Class 10 Science: Chemical Reactions, Acids, Bases, Salts, and Metals

Comprehensive review flashcards covering Chemical Reactions and Equations, Acids, Bases and Salts, and Metals and Non-metals.

What four visual or physical observations can help determine if a chemical reaction has taken place?

A change in state, a change in colour, evolution of a gas, or a change in temperature.

How should a magnesium ribbon be prepared before burning in Activity 1.1?

It should be cleaned by rubbing it with sandpaper, using a piece about $3\text{--}4\,\text{cm}$ long.

What is observed when a magnesium ribbon is burned in air?

It burns with a dazzling white flame and changes into a white powder of magnesium oxide ($MgO$).

In a word equation, where are reactants and products written?

Reactants are written on the left-hand side (LHS) with a plus sign ($+$), and products on the right-hand side (RHS) with a plus sign ($$). An arrow points towards the products.

What is a skeletal chemical equation?

An unbalanced chemical equation where the number of atoms of each element is not equal on both sides.

Why must a chemical equation be balanced?

To satisfy the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction.

What is the balanced chemical equation for the reaction of zinc with dilute sulphuric acid?

$$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$$

What is the hit-and-trial method of balancing equations?

A method where trial adjustments are made using the smallest whole number coefficients to balance the number of atoms on both sides.

What do the physical state notations $(g)$, $(l)$, $(aq)$, and $(s)$ denote in a chemical equation?

They denote gaseous, liquid, aqueous (solution in water), and solid physical states, respectively.

What does $(g)$ attached to $H_2O$ indicate in the equation $3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g)$?

It indicates that water is used in the form of steam.

What specific pressure condition is required for carbon monoxide and hydrogen gas to react to form methanol ($CH_3OH$)?

A pressure of $340\,\text{atm}$.

What is a combination reaction?

A reaction in which a single product is formed from two or more reactants.

What is the common name and formula for calcium oxide?

Quick lime, with the chemical formula $CaO$.

What is slaked lime and how is it produced?

Calcium hydroxide ($Ca(OH)_2$), produced by reacting calcium oxide ($CaO$) vigorously with water ($H_2O$).

What reaction creates a shiny finish on walls two to three days after whitewashing?

Calcium hydroxide ($Ca(OH)_2$) reacts slowly with atmospheric carbon dioxide ($CO_2$) to form a thin layer of calcium carbonate ($CaCO_3$).

What is the chemical formula for marble?

$$CaCO_3$$

What defines an exothermic chemical reaction?

A reaction in which heat is released along with the formation of products.

Write the balanced chemical equation for the combustion of methane gas.

$$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$$

Why is human respiration classified as an exothermic reaction?

Because glucose formed during digestion combines with oxygen in body cells to provide energy ($C_6H_{12}O_6(aq) + 6O_2(aq) \rightarrow 6CO_2(aq) + 6H_2O(l) + \text{energy}$).

What happens when green ferrous sulphate crystals ($FeSO_4 \cdot 7H_2O$) are heated?

They lose water, change colour, and decompose into solid ferric oxide ($Fe_2O_3$) and gases sulphur dioxide ($SO_2$) and sulphur trioxide ($SO_3$).

What is thermal decomposition?

A decomposition reaction brought about by heating.

What products are formed by the thermal decomposition of limestone ($CaCO_3$)?

Quick lime ($CaO$) and carbon dioxide ($CO_2$).

What brown fumes are emitted when lead nitrate powder ($Pb(NO_3)_2$) is heated?

Nitrogen dioxide gas ($NO_2$).

What is the balanced equation for the thermal decomposition of lead nitrate?

$$2Pb(NO_3)_2(s) \xrightarrow{\text{Heat}} 2PbO(s) + 4NO_2(g) + O_2(g)$$

During the electrolysis of water, which gases are evolved at the anode and cathode?

Oxygen gas is evolved at the anode, and hydrogen gas is evolved at the cathode.

Why is the volume of gas collected in one test tube during water electrolysis twice that of the other?

Water ($H_2O$) consists of two hydrogen atoms per oxygen atom, yielding double the volume of hydrogen gas compared to oxygen gas.

What causes white silver chloride ($AgCl$) to turn grey in sunlight?

Decomposition of silver chloride into silver ($Ag$) and chlorine ($Cl_2$) by sunlight.

What chemical reactions form the basis of black and white photography?

Photochemical decomposition of silver chloride ($2AgCl \xrightarrow{\text{Sunlight}} 2Ag + Cl_2$) and silver bromide ($2AgBr \xrightarrow{\text{Sunlight}} 2Ag + Br_2$).

What is an endothermic reaction?

A reaction in which energy is absorbed in the form of heat, light, or electricity.

What occurs when an iron nail is placed in blue copper sulphate solution for 20 minutes?

The iron nail becomes brownish and the blue colour of the copper sulphate solution fades due to the formation of iron sulphate ($FeSO_4$).

What is a displacement reaction?

A reaction in which a more reactive element displaces or removes a less reactive element from its compound.

What is a double displacement reaction?

A reaction in which there is an exchange of ions between the reactants.

What precipitate forms when sodium sulphate reacts with barium chloride?

A white precipitate of insoluble barium sulphate ($BaSO_4$).

What is oxidation in terms of oxygen?

The gain of oxygen by a substance during a reaction.

What is reduction in terms of oxygen?

The loss of oxygen by a substance during a reaction.

What product forms on the surface of copper powder when heated in air, and what is its colour?

Black copper(II) oxide ($CuO$).

What happens when hydrogen gas is passed over heated black copper(II) oxide ($CuO$)?

The black coating turns brown as $CuO$ is reduced back to copper metal ($Cu$).

What is corrosion?

The process in which a metal is attacked by substances around it, such as moisture and acids.

What colours are the corrosion coatings formed on silver and copper?

Silver forms a black coating (silver sulphide), and copper forms a green coating (basic copper carbonate).

What is rancidity?

The state of fats and oils becoming foul-smelling and tasting due to oxidation.

Why are chip bags flushed with nitrogen gas?

To replace oxygen and prevent fats and oils in chips from becoming oxidised and rancid.

From what plant group is litmus dye extracted, and what is its natural colour?

Extracted from lichen, a plant belonging to division Thallophyta; its natural colour is purple.

What are olfactory indicators?

Substances whose odour changes depending on whether they are in an acidic or basic medium.

What three olfactory indicators are mentioned in Activity 2.2?

Onion, vanilla, and clove.

What gas is produced when an acid reacts with a metal, and how is it confirmed?

Hydrogen gas ($H_2$), confirmed by bringing a burning candle near the gas bubbles, which burn with a pop sound.

What compound is produced when zinc metal reacts with sodium hydroxide solution?

Sodium zincate ($Na_2ZnO_2$) along with hydrogen gas.

What products are formed when metal carbonates or metal hydrogencarbonates react with acids?

A salt, carbon dioxide gas ($CO_2$), and water ($H_2O$).

How does lime water test for carbon dioxide gas?

Carbon dioxide turns lime water ($Ca(OH)_2$) milky due to the formation of insoluble calcium carbonate ($CaCO_3$).

What happens when excess carbon dioxide gas is passed through milky lime water?

The milkiness disappears because soluble calcium hydrogencarbonate ($Ca(HCO_3)_2$) is formed.

What is a neutralisation reaction?

The reaction between an acid and a base to give a salt and water ($\text{Base} + \text{Acid} \rightarrow \text{Salt} + \text{Water}$).

Why are metallic oxides classified as basic oxides?

Because they react with acids to produce salt and water, similar to how bases react with acids.

Why are non-metallic oxides classified as acidic oxides?

Because they react with bases to form salt and water.

Why should curd and sour foods not be kept in brass and copper vessels?

The acids in curd react with copper/brass to form toxic, poisonous metallic compounds.

Why do aqueous solutions of acids conduct electricity, while solutions of glucose and alcohol do not?

Acids dissociate in water to produce free ions ($H^+$ / $H_3O^+$) that carry electric current, whereas glucose and alcohol do not form ions.

What ion is responsible for the acidic properties of substances in water?

Hydrogen ion ($H^+(aq)$) or hydronium ion ($H_3O^+$).

Why does dry $HCl$ gas fail to change the colour of dry blue litmus paper?

$HCl$ can only separate into $H^+$ ions in the presence of water; dry $HCl$ gas contains no free $H^+$ ions.

What is an alkali?

A base that dissolves in water (such as $NaOH$ or $KOH$).

Why must concentrated acid always be added slowly to water, rather than water to acid?

The dilution process is highly exothermic; adding water to concentrated acid can cause local boiling, violent splashing, acid burns, and glass breakage.

What does 'pH' stand for, and what range does its scale cover?

'p' stands for 'potenz' (German for power); the scale measures hydrogen ion concentration generally from $0$ (very acidic) to $14$ (very alkaline).

What is the normal pH range within which the human body operates?

$$7.0\text{--}7.8$$

What is acid rain defined as?

Rainwater with a pH less than $5.6$.

What compound is used as an antacid to treat stomach indigestion?

Magnesium hydroxide (Milk of magnesia, $Mg(OH)_2$), a mild base.

At what pH threshold does tooth decay begin, and what is tooth enamel made of?

Tooth decay begins when mouth pH falls below $5.5$; tooth enamel is composed of calcium hydroxyapatite (a crystalline form of calcium phosphate).

What acid is present in bee stings and nettle stings?

Methanoic acid.

What plant leaf provides a traditional alkalising remedy for nettle stings?

The dock plant leaf.

What naturally occurring acids are found in vinegar, tamarind, and tomatoes?

Acetic acid in vinegar, tartaric acid in tamarind, and oxalic acid in tomatoes.

What is rock salt?

Large brown crystals of solid sodium chloride ($NaCl$) deposited when ancient seas dried up.

What is the chlor-alkali process?

The electrolysis of an aqueous sodium chloride solution (brine) to produce sodium hydroxide ($NaOH$), chlorine gas ($Cl_2$), and hydrogen gas ($H_2$).

At which electrodes are chlorine and hydrogen gases liberated in the chlor-alkali process?

Chlorine gas is liberated at the anode, and hydrogen gas is liberated at the cathode.

How is bleaching powder manufactured?

By reacting chlorine gas ($Cl_2$) with dry slaked lime ($Ca(OH)_2$), represented as $Ca(ClO)_2$.

What is the chemical formula for baking soda?

Sodium hydrogencarbonate ($NaHCO_3$).

What ingredients make up baking powder?

A mixture of baking soda ($NaHCO_3$) and a mild edible acid such as tartaric acid.

What is the chemical formula for washing soda?

Hydrated sodium carbonate ($Na_2CO_3 \cdot 10H_2O$).

What is water of crystallisation?

The fixed number of water molecules present in one formula unit of a salt.

How many water molecules of crystallisation are present in copper sulphate crystals?

Five water molecules ($CuSO_4 \cdot 5H_2O$).

What chemical transformation occurs when gypsum ($CaSO_4 \cdot 2H_2O$) is heated to $373\,\text{K}$?

It loses water molecules to become Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$).

What is metallic lustre?

The shining surface characteristic of metals in their pure state.

What is malleability, and which two metals are most malleable?

The property of metals to be beaten into thin sheets; gold and silver are the most malleable.

What is ductility, and how ductile is gold?

The ability of a metal to be drawn into thin wires; a wire of about $2\,\text{km}$ length can be drawn from one gram of gold.

Which two metals are the best conductors of heat, and which two are relatively poor conductors?

Best: Silver ($Ag$) and Copper ($Cu$); Poor: Lead ($Pb$) and Mercury ($Hg$).

What is sonority in metals?

The property of producing a ringing sound when striking a hard surface.

Which non-metal element is a liquid at room temperature?

Bromine.

Name two metals with such low melting points that they melt in the palm of your hand.

Gallium and caesium.

Which non-metal exhibits metallic lustre?

Iodine.

Compare the properties of carbon allotropes diamond and graphite.

Diamond is the hardest known natural substance with very high melting and boiling points; graphite is soft and conducts electricity.

Name three alkali metals soft enough to be cut easily with a knife.

Lithium, sodium, and potassium.

What are amphoteric oxides?

Metal oxides that react with both acids and bases to produce salt and water (e.g., $Al_2O_3$, $ZnO$).

Why are sodium and potassium stored submerged under kerosene oil?

They react so vigorously with air and moisture that they catch fire if left exposed.

What is anodising?

A process of forming a thick protective oxide layer on aluminium by electrolysis with dilute sulphuric acid.

What happens when potassium and sodium are placed in cold water?

They react violently, evolving hydrogen gas that immediately catches fire.

Why does calcium float when added to water?

Bubbles of hydrogen gas produced stick to the surface of the calcium metal, lifting it up.

Which metal reacts with hot water but not cold water, and also floats due to hydrogen bubbles?

Magnesium.

Which metals react only with steam (not cold/hot water) and which do not react with water at all?

React with steam: Aluminium, Iron, Zinc; Do not react with water: Lead, Copper, Silver, Gold.

Why is hydrogen gas NOT evolved when metals react with nitric acid ($HNO_3$)?

Because nitric acid is a strong oxidising agent that oxidises hydrogen gas to water and gets reduced to nitrogen oxides.

Which two metals CAN react with very dilute nitric acid to produce hydrogen gas?

Magnesium ($Mg$) and Manganese ($Mn$).

What is aqua regia?

A freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in a $3:1$ volume ratio that dissolves gold and platinum.

What is the reactivity series of metals?

A list of metals arranged in the order of their decreasing chemical activity.

What is the thermit reaction and its practical application?

The highly exothermic reaction of iron(III) oxide ($Fe_2O_3$) with aluminium powder ($Al$) producing molten iron used to join railway tracks.

What is galvanisation?

A method of protecting steel and iron from rusting by coating them with a thin layer of zinc.

What is 24 carat gold vs 22 carat gold?

24 carat gold is pure, soft gold; 22 carat gold contains 22 parts gold alloyed with 2 parts silver or copper to make it hard enough for jewellery.