Physical and Chemical Changes, Elements and Compounds Questions in English

(N/A) pure substance is defined as a substance that consists of only one type of particles (atoms or molecules).
For example: $Hydrogen$,$Water$,$Gold$,etc.,are considered pure substances.
Note: All elements and compounds are classified as pure substances because they have a fixed chemical composition and uniform properties throughout.

(N/A) Physical Change: These are changes where no new substance is formed and the chemical composition remains the same.
- Cutting of trees
- Melting of butter in a pan
- Boiling of water to form steam
- Dissolving common salt in water
- Making a fruit salad with raw fruits
Chemical Change: These are changes where new substances with different chemical properties are formed.
- Rusting of almirah (Iron reacts with oxygen and moisture to form iron oxide)
- Passing of electric current through water and the water breaking down into hydrogen and oxygen gases (Electrolysis)
- Burning of paper and wood (Combustion reaction)

(B) To confirm if a colorless liquid is pure water,we can determine its boiling point.
Pure water has a fixed boiling point of $100\,^{\circ}\text{C}$ at standard atmospheric pressure.
If the liquid boils exactly at $100\,^{\circ}\text{C}$,it is pure water.
If the boiling point is different,it indicates the presence of impurities.

(A) pure substance consists of particles of only one type of chemical nature. Its composition remains the same throughout.
$(a)$ Ice is solid water $(H_2O)$,which is a pure compound.
$(b)$ Milk is a mixture (colloid) of water,fats,and proteins.
$(c)$ Iron $(Fe)$ is an element,which is a pure substance.
$(d)$ Hydrochloric acid is typically an aqueous solution of hydrogen chloride gas $(HCl)$ in water,making it a mixture.
$(e)$ Calcium oxide $(CaO)$ is a pure compound.
$(f)$ Mercury $(Hg)$ is an element,which is a pure substance.
$(g)$ Brick is a mixture of various minerals and soil components.
$(h)$ Wood is a complex mixture of cellulose,lignin,and other organic compounds.
$(i)$ Air is a mixture of various gases like nitrogen,oxygen,argon,and carbon dioxide.
Therefore,the pure substances are $(a), (c), (e),$ and $(f)$.

(B) Rusting of iron is a process where iron reacts with oxygen and moisture to form hydrated iron$(III)$ oxide.
This process is known as corrosion.
Since a new substance is formed and the process is irreversible,it is classified as a chemical change.

(B) physical change is a change in which no new substance is formed and the chemical composition of the substance remains the same.
$(i)$ Melting of iron metal is a physical change because it only involves a change in the state of matter.
$(ii)$ Rusting of iron is a chemical change because iron reacts with oxygen and moisture to form iron oxide,which is a new substance.
$(iii)$ Bending of an iron rod is a physical change because it only involves a change in the shape of the metal.
$(iv)$ Drawing a wire of iron metal is a physical change because it involves a change in the shape/form of the metal without changing its chemical identity.
Therefore,$(i), (iii),$ and $(iv)$ are physical changes.

(C) chemical change is a process where one or more substances are altered into one or more new and different substances.
$(i)$ Decaying of wood: This is a chemical change because microorganisms break down the wood into new substances.
$(ii)$ Burning of wood: This is a chemical change because wood reacts with oxygen to form carbon dioxide,water vapor,and ash,which are new substances.
$(iii)$ Sawing of wood: This is a physical change because only the shape and size of the wood change,not its chemical composition.
$(iv)$ Hammering of a nail into a piece of wood: This is a physical change as it involves a mechanical action without altering the chemical identity of the wood or the nail.
Therefore,$(i)$ and $(ii)$ are chemical changes.

(B) The reaction $2A + B \to A_2B$ represents the formation of a chemical compound.
$(i)$ Incorrect: $A$ chemical compound $(A_2B)$ has properties entirely different from the constituent elements ($A$ and $B$).
$(ii)$ Correct: $A$ compound always has a fixed composition by mass.
$(iii)$ Incorrect: Since $A_2B$ is formed by the chemical combination of two substances in a fixed ratio,it is classified as a compound.
$(iv)$ Incorrect: $A_2B$ is a compound,not an element,as it consists of two different types of atoms chemically bonded together.
Therefore,statements $(i), (iii),$ and $(iv)$ are incorrect.

(A) $1$. According to the definition of elements,they are substances that cannot be broken down into simpler substances by simple chemical reactions. Since $X$ and $Y$ cannot be broken down,they are elements.
$2$. When two or more elements combine chemically in a fixed proportion,they form a compound. Here,$X$ and $Y$ combine to form $P$,so $P$ is a compound.
$3$. Compounds always have a fixed composition by mass.
$4$. Therefore,statements $(i)$,$(iii)$,and $(iv)$ are correct.

(N/A) Physical properties: $(a)$ and $(c)$. These describe the inherent characteristics or composition of the substance without changing its chemical identity.
Chemical properties: $(b)$ and $(d)$. These describe the ability of a substance to undergo a specific chemical change or reaction,resulting in the formation of new substances.

(B) Pure gold ($24$ carat) is extremely soft and ductile,making it unsuitable for making jewelry as it would easily deform. By alloying gold with small amounts of copper or silver,the metal gains significant hardness and structural strength. This allows the ornaments to maintain their shape and durability over time.

(N/A) The element is classified as a $Metal$.
Metals are elements that typically exhibit properties such as being sonorous (producing a ringing sound when struck) and ductile (can be drawn into thin wires).
Other characteristics expected to be possessed by this element include:
$1$. Lustre: They have a shiny appearance.
$2$. Malleability: They can be beaten into thin sheets.
$3$. Conductivity: They are good conductors of heat and electricity.
$4$. High melting and boiling points.

(A) It is a pure substance.
Even though the sucrose crystals are obtained from different sources (sugarcane and beetroot),their chemical composition is identical.
$A$ pure substance consists of particles of only one type of substance.
Since the chemical formula for sucrose is $C_{12}H_{22}O_{11}$ regardless of its origin,the mixture of these crystals remains a pure substance.

(N/A) This is a chemical change because a new substance is formed and the process is irreversible under normal conditions.
$(b)$ Yes,acidic and basic solutions can be prepared by dissolving the products in water:
$1.$ Basic solution: Calcium oxide $(CaO)$ reacts with water to form calcium hydroxide $(Ca(OH)_2)$,which is basic in nature.
$CaO + H_2O \rightarrow Ca(OH)_2$
$2.$ Acidic solution: Carbon dioxide $(CO_2)$ reacts with water to form carbonic acid $(H_2CO_3)$,which is acidic in nature.
$CO_2 + H_2O \rightarrow H_2CO_3$

(N/A) Iodine is a lustrous non-metal.
$(b)$ Bromine is the only non-metal that exists as a liquid at room temperature.
$(c)$ Graphite is an allotrope of carbon and is a good conductor of electricity due to the presence of free electrons.
$(d)$ Carbon is known to form the largest number of compounds due to its unique property of catenation.
$(e)$ Sulphur and phosphorus are non-metals other than carbon that exhibit allotropy.
$(f)$ Oxygen is a non-metal essential for the process of combustion.

(N/A) Based on the chemical composition,the substances are classified as follows:
Elements: These are pure substances consisting of only one type of atom. From the figure,the elements are: $Cu$ (Copper),$Zn$ (Zinc),$O_2$ (Oxygen),$F_2$ (Fluorine),$Hg$ (Mercury),and Diamond (an allotrope of Carbon).
Compounds: These are substances formed by the chemical combination of two or more elements in a fixed proportion. From the figure,the compounds are: $H_2O$ (Water) and $CaCO_3$ (Calcium carbonate).
Note: Substances like Sand,$NaCl(aq)$,and Wood are mixtures,not pure elements or compounds.

(A, C, E, F, G, I) compound is a substance formed when two or more chemical elements are chemically bonded together. Elements are pure substances consisting of only one type of atom.
$1$. Chlorine gas $(Cl_2)$ is an element.
$2$. Potassium chloride $(KCl)$ is a compound.
$3$. Iron $(Fe)$ is an element.
$4$. Iron sulphide $(FeS)$ is a compound.
$5$. Aluminium $(Al)$ is an element.
$6$. Iodine $(I_2)$ is an element.
$7$. Carbon $(C)$ is an element.
$8$. Carbon monoxide $(CO)$ is a compound.
$9$. Sulphur powder $(S)$ is an element.
Therefore,the substances that are not compounds are: Chlorine gas,Iron,Aluminium,Iodine,Carbon,and Sulphur powder.

(A) Physical changes: $(a), (b), (e)$. These are physical changes because no new substance is formed,and the chemical composition remains the same.
Chemical changes: $(c), (d)$. These are chemical changes because new substances are formed with different chemical properties. Burning of kerosene involves a combustion reaction,and adding lemon juice to tea causes a chemical reaction between the acid in the lemon and the pigments in the tea.

(N/A) Elements: These are pure substances consisting of only one type of atom. Examples: Sodium,silver,tin,and silicon.
Compounds: These are pure substances formed by the chemical combination of two or more elements in a fixed proportion. Examples: Calcium carbonate,methane,and carbon dioxide.
Mixtures: These are substances formed by the physical mixing of two or more components in any proportion. Examples: Soil,sugar solution,coal,air,soap,and blood.

(A, B, D, E, G) Chemical changes involve a change in the chemical composition of a substance,resulting in the formation of new substances.
$(a)$ Growth of a plant: Involves complex biochemical reactions.
$(b)$ Rusting of iron: Iron reacts with oxygen and moisture to form iron oxide.
$(d)$ Cooking of food: Involves chemical breakdown of nutrients.
$(e)$ Digestion of food: Enzymes break down complex molecules into simpler ones.
$(g)$ Burning of a candle: Combustion is a chemical reaction.
Mixing of iron filings and sand $(c)$ and freezing of water $(f)$ are physical changes because no new chemical substances are formed.

(A) $(i)$ Brass is a mixture (it is an alloy of copper and zinc).
$(ii)$ Diamond is an element (it is an allotrope of carbon,consisting only of carbon atoms).
$(iii)$ Sand is a compound (it is primarily silicon dioxide,$SiO_2$).

(N/A)

Physical Change	Chemical Change
$1$. No new substances are formed during a physical change.	$1$. New substances having different properties are formed.
$2$. The substance can easily be brought back to its original state,i.e.,it is a reversible process.	$2$. It is mostly an irreversible process.

(B) An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
$(b)$ Carbon,Silicon,and Chlorine are non-metals.

(N/A) Blood is considered a mixture because its components (such as plasma,red blood cells,white blood cells,and platelets) can be separated by physical methods like centrifugation,and their composition varies from person to person.
Graphite is considered an element because it is composed of only one kind of atom (carbon atoms) and it cannot be broken down into simpler substances by chemical or physical means.

(N/A) $1$. $A$ mixture of iron filings and sulphur powder retains the individual properties of its constituents. Iron particles are attracted by a magnet,and sulphur is soluble in carbon disulphide $(CS_2)$. This demonstrates that the components in a mixture do not lose their identity.
$2$. Iron sulphide $(FeS)$ is a compound formed by the chemical reaction between iron and sulphur. In a compound,the constituent elements lose their original properties. Therefore,iron sulphide is not attracted by a magnet,and sulphur does not dissolve in carbon disulphide.
$3$. Thus,it is justified that a mixture exhibits the properties of its constituents,whereas a compound possesses entirely new properties different from its constituent elements.

(N/A)

Parameters	Compounds vs Mixtures
Composition	Compounds have a fixed composition,whereas mixtures have a variable composition.
Melting and Boiling point	Compounds have sharp melting and boiling points,while mixtures do not.
Properties of constituents	In compounds,constituents lose their individual properties. In mixtures,constituents retain their original properties.
Separation	Compounds can only be separated by chemical methods,while mixtures can be separated by physical methods.

(N/A)

Element	Compound
$(1)$ It is the simplest form of matter that cannot be broken down into simpler substances by chemical reactions.	$(1)$ It is a substance composed of two or more elements chemically combined in a fixed proportion.
$(2)$ It consists of only one type of atom.	$(2)$ It can be separated into its constituent elements by chemical reactions.

Example of an element: $Copper (Cu)$.
Example of a compound: $Water (H_{2}O)$.

(N/A) The physical symptoms of goitre and hypothyroidism are as follows:
$(i)$ During hypothyroidism,the metabolic rate of the body decreases.
$(ii)$ Increased sleep and lethargy are observed along with increased body weight.
$(iii)$ Swelling of the neck (goitre) and sometimes swelling of the legs is observed.
$(iv)$ During goitre,the thyroid gland enlarges significantly.
$(b)$ Others can be informed about the importance of iodised salt through posters,banners,or by organizing small awareness programs in the locality.
$(c)$ Iodine is a non-metal with the symbol $I$.
$(d)$ Iodine is a non-metal that appears as a metallic grey solid and turns into a violet gas upon heating. It has a boiling point of about $457.4 \ K$ and a density of about $4.933 \ g/cm^3$. Iodine undergoes sublimation when heated.
$(e)$ There is no significant difference in the chemical structure of the base salt. The main difference is that normal salt is primarily $NaCl$,whereas iodised salt has a small amount of iodine compounds (like potassium iodide) added to $NaCl$,which is an essential micronutrient. This is necessary because iodine levels are low in many soils worldwide,leading to a deficiency in crops and subsequently in humans,causing goitre.

(II, III) Burning of a candle: When a candle burns,the wax melts,which is a physical change. Simultaneously,the wax reacts with oxygen to produce $CO_2$ and $H_2O$ vapor,which is a chemical change. Thus,both changes occur together.
$(b)$ The chemical changes are:
$(ii)$ Growth of plant: This involves complex metabolic processes and permanent changes in the organism's structure.
$(iii)$ Rusting of iron: This is a chemical reaction where iron reacts with oxygen and moisture to form hydrated iron$(III)$ oxide.

(N/A) $1$. Metals: Property: They are lustrous (shiny). Example: Iron $(Fe)$.
$2$. Non-metals: Property: They are poor conductors of heat and electricity. Example: Oxygen $(O_2)$.
$3$. Metalloids: Property: They exhibit properties intermediate between those of metals and non-metals. Example: Germanium $(Ge)$.

(C) Comparison between metals and non-metals:

Metals	Non-metals
$(1)$ Metals are malleable and ductile.	$(1)$ Non-metals are brittle.
$(2)$ Metals are sonorous,i.e.,they produce a ringing sound when struck.	$(2)$ Non-metals are not sonorous.
$(3)$ Metals are good conductors of heat and electricity.	$(3)$ Non-metals are poor conductors of heat and electricity (except diamond and graphite).
$(4)$ Metals are generally solids at room temperature (except mercury).	$(4)$ Non-metals can be solids,liquids,or gases at room temperature.

$(b)$ Elements that exhibit properties intermediate between those of metals and non-metals are called metalloids. Examples: Boron $(B)$ and Silicon $(Si)$.
$(c)$ Among the given elements,sodium $(Na)$ and mercury $(Hg)$ are metals.

(N/A) Heating of $NH_4Cl$ (ammonium chloride) and sugar are different processes because they involve different physical and chemical changes.
$1$. $NH_4Cl$ is a sublime substance. When heated,it undergoes sublimation,meaning it changes directly from a solid state to a gaseous state without passing through the liquid phase.
$2$. Sugar $(C_{12}H_{22}O_{11})$ is not a sublime substance. When heated,it undergoes a chemical change called dehydration and thermal decomposition,which results in charring (turning into black carbon residue) rather than sublimation.

(N/A) $(i)$ When iron filings and sulphur are heated together,they undergo a chemical reaction to form iron$(II)$ sulphide,represented by the formula $FeS$.
$(ii)$ When dilute hydrochloric acid $(HCl)$ is added to iron$(II)$ sulphide $(FeS)$,a chemical reaction occurs to produce hydrogen sulphide gas $(H_2S)$.
The chemical equation is: $FeS(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2S(g)$.
Two properties of hydrogen sulphide $(H_2S)$ gas are:
$(a)$ It is a colourless gas.
$(b)$ It has a characteristic smell of rotten eggs.

(N/A)

Elements	Compounds
$(i)$ Made up of only one type of atoms.	$(i)$ Made up of two or more types of atoms chemically combined in a fixed ratio.
$(ii)$ Cannot be broken down into simpler substances by chemical reactions.	$(ii)$ Can be broken down into simpler substances (elements) by chemical reactions.

Classification:
Elements: Iodine,$24$ carat gold,Oxygen gas,Carbon.
Compounds: Sodium chloride,Water.

(A) pure substance is defined as a substance that consists of only one type of particles. These particles can be atoms (in the case of an element) or molecules (in the case of a compound). All constituent particles in a pure substance have the same chemical properties.

(N/A) The elements which exhibit properties intermediate between those of metals and non-metals are called metalloids or semimetals.
Examples include Silicon $(Si)$,Germanium $(Ge)$,Arsenic $(As)$,Antimony $(Sb)$,and Tellurium $(Te)$.

(C) substance or compound is considered pure if it consists of only one type of particles and exhibits a fixed melting point and a fixed boiling point under standard atmospheric pressure.
If a substance is impure,it will typically show a range of temperatures for melting or boiling rather than a sharp,fixed value.

(B) compound is a substance formed when two or more chemical elements are chemically bonded together in a fixed proportion by mass.
Water $(H_2O)$ is a compound because it consists of hydrogen and oxygen atoms chemically combined in a fixed ratio of $2:1$ by volume or $1:8$ by mass.
Air is a mixture of various gases like nitrogen,oxygen,argon,and carbon dioxide.
Soda water is a mixture of carbon dioxide gas dissolved in water.
Pond water is a mixture containing water,dissolved minerals,organic matter,and microorganisms.
Therefore,water is the correct answer.

(C) An element is a pure substance consisting only of atoms that all have the same number of protons in their atomic nuclei.
$A$. Carbon dioxide $(CO_2)$ is a compound made of carbon and oxygen atoms.
$B$. Sulphur dioxide $(SO_2)$ is a compound made of sulphur and oxygen atoms.
$C$. Oxygen gas $(O_2)$ consists only of oxygen atoms,making it an element.
$D$. Nitrogen dioxide $(NO_2)$ is a compound made of nitrogen and oxygen atoms.
Therefore,the correct option is $C$.

(A) pure substance consists of particles of only one type of chemical nature. Elements and compounds are considered pure substances.
$CaO$ (Calcium oxide), $CO_2$ (Carbon dioxide), and $NaCl$ (Sodium chloride) are all chemical compounds with fixed compositions.
Brick is a mixture of various materials, mercury is an element (pure), but wood is a complex mixture.
Iron is an element (pure), $CaO$ is a compound (pure), but air is a mixture of gases.
Ice is pure water $(H_2O)$, but milk is a colloid (mixture of water, fats, proteins, etc.).
Therefore, the group containing only pure substances is $CaO, CO_2$ and $NaCl$.

(C) Rusting of iron is a process where iron reacts with oxygen and moisture in the air to form hydrated iron $(III)$ oxide $(Fe_2O_3 \cdot xH_2O)$.
This process is known as corrosion.
Since a new substance with different chemical properties is formed, it is a chemical change.
Therefore, rusting is corrosion and it is a chemical change.

(B) The statement is False.
When a candle burns,both physical and chemical changes occur simultaneously.
$1$. Physical Change: The wax near the flame melts due to heat,changing from a solid state to a liquid state. This is a physical change because the chemical composition of the wax remains the same.
$2$. Chemical Change: The melted wax (vaporized) reacts with oxygen in the air to produce carbon dioxide,water vapor,heat,and light. This is a chemical change because new substances are formed.

(FALSE) The statement is False.
Non-metals are generally non-lustrous (dull) and are poor conductors of heat and electricity (insulators). Exceptions exist,such as graphite,which is a good conductor of electricity,but as a general rule,the statement is incorrect.

(A) The statement is True.
$1$. Malleability: Metals can be beaten into thin sheets.
$2$. Ductility: Metals can be drawn into thin wires.
$3$. Sonorous: Metals produce a ringing sound when struck.
$4$. Lustrous: Metals have a shiny appearance.

(TRUE) The statement is True.
$A$ compound is defined as a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass. The properties of a compound are entirely different from the properties of its constituent elements.

(B) Carbon exists in nature in two primary states: combined forms and elemental forms.
In combined forms,it is found as carbon dioxide,carbonates,and hydrogen carbonates.
In elemental forms,carbon exists as allotropes,which include diamond and graphite.
Therefore,the correct option is $B$.