrogen dioxide (N_O2). This happens because the lead nitrate breaks down into lead oxide (PbO), nitrogen dioxide (NO₂), and oxygen (O₂). The brown color of the fumes is characteristic of nitrogen dioxide, making option (a) the correct answer.

Ans: (c)
Sol: Silver salt (AgCl) are used in black and white photography. silver salt (AgCl) is photo sensitive compound, it decomposes into elemental chlorine (Cl₂) and Ag(metal).
AgBr is also used as black and white photography.

Ans: 

AgCI decomposes on absorbing light energy.

Copper metal undergoes oxidation. Zn displaces Cu from CuSO₄ solution. Colour changes from blue to colourless.

Ans: 

(a) 'M' is Mg and 'N' is MgO

(c) 'M' undergoes oxidation in this reaction because Mg gain oxygen to form MgO.

Ans: The equation which contains an equal number of atoms of each element on both sides of the arrow is called a balanced chemical equation.

Ans: The reaction between CaO and H₂O to form Ca(OH)₂ is an exothermic and combination reaction.

Ans: Reactants: Nitric acid, calcium hydroxide (slaked lime)
Products: Calcium nitrate, water

Ans: (d)
Assertion (A): Burning of natural gas is an endothermic process. This is incorrect. Burning (or combustion) of natural gas, which mainly consists of methane, is an exothermic process, meaning it releases heat.
Reason (R): Methane gas combines with oxygen to produce carbon dioxide and water. This statement is correct. During combustion, methane reacts with oxygen to produce carbon dioxide and water as products.
Since the assertion is false, but the reason is true, the correct answer is (d) (A) is false but (R) is true.

Ans: (b)
Sol: Formation of crystals is a physical change while others are chemical change.

Ans: (a)
In this question, both the Assertion (A) and the Reason (R) are true. The balanced chemical equation 3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + 4H₂ (g) correctly represents the reaction of steam with iron, forming iron(II,III) oxide (Fe₃O₄) and hydrogen gas (H₂). The Reason (R) states that the law of conservation of mass holds, meaning that mass is neither created nor destroyed in a chemical reaction, which is indeed reflected in the balanced equation. Therefore, option (a) is correct, as the Reason accurately explains the Assertion.

Ans: (d)

When hydrogen sulfide (H₂S) is passed through a blue solution of copper sulfate (CuSO₄), it forms a black precipitate of copper sulfide (CuS) while sulfuric acid (H₂SO₄) remains in the solution. This process involves the exchange of ions between the reactants, characteristic of a double displacement reaction. In this type of reaction, the ions from both compounds swap partners, which makes option (d) the correct answer.

Ans: (d)
In a double displacement reaction, like the one between sodium sulfate and barium chloride, the ions in the reactants swap places. This results in the formation of an insoluble salt (barium sulfate), which is a solid that separates out (precipitate) from the solution. So, options (B), (C), and (D) are all correct because they describe the ion exchange and the formation of a precipitate.

Ans: 

AgCl decomposes on absorbing light energy.

Copper metal undergoes oxidation.

Zn displaces Cu from CuSO₄ solution. Colour changes from blue to colourless.

Ans: (b)
When copper is left exposed to air, it reacts with oxygen and carbon dioxide, leading to the formation of a green coating called copper carbonate (CuCO₃). This green layer is a sign of corrosion and happens over time as the copper oxidizes, which is why option (b) is correct.

Ans: Food materials containing fats and oils change their taste and smell due to a process called rancidity. Rancidity is a process in which air reacts with fats and oils which changes the smell and taste of food. Methods of prevention : Vacuum packing, refrigeration of food materials, placing of food materials away from direct sunlight.

Ans: (a) At anode: Oxygen gas is liberated. At cathode: Hydrogen gas is liberated.

(b) In the test tube covering the cathode, the amount of gas collected is double than that of the gas collected in the test tube covering the anode due to stochiometry.

2H₂O → 2H₂ + O₂

_{(c) Addition of a few drops of sulphuric acid make water a good conductor of electricity.} 

_{Ans: When copper powder is heated in a China dish, the reddish brown surface of copper powder becomes coated with a black substance which is copper oxide.
 
When hydrogen gas is passed over CuO, the black coating on the surface turned reddish brown due to the formation of Cu.}

Ans: (i) Compound A is calcium oxide (CaO) and compound B is calcium hydroxide (Ca(OH)₂).
(ii) The chemical equation for the reaction of A (calcium oxide) with water is:
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
(iii) The reaction can be classified as a combination reaction and an exothermic reaction. It is a combination reaction because two substances combine to form a new compound, and it is exothermic because it evolves a large amount of heat.

Ans: (i) The type of reaction in which the reaction mixture becomes warm is an exothermic reaction. An example of such a reaction is the combustion of methane (CH₄):
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + heat
(ii) The type of reaction in which an insoluble substance is formed is a precipitation reaction. An example of such a reaction is the reaction between silver nitrate (AgNO₃) and sodium chloride (NaCl):
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
In this reaction, silver chloride (AgCl) is formed as an insoluble substance, which precipitates out of the solution.

Ans: (a) The compound precipitated is lead iodide (PbI₂), which is yellow in color.
(b) The balanced chemical equation for the reaction is:
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
(c) The type of this reaction is a precipitation reaction. In this reaction, a solid (precipitate) is formed when two solutions are mixed together. In this case, lead iodide is formed as a yellow precipitate.

Ans: (A) Electrolytic decomposition of water/ electrolysis of water. 
(B) The gas collected at cathode is hydrogen which is double the volume of oxygen collected at anode. 
(C) 
(D) Water is not a good conductor of electricity sulphuric acid is added in the water to make it a good conductor of electricity.

Ans: 3BaCl₂ + Al₂(SO₄)₃ → 2AICI₃ + 3BaSO₄

Ans: Copper is less reactive than Fe, hence copper cannot replace iron from ferrous sulphate solution, so no colour change is observed. Aluminium is more reactive than Fe, hence displace iron from ferrous sulphate solution.
2Al + 3FeSO₄ → 3Fe + Al₂(SO₄)₃ 
Green colour of solution due to FeSO₄ changes to colourless.
This is an example of single displacement reaction.

Ans: (i) Test tube should be dried properly.
(ii) Hold the test tube in a test tube holder.

Ans: When silver chloride (AgCl) is exposed to sunlight, it undergoes a decomposition reaction and gets converted into silver (Ag) and chlorine gas (Cl₂). The balanced chemical equation for this reaction is:

2AgCl(s) → 2Ag(s) + Cl₂(g)

The observation in this case would be that the white silver chloride gradually turns grayish-white or silver-colored due to the formation of silver, and the chlorine gas may be seen as a pale yellowish-green gas. This is an example of a photodecomposition reaction, where light energy is used to break down the compound into its elements.

Ans: (a) The type of reaction taking place is a single displacement reaction. Zinc (Zn) displaces silver (Ag) from silver nitrate (AgNO₃) to form zinc nitrate (Zn(NO₃)₂) and silver (Ag). The balanced chemical equation for this reaction is:
Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(b) The type of reaction taking place is a double displacement reaction or a precipitation reaction. Potassium iodide (KI) reacts with lead nitrate (Pb(NO₃)₂) to produce potassium nitrate (KNO₃) and lead iodide (PbI₂). The balanced chemical equation for this reaction is:
2KI(aq) + Pb(NO₃)₂(aq) → 2KNO₃(aq) + PbI₂(s)

Ans: (a) The color of the precipitate formed is yellow. The compound precipitated is lead iodide (PbI2)
(b) The balanced chemical equation for this reaction is:
2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq)
(c) The two types of reactions in which this reaction can be placed are a double displacement reaction and a precipitation reaction. In a double displacement reaction, the positive and negative ions of two compounds switch places to form new compounds. In a precipitation reaction, a solid precipitate is formed when two solutions are mixed together.

Ans: (a) Two observations during the heating of ferrous sulfate crystals are:
1. The crystals lose water and become anhydrous.
2. The color of the crystals changes from green to a reddish-brown color.
(b) The type of chemical reaction taking place is a thermal decomposition reaction.
(c) The balanced chemical equation for the reaction is:
FeSO₄⋅7H₂O(s) → Fe₂O₃(s) + SO₂(g) + H₂O(g)
The products formed are ferric oxide (Fe₂O₃), sulfur dioxide (SO₂), and water (H₂O).

Ans: (a) The black substance is formed because copper reacts with oxygen in the air to form copper oxide.
(b) The black substance is copper oxide (CuO).
(c) The chemical equation for the reaction is:
2Cu(s) + O₂(g) → 2CuO(s)
(d) The black coating on the surface can be turned reddish-brown by reducing it back to copper. This can be done by passing hydrogen gas over the hot copper oxide. The chemical equation for this reaction is:
CuO(s) + H₂(g) → Cu(s) + H₂O(g)
The reduction reaction converts the black copper oxide back to reddish-brown copper.

Ans: (A) Take a small amount of lead nitrate powder in a boiling tube. Hold the boiling tube with a pair of tongs and heat it over the flame first gently and then strongly. 
(B) Labelled diagram of the experimental setup:

Two main observations:

(i) We observe emission of brown fumes of a gas which is nitrogen dioxide.
(ii) The white colour of lead nitrate changes to yellow colour as lead oxide is formed.
(C) Balanced equation:

Ans: Decomposition reaction with heat energy supplied:

ZnCO₃(s) → ZnO(s) + CO₂(g)

Decomposition reaction with light energy supplied:

2H₂O₂(l) → 2H₂O(l) + O₂(g)

Decomposition reaction with electrical energy supplied:

2AgCl(s) → 2Ag(s) + Cl₂(g)

Ans: (i) On touching the test tube from outside, you will feel the test tube becoming cold.
(ii) The inference about the type of reaction that occurred is that it is an endothermic reaction. In endothermic reactions, heat is absorbed from the surroundings, resulting in a decrease in temperature
(iii) The balanced chemical equation for the reaction is:
Ba(OH)₂(s) + 2NH₄Cl(s) → BaCl₂(aq) + 2NH₃(g) + 2H₂O(l)

Ans: (a) Consider the following displacement reaction:
Zn_(s)+ CuSO_4(aq) → ZnSO_4(aq) + Cu_(s)
Here, Zn has changed into ZnSO₄ (i.e., Zn²⁺ ions) by loss of electrons. Hence, Zn has been oxidised. CuSO₄ (i.e., Cu²⁺) has changed into Cu by gain of electrons. Hence, CuSO₄ has been reduced. Thus, the above reaction is a displacement reaction as well as a redox reaction.
(b) (i) Neutralisation reaction
(ii) Oxidation reaction.

Ans: (i) The type of chemical reaction represented by the equation CaO + H₂O → Ca(OH)₂ is a combination or synthesis reaction.
(ii) The type of chemical reaction represented by the equation 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄ is a double displacement or precipitation reaction.
(iii) The type of chemical reaction represented by the equation 2FeSO₄ + Heat → Fe₂O3 + SO₂ + SO₃ is a thermal decomposition reaction. 

Ans: (a) The chemical reaction involved is a double displacement reaction or a precipitation reaction. The reaction between potassium chloride (KCl) and silver nitrate (AgNO₃) produces silver chloride (AgCl), which is an insoluble white substance. The balanced chemical equation for this reaction is:
KCl(aq) + AgNO₃(aq) → AgCl(s) + KNO₃(aq)
(b) The chemical reaction involved is a thermal decomposition reaction. Ferrous sulfate (FeSO₄) when heated decomposes to form ferric oxide (Fe₂O₃), sulfur dioxide (SO₂), and water (H₂O). The balanced chemical equation for this reaction is:
FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + H₂O(g)

Ans: When a strip of copper metal is placed in a solution of silver nitrate, copper displaces silver from silver nitrate solution as copper is a more reactive metal than silver. Copper nitrate is formed with a shiny greyish white deposit of silver on the copper strip.

Ans: The law which should be kept in mind when we balance chemical equations is the law of conservation of mass which states that “Matter can neither be created nor be destroyed”. It means that the total mass of atoms of reactants is equal to total mass of atoms of products, as atoms can neither be created nor be destroyed.