Class 10 Chemistry — Important Questions with Answers

What it is

A more reactive metal displaces a less reactive one from its salt solution.

Answer

Iron is more reactive than copper, so it displaces copper from copper sulphate: Fe + CuSO4 -> FeSO4 + Cu. The blue colour of CuSO4 fades as it is replaced by the pale green FeSO4, and a brown copper coating forms on the nail. This is a displacement reaction.

What it is

In a redox reaction one substance gains oxygen (oxidised) while another loses it (reduced), together.

Answer

Hydrogen gains oxygen to form water, so H2 is oxidised. Copper oxide loses oxygen to form copper, so CuO is reduced. Since oxidation and reduction happen together, this is a redox reaction; here H2 acts as the reducing agent and CuO as the oxidising agent.

What it is

Most reactions are one of four types: combination, decomposition, displacement or double displacement.

Answer

1) Combination: two or more substances combine to form a single product, e.g. CaO + H2O -> Ca(OH)2. 2) Decomposition: a single compound breaks into two or more substances, often on heating, e.g. CaCO3 -> CaO + CO2. 3) Displacement: a more reactive element displaces a less reactive one from its compound, e.g. Fe + CuSO4 -> FeSO4 + Cu. 4) Double displacement: two compounds exchange their ions, e.g. Na2SO4 + BaCl2 -> BaSO4 + 2NaCl.

What it is

A balanced chemical equation has the same number of atoms of each element on both sides, as required by the law of conservation of mass.

Answer

Balanced, it is 2H2 + O2 -> 2H2O. That gives 4 hydrogen atoms and 2 oxygen atoms on each side. We only adjust the coefficients in front of each formula, because changing a subscript would change the substance itself.

What it is

In a combination reaction two or more substances form a single product; in a decomposition reaction a single substance breaks into two or more products.

Answer

A combination reaction is one in which two or more reactants combine to form a single product; for example, calcium oxide reacts with water to form calcium hydroxide: CaO + H2O -> Ca(OH)2. A decomposition reaction is one in which a single compound breaks down into two or more simpler substances, usually on heating (thermal), by light (photolytic) or by electricity (electrolytic); for example, calcium carbonate on heating gives calcium oxide and carbon dioxide: CaCO3 -> CaO + CO2.

What it is

In displacement a more reactive element replaces a less reactive one; in double displacement two compounds exchange ions.

Answer

A displacement reaction is one in which a more reactive element displaces a less reactive element from its compound; for example, iron displaces copper: Fe + CuSO4 -> FeSO4 + Cu. A double displacement reaction is one in which two compounds react by exchanging their ions to form two new compounds; for example, sodium sulphate reacts with barium chloride to form a white precipitate of barium sulphate: Na2SO4 + BaCl2 -> BaSO4 + 2NaCl. Reactions that form an insoluble precipitate are called precipitation reactions.

What it is

Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen. Both happen together in a redox reaction.

Answer

Oxidation is the gain of oxygen or loss of hydrogen (loss of electrons) by a substance; reduction is the loss of oxygen or gain of hydrogen (gain of electrons). In a redox reaction both occur simultaneously. In CuO + H2 -> Cu + H2O, copper oxide (CuO) loses oxygen to become copper, so it is reduced; hydrogen (H2) gains oxygen to become water, so it is oxidised. CuO acts as the oxidising agent and H2 as the reducing agent.

What it is

Corrosion is the slow oxidation of a metal by the environment; rancidity is the oxidation of fats and oils in food.

Answer

Corrosion is the process in which a metal is slowly eaten away by the action of air, moisture or chemicals on its surface; the rusting of iron (forming reddish-brown Fe2O3.xH2O) is a common example. It can be prevented by painting, oiling, galvanising (coating with zinc) or electroplating. Rancidity is the development of an unpleasant smell and taste in fats and oils in food due to their oxidation. It can be prevented by adding antioxidants, storing food in airtight containers, refrigeration, or flushing packets with nitrogen gas.

What it is

A balanced chemical equation has equal numbers of each kind of atom on both sides, satisfying the law of conservation of mass.

Answer

Count and balance each atom by adjusting coefficients. Start with iron: put 3 before Fe to match Fe3O4. Balance oxygen: Fe3O4 has 4 oxygen atoms, so put 4 before H2O. Now hydrogen: 4 H2O gives 8 hydrogen atoms, so put 4 before H2. The balanced equation is 3Fe + 4H2O -> Fe3O4 + 4H2. Now both sides have 3 Fe, 8 H and 4 O atoms, satisfying the law of conservation of mass.

What it is

pH measures how acidic or basic a solution is, from 0 (acid) to 14 (base), with 7 neutral.

Answer

The pH scale runs from 0 to 14 and measures the concentration of hydrogen ions (H+) in a solution. A pH of 7 is neutral, below 7 is acidic and above 7 is basic. As a solution becomes more acidic, the H+ concentration increases and the pH value decreases.

What it is

A neutralisation reaction is an acid reacting with a base to give a salt and water.

Answer

A neutralisation reaction is the reaction between an acid and a base to form a salt and water. The general equation is: Acid + Base -> Salt + Water (for example, HCl + NaOH -> NaCl + H2O). An everyday use is taking an antacid (a mild base) to neutralise excess acid in the stomach and relieve acidity.

What it is

Acids are sour, turn blue litmus red and release H+ ions; bases are bitter, turn red litmus blue and release OH- ions.

Answer

Acids are sour in taste, turn blue litmus red, and produce hydrogen ions (H+) in aqueous solution; they react with active metals to release hydrogen gas. Bases are bitter in taste, feel soapy, turn red litmus blue, and produce hydroxide ions (OH-) in aqueous solution. Acids and bases neutralise each other to form salt and water.

What it is

The pH scale measures how acidic or basic a solution is, from 0 to 14.

Answer

The pH scale is a scale from 0 to 14 used to measure the hydrogen-ion concentration, and hence the acidity or basicity, of a solution. A pH of 7 is neutral (pure water); a pH less than 7 indicates an acidic solution, and a pH greater than 7 indicates a basic (alkaline) solution. pH is important in everyday life: our body works within a narrow pH range, tooth decay occurs when the mouth's pH falls below 5.5, plants grow best in soil of a particular pH, and antacids (bases) relieve acidity in the stomach.

What it is

A neutralisation reaction is the reaction between an acid and a base to form salt and water.

Answer

A neutralisation reaction is a reaction in which an acid reacts with a base to form a salt and water, so that the acidic and basic properties are cancelled out. For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water: HCl + NaOH -> NaCl + H2O. This is why an antacid (a mild base) neutralises excess acid in the stomach.

What it is

Common salt (NaCl) is the raw material for making sodium hydroxide, chlorine, bleaching powder and other chemicals.

Answer

When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes in the chlor-alkali process to give three products: sodium hydroxide (NaOH), chlorine gas (Cl2) at the anode, and hydrogen gas (H2) at the cathode. Bleaching powder (calcium oxychloride, CaOCl2) is made by passing chlorine over dry slaked lime, and it is used for bleaching cotton and paper, for disinfecting drinking water, and as an oxidising agent.

What it is

Baking soda is sodium hydrogencarbonate (NaHCO3) and washing soda is sodium carbonate (Na2CO3.10H2O).

Answer

Baking soda is sodium hydrogencarbonate, with the formula NaHCO3; it is a mild, non-corrosive base used in cooking (as it releases carbon dioxide that makes cakes and bread rise), as an antacid, and in soda-acid fire extinguishers. Washing soda is sodium carbonate decahydrate, with the formula Na2CO3.10H2O; it is used for washing clothes, for softening hard water, and in the manufacture of glass, soap and paper.

What it is

Water of crystallisation is the fixed number of water molecules in a crystal; Plaster of Paris is made by heating gypsum.

Answer

Water of crystallisation is the fixed number of water molecules chemically combined in one formula unit of a crystalline salt; for example, copper sulphate crystals are CuSO4.5H2O (blue), which turn white on heating as they lose this water. Plaster of Paris (calcium sulphate hemihydrate, CaSO4.1/2 H2O) is prepared by heating gypsum (CaSO4.2H2O) to about 100 degrees Celsius. When mixed with water, it sets into a hard solid (gypsum again), so it is used by doctors to support fractured bones and to make casts, moulds and decorative items.

What it is

Metals higher in the reactivity series displace those lower down, never the reverse.

Answer

In the reactivity series zinc lies above copper, so zinc is more reactive and loses electrons more readily. Therefore zinc displaces the less reactive copper: Zn + CuSO4 -> ZnSO4 + Cu. Copper, being less reactive than zinc, cannot displace zinc, so Cu + ZnSO4 does not react.

What it is

The reactivity series lists metals from most reactive (potassium) to least reactive (gold).

Answer

The reactivity series is a list of metals arranged in decreasing order of their reactivity, from potassium and sodium at the top down to silver and gold at the bottom. A metal higher in the series can displace a metal lower down from its salt solution. The series is useful for predicting displacement reactions, choosing the method to extract a metal from its ore, and explaining why very reactive metals corrode easily while gold and silver stay shiny.

What it is

Metals are lustrous, malleable, ductile, sonorous and good conductors; non-metals generally are not.

Answer

Metals are generally lustrous (shiny), malleable (can be beaten into sheets), ductile (can be drawn into wires), sonorous (produce a ringing sound), hard and good conductors of heat and electricity, and most are solids with high melting points. Non-metals are generally dull, brittle (if solid), non-sonorous and poor conductors, and may be solids, liquids or gases. Exceptions include mercury (a liquid metal), sodium (a soft metal), graphite (a non-metal that conducts electricity) and diamond (a very hard non-metal).

What it is

The reactivity series arranges metals in order of decreasing reactivity.

Answer

The reactivity series is a list of metals arranged in the order of their decreasing chemical reactivity, with the most reactive metals (like potassium and sodium) at the top and the least reactive (like gold and silver) at the bottom. It helps predict displacement reactions: a metal higher in the series can displace a metal lower in the series from its salt solution. For example, zinc is above copper, so zinc displaces copper: Zn + CuSO4 -> ZnSO4 + Cu. Metals below hydrogen (like copper and silver) do not displace hydrogen from dilute acids.

What it is

Ionic compounds form by transfer of electrons and are hard solids with high melting points that conduct electricity when molten or in solution.

Answer

Ionic compounds are formed by the transfer of electrons from a metal to a non-metal, producing oppositely charged ions held by strong electrostatic forces. Their properties are: they are usually hard, crystalline solids; they have high melting and boiling points because a lot of energy is needed to break the strong ionic bonds; they are generally soluble in water but insoluble in organic solvents like kerosene; and they do not conduct electricity in the solid state but conduct well when molten or dissolved in water, because the ions become free to move.

What it is

Metals are extracted from their ores by methods that depend on their position in the reactivity series.

Answer

The method of extracting a metal from its ore depends on its reactivity. Highly reactive metals at the top of the series (such as sodium, calcium and aluminium) are extracted by the electrolysis of their molten ores, since their compounds are very stable. Moderately reactive metals in the middle (such as iron, zinc and lead) are extracted by reducing their oxides with carbon; their ores are first converted to oxides by roasting (sulphide ores) or calcination (carbonate ores). The least reactive metals at the bottom (such as gold and silver) often occur free in nature or are obtained by simple heating.

What it is

Corrosion is the gradual attack of a metal by air and moisture; it is prevented by coating or alloying.

Answer

The rusting of iron requires the presence of both oxygen (air) and moisture (water); in their absence, iron does not rust. Rust is hydrated iron(III) oxide. Rusting can be prevented by methods that keep out air and moisture or make the iron less reactive: for example, galvanisation, which coats iron with a layer of zinc; painting, greasing or oiling the surface; and alloying, such as mixing iron with chromium and nickel to make stainless steel, which does not rust.

What it is

Carbon forms millions of compounds because it makes four strong covalent bonds and chains (catenation).

Answer

Carbon has four valence electrons, so it shares electrons to form four strong covalent bonds rather than gaining or losing electrons. Two special properties help: catenation (carbon atoms link with other carbon atoms to form long chains, branches and rings) and tetravalency (it can bond with four other atoms such as hydrogen, oxygen, nitrogen). Together these give a huge number of stable compounds.

What it is

A homologous series is a family of carbon compounds with the same general formula, differing by CH2.

Answer

A homologous series is a group of organic compounds having the same general formula and the same functional group, in which each successive member differs from the previous one by a -CH2- unit. Two characteristics are: (1) all members have similar chemical properties because of the same functional group, and (2) their physical properties such as boiling point change gradually as the molecule gets bigger. The alkanes (CH4, C2H6, C3H8, ...) are an example.

What it is

Carbon forms millions of compounds because of catenation and its tetravalency.

Answer

Carbon forms an extremely large number of compounds for two main reasons. First, catenation: carbon atoms can form strong covalent bonds with other carbon atoms, creating long straight chains, branched chains and rings. Second, tetravalency: a carbon atom has four valence electrons, so it can form four strong covalent bonds with other atoms such as hydrogen, oxygen, nitrogen, sulphur and the halogens. Together these give a huge variety of stable molecules.

What it is

Alkanes form a homologous series of saturated hydrocarbons with the general formula CnH(2n+2).

Answer

Alkanes are saturated hydrocarbons (only single C-C and C-H bonds) with the general formula CnH(2n+2), where n is the number of carbon atoms. So n = 1 gives CH4 (methane), n = 2 gives C2H6 (ethane), n = 3 gives C3H8 (propane) and n = 4 gives C4H10 (butane). Each successive member differs by one CH2 unit - this family is called a homologous series.

What it is

Carbon forms covalent bonds by sharing electrons, and its properties of catenation and tetravalency let it form a huge number of compounds.

Answer

Carbon forms a very large number of compounds mainly for two reasons. First, catenation: carbon atoms have a strong ability to bond with other carbon atoms to form long chains, branched chains and rings. Second, tetravalency: a carbon atom has four valence electrons, so it can form four strong covalent bonds by sharing electrons with other carbon atoms or with atoms of other elements such as hydrogen, oxygen, nitrogen and the halogens. These bonds are strong and stable, giving a vast variety of compounds.

What it is

Saturated hydrocarbons (alkanes) have only single carbon-carbon bonds; unsaturated hydrocarbons (alkenes, alkynes) have double or triple bonds.

Answer

Saturated hydrocarbons, called alkanes, contain only single covalent bonds between carbon atoms and hold the maximum number of hydrogen atoms; they are relatively unreactive and burn with a clean blue flame, for example methane (CH4) and ethane (C2H6). Unsaturated hydrocarbons contain at least one carbon-carbon double bond (alkenes, such as ethene C2H4) or triple bond (alkynes, such as ethyne C2H2); they are more reactive, undergo addition reactions, and burn with a sooty yellow flame.

What it is

A homologous series is a family of compounds with the same functional group and general formula, differing by CH2.

Answer

A homologous series is a group (family) of organic compounds having the same general formula and the same functional group, in which each successive member differs from the previous one by a -CH2- unit. Its characteristics are: all members have the same functional group and hence similar chemical properties; consecutive members differ by a CH2 group (a mass difference of 14 units); and their physical properties, such as melting and boiling points, change gradually with increasing molecular mass. Examples include the alkanes and the alcohols.

What it is

Ethanol (C2H5OH) is an alcohol and ethanoic acid (CH3COOH) is a carboxylic acid; they have distinct properties and uses.

Answer

Ethanol (C2H5OH) contains the alcohol functional group -OH; it is a colourless liquid used as a solvent, in sanitisers and in alcoholic drinks, and it reacts with sodium to release hydrogen. Ethanoic acid (CH3COOH), commonly known as acetic acid, contains the carboxylic acid functional group -COOH; its dilute solution is called vinegar, it has a sour taste and pungent smell, turns blue litmus red, and reacts with sodium carbonate to give carbon dioxide. Ethanol and ethanoic acid react together (esterification) to form a sweet-smelling ester.

What it is

Soaps and detergents clean by forming micelles that trap oily dirt so it can be washed away in water.

Answer

A soap molecule has two ends: a long hydrocarbon tail that is hydrophobic (water-repelling but oil-attracting) and an ionic head that is hydrophilic (water-attracting). When soap is added to water containing oily dirt, the hydrophobic tails attach to the oil droplet while the hydrophilic heads point outward into the water, forming a spherical cluster called a micelle. This traps the oily dirt inside, keeps it suspended in water, and it is then rinsed away. Detergents work similarly but also clean well in hard water.

What it is

Mendeleev arranged elements in order of increasing atomic mass and grouped those with similar properties.

Answer

Mendeleev's periodic law states that the properties of elements are a periodic function of their atomic masses. His achievements were: he arranged the then-known elements in a table according to increasing atomic mass and grouped elements with similar properties together; and he left gaps for elements that had not yet been discovered, boldly predicting their properties (for example eka-silicon, later found to be germanium), and these predictions turned out to be remarkably accurate. A limitation was that the position of isotopes and some pairs (like argon and potassium) could not be explained.

What it is

The modern periodic law states that the properties of elements are a periodic function of their atomic numbers.

Answer

The modern periodic law states that the properties of elements are a periodic function of their atomic numbers. In the modern periodic table, elements are arranged in order of increasing atomic number. The table has 18 vertical columns called groups and 7 horizontal rows called periods. Elements in the same group have the same number of valence electrons and hence similar chemical properties, while across a period the properties change gradually. This arrangement removed the anomalies of Mendeleev's table, such as the placement of isotopes.

What it is

Across a period from left to right, atomic size decreases, metallic character decreases and non-metallic character increases.

Answer

As we move from left to right across a period, the atomic size (atomic radius) decreases. This is because the number of protons in the nucleus increases, so the nuclear charge increases and pulls the electrons of the same outermost shell more strongly, making the atom smaller. The metallic character decreases and the non-metallic character increases across a period, because elements on the left lose electrons easily (metals) while those on the right gain electrons easily (non-metals). Thus periods begin with metals and end with non-metals and a noble gas.

What it is

Down a group, atomic size increases and metallic character increases, while valence electrons stay the same.

Answer

On moving down a group, the atomic size increases because a new electron shell is added at each step, so the outermost electrons are farther from the nucleus. The metallic character also increases down a group, because the outermost electrons are held less tightly and are lost more easily. The number of valence electrons remains the same for all elements in a group, which is why they show similar chemical properties. Thus the most reactive metals lie at the bottom-left of the periodic table.