NCERT Class 10 Science Solutions: Acids, Bases and Salts
Question:
A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains ______.
A. NaCl
B. HCl
C. LiCl
D. KCl
Concept in a Minute:
Acids react with carbonates to produce carbon dioxide gas. Carbon dioxide gas turns lime water (calcium hydroxide solution) milky due to the formation of insoluble calcium carbonate.
Explanation:
The question states that a solution reacts with crushed egg-shells to give a gas that turns lime-water milky. Egg-shells are primarily made of calcium carbonate (CaCO3). The reaction of an acid with a carbonate produces carbon dioxide (CO2), water (H2O), and a salt. The chemical equation for the reaction of a generic acid (HX) with calcium carbonate is:
2HX + CaCO3 -> CaX2 + H2O + CO2 (g)
The gas produced is carbon dioxide. When carbon dioxide is passed through lime water (Ca(OH)2), it reacts to form calcium carbonate (CaCO3), which is a white precipitate, making the lime water appear milky. The reaction is:
Ca(OH)2 (aq) + CO2 (g) -> CaCO3 (s) + H2O (l)
Now let’s consider the given options:
A. NaCl (Sodium Chloride): NaCl is a salt and does not react with calcium carbonate to produce carbon dioxide.
B. HCl (Hydrochloric Acid): HCl is a strong acid. It reacts with calcium carbonate (egg-shells) to produce calcium chloride, water, and carbon dioxide gas. This fits the description in the question.
HCl + CaCO3 -> CaCl2 + H2O + CO2 (g)
The CO2 produced will turn lime water milky.
C. LiCl (Lithium Chloride): LiCl is a salt and does not react with calcium carbonate to produce carbon dioxide.
D. KCl (Potassium Chloride): KCl is a salt and does not react with calcium carbonate to produce carbon dioxide.
Therefore, the solution must be an acid that reacts with calcium carbonate to produce carbon dioxide. Among the given options, only HCl is an acid.
The final answer is $\boxed{B}$.
Question:
Which one of the following types of medicines is used for treating indigestion?
A. Antibiotic
B. Analgesic
C. Antacid
D. Antiseptic
Concept in a Minute:
Indigestion is a common problem characterized by discomfort or pain in the upper abdomen, often due to an excess of stomach acid. Medicines used to treat such conditions are specifically designed to neutralize or reduce this excess acid.
Explanation:
The question asks for the type of medicine used to treat indigestion. Let’s consider the options:
A. Antibiotics are used to kill bacteria and treat infections. They do not address stomach acid.
B. Analgesics are pain relievers. While pain might be a symptom of indigestion, analgesics don’t treat the underlying cause.
C. Antacids are substances that neutralize stomach acid, providing relief from symptoms like heartburn and indigestion. This directly addresses the cause of indigestion.
D. Antiseptics are used to kill or inhibit the growth of microorganisms on living tissues, preventing infection. They are not related to treating indigestion.
Therefore, antacids are the correct type of medicine for treating indigestion.
The final answer is $\boxed{C}$.
Question:
A solution turns red litmus blue, its pH is likely to be ______.
A. 1
B. 4
C. 5
D. 10
Concept in a Minute:
Acids turn blue litmus red. Bases turn red litmus blue. The pH scale ranges from 0 to 14. pH values below 7 are acidic, pH 7 is neutral, and pH values above 7 are basic.
Explanation:
The question states that a solution turns red litmus blue. This is a characteristic property of a basic solution. Basic solutions have a pH greater than 7. Let’s examine the given options:
A. 1: This pH is highly acidic. Acids turn red litmus red, not blue.
B. 4: This pH is acidic. Acids turn red litmus red.
C. 5: This pH is acidic. Acids turn red litmus red.
D. 10: This pH is basic. Bases turn red litmus blue.
Therefore, a solution with a pH of 10 is likely to turn red litmus blue.
The final answer is $\boxed{\text{10}}$.
Question:
Write a word equation and then a balanced equation for the reaction taking place when:
Dilute hydrochloric acid reacts with magnesium ribbon.
Concept in a Minute:
Chemical reactions involve reactants transforming into products. Word equations represent these reactions using the names of substances. Balanced equations use chemical formulas and coefficients to ensure the law of conservation of mass is upheld (number of atoms of each element is equal on both sides). Acids react with reactive metals to produce a salt and hydrogen gas.
Explanation:
When dilute hydrochloric acid reacts with magnesium ribbon, magnesium, a reactive metal, displaces hydrogen from the acid. This results in the formation of magnesium chloride (a salt) and hydrogen gas.
Word Equation:
Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen
Balanced Equation:
To balance the equation, we need to ensure the number of atoms of each element is the same on both the reactant and product sides.
1. Write the unbalanced equation using chemical formulas:
Mg + HCl → MgCl₂ + H₂
2. Count the atoms of each element on both sides:
Reactant side:
Mg: 1
H: 1
Cl: 1
Product side:
Mg: 1
Cl: 2
H: 2
3. Balance the equation. We see that chlorine and hydrogen are not balanced. To balance chlorine, we put a coefficient of 2 in front of HCl. This also balances hydrogen.
Balanced Equation:
Mg + 2HCl → MgCl₂ + H₂
4. Re-count the atoms to verify:
Reactant side:
Mg: 1
H: 2 (from 2HCl)
Cl: 2 (from 2HCl)
Product side:
Mg: 1
Cl: 2
H: 2
The equation is now balanced.
Question:
Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Concept in a Minute:
– Water undergoes autoionization to produce H$^+$ and OH$^-$ ions.
– In any aqueous solution, the product of the concentration of H$^+$ and OH$^-$ ions is a constant (Kw = [H$^+$][OH$^-$] = 1.0 x 10$^{-14}$ at 25$^\circ$C).
– A solution is acidic if [H$^+$] > [OH$^-$], neutral if [H$^+$] = [OH$^-$], and basic if [H$^+$] < [OH$^-$].
Explanation:
Yes, basic solutions also have H$^+$ (aq) ions. This is because water itself undergoes autoionization, producing both H$^+$ and OH$^-$ ions in equal amounts. Even in a basic solution, where the concentration of OH$^-$ ions is significantly higher than that of H$^+$ ions, a small, measurable concentration of H$^+$ ions will always be present due to this autoionization.
These solutions are considered basic not because the H$^+$ ions are absent, but because the concentration of hydroxide ions (OH$^-$) is much greater than the concentration of hydrogen ions (H$^+$). In any aqueous solution, the product of the concentrations of H$^+$ and OH$^-$ ions is a constant (Kw). If the concentration of OH$^-$ ions increases (making the solution basic), the concentration of H$^+$ ions must decrease proportionally to maintain this constant product. Therefore, in a basic solution, [OH$^-$] > [H$^+$], which defines it as basic.
Question:
What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Concept in a Minute:
Acids and bases.
pH scale.
Relationship between H+ ion concentration and acidity/alkalinity.
Explanation:
The concentration of H+ (aq) ions directly determines whether a solution is acidic, basic, or neutral.
– If the concentration of H+ ions is high, the solution is acidic.
– If the concentration of H+ ions is low (meaning a high concentration of OH- ions), the solution is basic or alkaline.
– If the concentration of H+ ions is equal to the concentration of OH- ions, the solution is neutral.
The pH scale is a logarithmic scale used to express the acidity or alkalinity of a solution. It is defined as the negative logarithm of the H+ ion concentration: pH = -log[H+].
– A lower pH value (typically below 7) indicates a higher concentration of H+ ions and thus an acidic solution.
– A higher pH value (typically above 7) indicates a lower concentration of H+ ions (and a higher concentration of OH- ions) and thus a basic or alkaline solution.
– A pH of 7 indicates a neutral solution where the concentration of H+ and OH- ions is equal.
Therefore, an increase in the concentration of H+ (aq) ions makes the solution more acidic, while a decrease in the concentration of H+ (aq) ions makes the solution less acidic or more basic.
Question:
Why does distilled water not conduct electricity, whereas rain water does?
Concept in a Minute:
Electrical conductivity in water is primarily due to the presence of dissolved ions. Pure water (distilled water) has very few ions. Rainwater collects dissolved minerals and gases from the atmosphere, which dissociate into ions, making it conductive.
Explanation:
Distilled water is pure H₂O. In pure water, the dissociation of water molecules into hydrogen ions (H⁺) and hydroxide ions (OH⁻) is extremely limited. These ions are responsible for carrying electric charge and thus conducting electricity. Since the concentration of ions in distilled water is very low, it is a poor conductor of electricity.
Rainwater, on the other hand, absorbs gases like carbon dioxide (CO₂) from the atmosphere. When carbon dioxide dissolves in water, it forms carbonic acid (H₂CO₃), which then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). Additionally, rainwater can pick up dissolved minerals from dust particles and pollutants in the atmosphere, which also contain ions. The presence of these dissolved ions (like H⁺, OH⁻, HCO₃⁻, and ions from dissolved salts) allows rainwater to conduct electricity. The more impurities and dissolved salts present in rainwater, the better its electrical conductivity.
Question:
Give two important uses of washing soda.
Concept in a Minute:
Washing soda is a common chemical compound with the formula Na2CO3·10H2O. Its properties, such as its alkaline nature and ability to remove hardness of water, lead to its various uses.
Explanation:
Washing soda, or sodium carbonate decahydrate, has two important uses:
1. Water Softening: Washing soda is extensively used in laundries to soften hard water. Hard water contains dissolved calcium and magnesium salts which interfere with the cleansing action of soap. Washing soda reacts with these dissolved salts to form insoluble precipitates, thereby removing them from the water and making it soft. This allows soap to lather effectively and clean clothes efficiently. The reaction is generally:
Na2CO3(aq) + Ca2+(aq) → CaCO3(s) + 2Na+(aq)
Na2CO3(aq) + Mg2+(aq) → MgCO3(s) + 2Na+(aq)
2. Manufacture of other chemicals: Washing soda is an important industrial chemical used in the manufacture of other useful compounds such as glass, soap, and detergents. It is also used in the production of sodium compounds like borax. For instance, in glass manufacturing, sodium carbonate acts as a flux, lowering the melting point of silica.
Question:
How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Concept in a Minute:
Acids in water dissociate to produce hydrogen ions (H⁺). These hydrogen ions then react with water molecules to form hydronium ions (H₃O⁺). Dilution means adding more solvent (usually water) to a solution. The concentration of a substance is the amount of solute per unit volume of solvent.
Explanation:
When a solution of an acid is diluted, more water is added to it. This addition of water increases the total volume of the solution. Although the total number of hydronium ions (H₃O⁺) present in the solution remains the same (as no more acid is added), the volume of the solution has increased. Concentration is defined as the amount of solute (in this case, H₃O⁺ ions) per unit volume. Since the volume has increased and the amount of H₃O⁺ ions has not, the concentration of hydronium ions decreases. Mathematically, if the number of moles of H₃O⁺ is ‘n’ and the initial volume is ‘V₁’, the initial concentration is C₁ = n/V₁. When diluted to a new volume V₂, the new concentration C₂ = n/V₂. Since V₂ > V₁, it follows that C₂ < C₁. Therefore, diluting an acidic solution leads to a decrease in the concentration of hydronium ions.
Question:
You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
Concept in a Minute:
The pH scale is a measure of the acidity or basicity of an aqueous solution. It is inversely related to the hydrogen ion concentration ([H+]). A lower pH indicates a higher [H+] and thus a more acidic solution. A higher pH indicates a lower [H+] and thus a more basic solution. The neutral pH is 7.
Explanation:
The pH scale ranges from 0 to 14.
pH = -log[H+]
Therefore, [H+] = 10^-pH
Solution A has a pH of 6.
The hydrogen ion concentration in solution A is [H+]_A = 10^-6 M.
Solution B has a pH of 8.
The hydrogen ion concentration in solution B is [H+]_B = 10^-8 M.
Comparing the hydrogen ion concentrations:
10^-6 M is greater than 10^-8 M.
So, solution A has more hydrogen ion concentration.
Acidity and Basicity:
A solution with a pH less than 7 is acidic.
A solution with a pH greater than 7 is basic.
A solution with a pH of 7 is neutral.
Solution A has a pH of 6, which is less than 7. Therefore, solution A is acidic.
Solution B has a pH of 8, which is greater than 7. Therefore, solution B is basic.
Answer:
Solution A has more hydrogen ion concentration.
Solution A is acidic and solution B is basic.
Question:
What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Concept in a Minute:
Thermal decomposition of sodium bicarbonate (sodium hydrogen carbonate). Properties of carbonates and bicarbonates. Balancing chemical equations.
Explanation:
When a solution of sodium bicarbonate is heated, it undergoes thermal decomposition. Sodium bicarbonate, also known as sodium hydrogen carbonate (NaHCO3), is an unstable salt and decomposes upon heating into sodium carbonate (Na2CO3), water (H2O), and carbon dioxide gas (CO2). This is a common reaction observed in kitchens when baking soda (sodium bicarbonate) is used in recipes that require heating. The water evaporates, and the carbon dioxide gas produced causes leavening, making baked goods rise.
The chemical equation for the reaction is:
2NaHCO3(s) heat-> Na2CO3(s) + H2O(g) + CO2(g)
In this equation:
– NaHCO3 represents sodium bicarbonate.
– Na2CO3 represents sodium carbonate.
– H2O represents water.
– CO2 represents carbon dioxide.
– The arrow (->) indicates the direction of the reaction.
– ‘heat’ above the arrow signifies that heat is applied to cause the decomposition.
– (s) denotes a solid state, and (g) denotes a gaseous state.
Question:
Give two important uses of baking soda.
Concept in a Minute:
Baking soda, chemically known as sodium bicarbonate (NaHCO3), is a versatile compound with various applications due to its mild alkaline nature and its ability to release carbon dioxide gas when heated or reacted with an acid. Key properties relevant to its uses include its leavening action in baking and its antacid properties.
Explanation:
Two important uses of baking soda are:
1. In the kitchen as a leavening agent: When baking soda is mixed with an acid (like buttermilk, yogurt, or vinegar) and heated, it decomposes to produce carbon dioxide gas. This gas gets trapped in the batter or dough, causing it to rise and become fluffy. This is why it’s a crucial ingredient in cakes, cookies, and bread.
2. As an antacid: Baking soda is mildly alkaline. When taken orally, it can neutralize excess acid in the stomach, providing relief from indigestion and heartburn. The reaction is: NaHCO3 (sodium bicarbonate) + HCl (stomach acid) → NaCl (sodium chloride) + H2O (water) + CO2 (carbon dioxide). The carbon dioxide produced can cause belching.
Question:
Why do acids not show acidic behaviour in the absence of water?
Concept in a Minute:
Acids show acidic behaviour due to the presence of hydrogen ions (H+) in aqueous solutions. Acids dissociate in water to release H+ ions.
Explanation:
Acids do not show acidic behaviour in the absence of water because they need water to dissociate and release hydrogen ions (H+). In solid form or when dissolved in non-aqueous solvents, acids like hydrochloric acid (HCl) exist as covalent molecules. The hydrogen chloride molecule (HCl) is bonded covalently. When HCl is dissolved in water, the water molecules interact with the HCl molecules, leading to the breaking of the covalent bond and the formation of ions: H+ and Cl-. The H+ ions are then hydrated by water molecules to form hydronium ions (H3O+). It is the presence of these mobile, hydrated hydrogen ions that causes the characteristic acidic properties such as sour taste, ability to conduct electricity, and reaction with bases to form salt and water. Without water, this dissociation and formation of free hydrogen ions cannot occur, and thus the acidic behaviour is not observed.
Question:
Why does an aqueous solution of an acid conduct electricity?
Concept in a Minute:
Acids in aqueous solution dissociate into ions, which are charged particles. Electricity is the flow of charged particles.
Explanation:
When an acid is dissolved in water (aqueous solution), it undergoes a process called dissociation. This means the acid molecule breaks down into positively charged ions (cations) and negatively charged ions (anions). For example, hydrochloric acid (HCl) dissociates in water to form hydrogen ions (H+) and chloride ions (Cl-).
HCl(aq) → H+(aq) + Cl-(aq)
These mobile ions in the solution are responsible for carrying electrical charge. When an electric potential difference (voltage) is applied across the solution, these ions are attracted to the electrodes with opposite charges. The movement of these charged ions through the solution constitutes an electric current, hence an aqueous solution of an acid conducts electricity. Strong acids dissociate almost completely, leading to a higher concentration of ions and thus better conductivity, while weak acids only partially dissociate.
Question:
What is a neutralization reaction? Give an example.
Concept in a Minute:
A neutralization reaction is a chemical reaction where an acid and a base react to form salt and water. The key concept is that the acidic properties of the acid are cancelled out by the basic properties of the base.
Explanation:
A neutralization reaction is a type of double displacement reaction. In this reaction, the hydrogen ion (H+) from an acid reacts with the hydroxide ion (OH-) from a base to form water (H2O). The remaining ions combine to form a salt. The general equation for a neutralization reaction is:
Acid + Base → Salt + Water
Example:
Consider the reaction between hydrochloric acid (HCl), which is an acid, and sodium hydroxide (NaOH), which is a base.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
In this reaction, hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to produce sodium chloride (NaCl), which is a salt, and water (H2O). The acidic nature of HCl is neutralized by the basic nature of NaOH, and vice-versa.
Question:
Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Concept in a Minute:
Acids react with reactive metals to produce a salt and a gas. This gas is flammable and burns with a pop sound.
Explanation:
When an acid reacts with a metal, a chemical reaction occurs where the metal displaces hydrogen from the acid. This process typically liberates hydrogen gas.
Illustrative Example:
Let’s consider the reaction between dilute hydrochloric acid (HCl) and zinc metal (Zn).
The balanced chemical equation is:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
In this reaction, zinc chloride (ZnCl2) is formed as a salt, and hydrogen gas (H2) is liberated.
Testing for the Presence of Hydrogen Gas:
To test for the presence of hydrogen gas, a burning candle or a matchstick is brought near the gas evolved.
Observation: The gas burns with a characteristic ‘pop’ sound, indicating the presence of hydrogen gas. This is because hydrogen is a highly flammable gas and it combusts rapidly in the presence of a flame.
Question:
Write an equation to show the reaction between Plaster of Paris and water.
Concept in a Minute:
Plaster of Paris is a chemical compound commonly used in construction and medicine. Its reaction with water is a setting process that involves hydration.
Explanation:
Plaster of Paris, chemically known as calcium sulfate hemihydrate (CaSO₄·½H₂O), reacts with water to form gypsum, which is calcium sulfate dihydrate (CaSO₄·2H₂O). This reaction is exothermic and results in the hardening of the mixture.
The balanced chemical equation for the reaction is:
2CaSO₄·½H₂O(s) + 3H₂O(l) → 2CaSO₄·2H₂O(s)
In this equation:
– CaSO₄·½H₂O represents Plaster of Paris (calcium sulfate hemihydrate).
– H₂O represents water.
– CaSO₄·2H₂O represents gypsum (calcium sulfate dihydrate).
The ‘(s)’ indicates a solid state, and ‘(l)’ indicates a liquid state. The coefficients (2 and 3) are used to balance the number of atoms of each element on both sides of the equation, ensuring that the law of conservation of mass is upheld.
Question:
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Concept in a Minute:
Exothermic reactions, heat of dilution, safety precautions when handling acids.
Explanation:
Diluting a concentrated acid with water is a highly exothermic process, meaning it releases a significant amount of heat.
If water is added to the acid, the small amount of water will be instantly heated to a very high temperature by the concentrated acid. This sudden increase in temperature can cause the water to boil rapidly and splash the corrosive acid out of the container, leading to severe burns and injuries.
However, when acid is added to water, the larger volume of water can absorb the heat generated more effectively. This gradual release of heat prevents the water from boiling and splashing, making the dilution process much safer. The recommended procedure ensures that the heat is dissipated into the larger volume of water, minimizing the risk of an uncontrolled exothermic reaction.
Question:
Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
Concept in a Minute:
Soil pH, acidity, alkalinity, neutralization reactions, calcium compounds and their role in agriculture.
Explanation:
A farmer would treat the soil of his fields with quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) when the soil is acidic. These substances are alkaline in nature and react with the acids present in the soil, neutralizing them. This process is known as liming. Acidic soils can hinder the growth of many crops as essential nutrients may not be available to plants in such conditions, and toxic substances like aluminum can become more soluble. Adding lime raises the soil pH, making it more neutral or slightly alkaline, which is favorable for the growth of most plants and improves the availability of nutrients. Calcium compounds are used because they are readily available and effective in raising soil pH.
Question:
A milkman adds a very small amount of baking soda to fresh milk. Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
Concept in a Minute:
Acids and bases, pH scale, properties of baking soda (sodium bicarbonate) as a base.
Explanation:
Fresh milk is slightly acidic, with a pH around 6. This acidity is due to the presence of lactic acid, which is produced by the bacterial fermentation of lactose (milk sugar). Baking soda, chemically known as sodium bicarbonate (NaHCO3), is a mild base. When a small amount of baking soda is added to the fresh milk, it reacts with the lactic acid present in the milk. This reaction neutralizes the acid, reducing the concentration of hydrogen ions (H+) in the milk. As the acidity decreases and the solution becomes less acidic, the pH value increases. Since baking soda is a base, it shifts the pH from the acidic range towards a neutral or slightly alkaline range. Therefore, the pH of the fresh milk shifts from 6 to slightly alkaline.
Question:
Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Concept in a Minute:
Acids release hydrogen ions (H+) in aqueous solution. This release is facilitated by the dissociation of polar covalent bonds in the acid molecule.
Explanation:
In aqueous solutions, acids like HCl and HNO3 dissociate to release hydrogen ions (H+). HCl dissociates into H+ and Cl- ions, and HNO3 dissociates into H+ and NO3- ions. These free H+ ions are responsible for the acidic properties. In contrast, alcohols (like ethanol, C2H5OH) and glucose (C6H12O6) are covalent compounds. Although they contain hydrogen atoms, these hydrogen atoms are not readily released as H+ ions in water because the O-H bonds in alcohols are less polar than the H-Cl or H-N bonds in mineral acids. Glucose does not have an acidic functional group that can release H+ ions. Therefore, their aqueous solutions do not exhibit acidic character.
Question:
Why should curd and sour substances not be kept in brass and copper vessels?
Concept in a Minute:
Acids react with metals to produce salts and hydrogen gas. Some metals, like brass and copper, react with acids to form poisonous compounds.
Explanation:
Curd and sour substances contain acids (like lactic acid). Brass and copper are reactive metals. When acids come into contact with brass and copper vessels, a chemical reaction occurs. The acid reacts with the metal to form metal salts. Some of these metal salts, particularly those of copper and brass (which is an alloy of copper and zinc), are toxic. Therefore, keeping curd and sour substances in brass and copper vessels can lead to the formation of poisonous compounds, making the food unsafe for consumption. Traditionally, earthenware or stainless steel vessels are preferred for storing such substances to avoid this reaction.
Question:
What is the common name of the compound CaOCl2?
Concept in a Minute:
Nomenclature of inorganic compounds, specifically common names.
Explanation:
The compound CaOCl2 is a chemical compound that acts as a bleaching agent. It is formed from calcium oxide (CaO) and chlorine (Cl2) or calcium hydroxide (Ca(OH)2) and chlorine. The common name for CaOCl2 is bleaching powder. This name reflects its primary use as a bleaching agent, particularly in textiles and paper industries.
Step-by-step derivation:
1. Identify the chemical formula given: CaOCl2.
2. Recall or research the common names associated with inorganic chemical compounds.
3. Recognize that CaOCl2 is a well-known compound with a specific common name.
4. Access knowledge base or chemical literature to find the common name for CaOCl2.
5. The common name for CaOCl2 is bleaching powder.
Question:
Plaster of Paris should be stored in a moisture-proof container. Explain why?
Concept in a Minute:
Chemical reaction between Plaster of Paris and water, hydration, formation of gypsum.
Explanation:
Plaster of Paris (calcium sulfate hemihydrate, CaSO₄·½H₂O) reacts with moisture (water) present in the atmosphere. This reaction, called hydration, causes Plaster of Paris to set and harden by forming calcium sulfate dihydrate (gypsum, CaSO₄·2H₂O). The chemical equation is:
2(CaSO₄·½H₂O) + 3H₂O → 2(CaSO₄·2H₂O)
If Plaster of Paris is stored in a moisture-proof container, it prevents contact with atmospheric moisture, thus preventing this setting and hardening reaction. This ensures that the Plaster of Paris remains in its powdery form, ready for use when needed. If exposed to moisture, it would harden in the container, rendering it useless.
Question:
How is the concentration of hydroxide ions (OH−) affected when excess base is dissolved in a solution of sodium hydroxide?
Concept in a Minute:
Dissociation of bases in water. For strong bases like sodium hydroxide (NaOH), dissociation is nearly complete.
Explanation:
Sodium hydroxide (NaOH) is a strong base. When dissolved in water, it dissociates almost completely into sodium ions (Na⁺) and hydroxide ions (OH⁻):
NaOH(aq) → Na⁺(aq) + OH⁻(aq)
The question states that “excess base is dissolved in a solution of sodium hydroxide.” This implies that more sodium hydroxide is being added to an existing solution of sodium hydroxide. Since sodium hydroxide is a strong electrolyte, it will further dissociate, increasing the concentration of hydroxide ions already present in the solution. Therefore, the concentration of hydroxide ions (OH⁻) will increase.
Question:
A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a long time to set as curd?
Concept in a Minute:
Acids and bases, pH, lactic acid, fermentation
Explanation:
Fresh milk is slightly acidic. When a milkman adds baking soda (sodium bicarbonate), which is a base, it neutralizes the natural acidity of the milk. Curd formation is a process of fermentation where lactic acid bacteria convert lactose (milk sugar) into lactic acid. This lactic acid production lowers the pH of the milk, causing the milk proteins to coagulate and form curd. By adding baking soda, the milk’s pH is raised, making it more alkaline. This increased pH inhibits the growth of lactic acid bacteria and slows down the production of lactic acid, thus taking a much longer time for the milk to set as curd.
Question:
Fresh milk from cows has a pH of 6. So how does the pH change when this milk is turned to curd? Explain your answer.
Concept in a Minute:
Acids and Bases: pH scale, meaning of pH, acidic and basic substances.
Fermentation: The process where microorganisms convert sugars into acids.
Explanation:
When fresh milk is turned into curd, the pH changes from 6 to a lower value. This is because lactic acid bacteria, which are responsible for curdling milk, convert lactose (the sugar present in milk) into lactic acid. Lactic acid is an acid, and the presence of more acid in the milk lowers its pH. Milk with a pH of 6 is slightly acidic. As lactic acid is produced during the fermentation process, the concentration of hydrogen ions increases, leading to a decrease in pH, making the curd more acidic than fresh milk.
Question:
Dry HCl gas does not change the colour of dry blue litmus paper. Why?
Concept in a Minute:
Acids show acidic properties only in the presence of water. Litmus paper is an indicator that changes color in the presence of acids or bases.
Explanation:
Dry HCl gas is a covalent compound and does not dissociate into ions in the absence of water. For HCl to exhibit acidic properties, it needs to dissolve in water and form hydrochloric acid. In its dry gaseous state, HCl does not produce H+ ions. Dry blue litmus paper is a pH indicator. Since dry HCl gas does not produce H+ ions, it does not create an acidic environment that would cause the litmus paper to change color. Therefore, dry HCl gas does not change the color of dry blue litmus paper.
Question:
Name the sodium compound which is used for softening hard water.
Concept in a Minute:
Hard water contains dissolved minerals, primarily calcium and magnesium ions. Softening hard water involves removing these ions. Sodium compounds are often used in water softening processes.
Explanation:
The sodium compound used for softening hard water is Sodium Carbonate (Na₂CO₃).
When sodium carbonate is added to hard water, it reacts with the dissolved calcium and magnesium ions to form insoluble precipitates of calcium carbonate and magnesium carbonate, which can then be removed by filtration. The reactions are:
Na₂CO₃(aq) + Ca²⁺(aq) → CaCO₃(s) + 2Na⁺(aq)
Na₂CO₃(aq) + Mg²⁺(aq) → MgCO₃(s) + 2Na⁺(aq)
Question:
What is a neutralisation reaction?
Concept in a Minute:
Acids react with bases to form salt and water. This is a chemical reaction.
Explanation:
A neutralisation reaction is a chemical reaction in which an acid and a base react quantitatively with each other. In a reaction in water, neutralization results in there being no excess of hydrogen or hydroxide ions present in the solution. The product of neutralization is generally a salt and water. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), it forms sodium chloride (NaCl) and water (H2O). The general equation for a neutralisation reaction is:
Acid + Base → Salt + Water
Question:
Name the substance which on treatment with chlorine yields bleaching powder?
Concept in a Minute:
The question asks to identify a substance that produces bleaching powder when reacted with chlorine. This involves understanding a fundamental chemical reaction related to the preparation of bleaching powder.
Explanation:
Bleaching powder, chemically known as calcium hypochlorite, is produced by the action of chlorine gas on slaked lime. Slaked lime is the common name for calcium hydroxide, Ca(OH)2. The reaction is as follows:
Ca(OH)2 (s) + Cl2 (g) → CaOCl2 (s) + H2O (l)
Therefore, the substance which on treatment with chlorine yields bleaching powder is slaked lime (calcium hydroxide).
Next Chapter: Carbon and Its Compounds
Refer Acids, Bases and Salts Notes
Practice Acids, Bases and Salts Extra Questions
Conquer Maths & Science – with LearnTheta’s AI-Practice!
✅ All Topics at One Place
🤖 Adaptive Question Practice
📊 Progress and Insights