11 state one commercial use of calcium oxide
12 the following data gives the ph values of some solutions
| solution | ph |
| --- | --- |
| p | 14.0 |
| q | 6.8 |
| r | 2.5 |
(a) what colour change would occur in solution p on addition of two drops of phenolphthalein indicator?
(d) state the ph value of a resulting solution when equal moles of solution p and r react
13 a beekeeper found that when stung by a bee, application of a little solution of sodium hydrogen carbonate helped to relieve the irritation of the affected area. explain
14 complete the table below to show the colour of the given indicator in acidic and basic solutions:
| indicator | colour in acidic solution | basic solution |
| --- | --- | --- |
| methyl orange | pink | |
| phenolphthalein | | pink |
15(a) what is rust?
(b) give two methods that can be used to prevent rusting
(c) name one substance which speeds up the rusting process
16 oxygen is obtained on large scale by the fractional distillation of air as shown on the flow chart bellow.
flow chart: air - filtration and purification - compress - expansion engine - liquid air - fractional distillation; with temperatures -196°c, -186°c, -183°c connected back
a) identify the substance that is removed at the filtration stage
b) explain why carbon (iv) oxide and water are removed before liquefaction of air
c) identify the component that is collected at -186°c
17 in an experiment, a piece of magnesium ribbon was cleaned with steel wool. 2.4g of the clean magnesium ribbon was placed in a crucible and completely burnt in oxygen. after cooling product weighed 4.0g
a) explain why it is necessary to clean magnesium ribbon

Question

11 state one commercial use of calcium oxide
12 the following data gives the ph values of some solutions
| solution | ph |
| --- | --- |
| p | 14.0 |
| q | 6.8 |
| r | 2.5 |
(a) what colour change would occur in solution p on addition of two drops of phenolphthalein indicator?
(d) state the ph value of a resulting solution when equal moles of solution p and r react
13 a beekeeper found that when stung by a bee, application of a little solution of sodium hydrogen carbonate helped to relieve the irritation of the affected area. explain
14 complete the table below to show the colour of the given indicator in acidic and basic solutions:
| indicator | colour in acidic solution | basic solution |
| --- | --- | --- |
| methyl orange | pink |  |
| phenolphthalein |  | pink |
15(a) what is rust?
(b) give two methods that can be used to prevent rusting
(c) name one substance which speeds up the rusting process
16 oxygen is obtained on large scale by the fractional distillation of air as shown on the flow chart bellow.
flow chart: air -> filtration and purification -> compress -> expansion engine -> liquid air -> fractional distillation; with temperatures -196°c, -186°c, -183°c connected back
a) identify the substance that is removed at the filtration stage
b) explain why carbon (iv) oxide and water are removed before liquefaction of air
c) identify the component that is collected at -186°c
17 in an experiment, a piece of magnesium ribbon was cleaned with steel wool. 2.4g of the clean magnesium ribbon was placed in a crucible and completely burnt in oxygen. after cooling product weighed 4.0g
a) explain why it is necessary to clean magnesium ribbon

Accepted Answer

### Question 11 # Brief Explanations: Calcium Oxide (CaO) is used in many commercial applications. One common commercial use is in the production of cement. It is also used for drying purposes (as a desiccant), in the steel industry for purifying iron, and in agriculture to improve soil pH (liming). # Answer: One commercial use of Calcium Oxide is in the manufacture of cement (or as a desiccant, in steel production, or for soil liming). ### Question 12(a) # Brief Explanations: Solution P has a pH of 14.0, which means it is a strong base. Phenolphthalein indicator is colorless in acidic and neutral solutions but turns pink (or magenta) in basic solutions (pH > 8.2). Since solution P is a strong base (pH = 14), adding phenolphthalein will cause it to turn pink (or magenta). # Answer: The color change in solution P (a strong base, pH = 14) on adding phenolphthalein indicator will be from colorless to pink (or magenta). ### Question 12(d) # Explanation: ## Step 1: Identify the nature of solutions P and R Solution P has a pH of 14.0, so it is a strong base (e.g., a strong alkali like NaOH or KOH). Solution R has a pH of 2.5, so it is a strong acid (e.g., HCl or H₂SO₄). ## Step 2: Reaction between equal moles of strong acid and strong base When equal moles of a strong acid and a strong base react, they undergo a neutralization reaction: $ \text{Acid} + \text{Base} ightarrow \text{Salt} + \text{Water} $. For a strong acid and strong base, the resulting solution is neutral (since the H⁺ ions from the acid and OH⁻ ions from the base react completely to form water, and the salt formed is neutral). A neutral solution has a pH of 7.0. # Answer: The pH value of the resulting solution when equal moles of solution P (strong base) and R (strong acid) react is 7.0. ### Question 13 # Brief Explanations: Bee stings inject an acidic substance (like formic acid) into the skin, causing irritation. Sodium hydrogen carbonate (NaHCO₃) is a weak base (bicarbonate). When applied to the affected area, the bicarbonate ion ($ \text{HCO}_3^- $) reacts with the acidic substance (H⁺ from the formic acid) in a neutralization reaction: $ \text{HCO}_3^- + \text{H}^+ ightarrow \text{H}_2\text{O} + \text{CO}_2 \uparrow $. This reaction neutralizes the acid, reducing the irritation. # Answer: Bee stings inject an acidic substance (e.g., formic acid) into the skin. Sodium hydrogen carbonate (NaHCO₃) is a weak base. It neutralizes the acidic substance via the reaction $ \text{HCO}_3^- + \text{H}^+ ightarrow \text{H}_2\text{O} + \text{CO}_2 \uparrow $, thus relieving the irritation. ### Question 14 # Brief Explanations: - **Methyl orange**: In acidic solutions (pH < 3.1), methyl orange is red; in basic solutions (pH > 4.4), it is yellow. The table initially has "Pink" for acidic, which is incorrect. The correct color in basic solution for methyl orange is yellow. - **Phenolphthalein**: In acidic and neutral solutions (pH < 8.2), phenolphthalein is colorless; in basic solutions (pH > 8.2), it is pink. The table initially has "Pink" for basic (correct) and a blank for acidic, so the color in acidic solution is colorless. So, the completed table is: | Indicator | Colour in acidic solution | Basic solution | |-----------------|---------------------------|----------------| | Methyl orange | Red (correcting the initial "Pink") | Yellow | | Phenolphthalein | Colorless | Pink | (Note: The initial "Pink" for methyl orange in acidic is incorrect; the correct color in acidic is red, and in basic is yellow. For phenolphthalein, acidic is colorless, basic is pink (as given initially for basic is correct).) # Answer: | Indicator | Colour in acidic solution | Basic solution | |-----------------|---------------------------|----------------| | Methyl orange | Red | Yellow | | Phenolphthalein | Colorless | Pink | ### Question 15(a) # Brief Explanations: Rust is the common name for the hydrated iron(III) oxide formed when iron (or steel) reacts with oxygen and water (moisture) in the presence of air. The chemical formula for rust is typically $ \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} $ (where $ n $ is the number of water molecules, usually around 1 - 3), or more simply $ \text{Fe}_2\text{O}_3 $ (iron(III) oxide) with water of hydration. It is a reddish-brown, flaky substance that forms on the surface of iron or steel objects exposed to air and moisture. # Answer: Rust is the hydrated iron(III) oxide ($ \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} $) formed when iron (or steel) reacts with oxygen and water (moisture) in the air. It is a reddish-brown, flaky substance. ### Question 15(b) # Brief Explanations: Two methods to prevent rusting are: 1. **Painting or Coating with a Protective Layer**: Applying paint, oil, grease, or a layer of another metal (like zinc in galvanization) to the iron/steel surface. This prevents air (oxygen) and water from coming into contact with the iron, thus inhibiting the rusting reaction. 2. **Galvanization (Zinc Coating)**: Coating iron or steel with a layer of zinc. Zinc is more reactive than iron, so it oxidizes (corrodes) preferentially, protecting the iron from rusting (sacrificial protection). Another method is **Alloying** (e.g., making stainless steel by adding chromium and nickel to iron), which makes the iron more resistant to rusting. # Answer: Two methods to prevent rusting are: 1. Painting (or applying a protective coating like oil/grease) to the iron/steel surface to prevent contact with air and water. 2. Galvanization (coating iron/steel with zinc) or alloying (e.g., making stainless steel). ### Question 15(c) # Brief Explanations: Substances that speed up (catalyze) the rusting process are typically electrolytes (substances that conduct electricity when dissolved in water), such as salts (e.g., sodium chloride, $ \text{NaCl} $) or acids. For example, salt water (containing $ \text{NaCl} $) increases the rate of rusting because the dissolved ions ( $ \text{Na}^+ $ and $ \text{Cl}^- $) increase the electrical conductivity of the water on the iron surface, facilitating the electrochemical reaction of rusting. Acids (like $ \text{H}_2\text{SO}_4 $ or $ \text{HCl} $) can also speed up rusting by providing more H⁺ ions, which participate in the rusting reaction. # Answer: One substance that speeds up the rusting process is sodium chloride (NaCl, common salt) or an acid (like $ \text{H}_2\text{SO}_4 $ or $ \text{HCl} $). ### Question 16(a) # Brief Explanations: At the filtration and purification stage of air before liquefaction, solid particles (dust, pollen, etc.) and some gaseous impurities (like dust, smoke, and other suspended particles) are removed. Additionally, water vapor and carbon dioxide are often removed later, but at the filtration stage, the primary substance removed is solid particulate matter (dust, dirt, etc.) from the air. # Answer: The substance removed at the filtration stage is solid particulate matter (dust, dirt, suspended solid impurities) from the air. ### Question 16(b) # Brief Explanations: Carbon (IV) oxide ($ \text{CO}_2 $) and water ($ \text{H}_2\text{O} $) are removed before liquefaction of air because: - $ \text{CO}_2 $ and $ \text{H}_2\text{O} $ would freeze (solidify) at the low temperatures required for air liquefaction (around -196°C for nitrogen, -183°C for oxygen). Solid $ \text{CO}_2 $ (dry ice) and ice (frozen water) would block the pipes and equipment used in the liquefaction and fractional distillation process. - Removing these substances ensures that the liquefaction process proceeds smoothly without blockages and that the resulting liquid air is pure for separation into its components. # Answer: $ \text{CO}_2 $ and $ \text{H}_2\text{O} $ are removed before liquefaction because they would freeze (solidify) at the low temperatures of air liquefaction, blocking pipes and equipment. ### Question 16(c) # Explanation: ## Step 1: Recall boiling points of air components Air is mainly composed of nitrogen (boiling point ≈ -196°C), oxygen (boiling point ≈ -183°C), and argon (boiling point ≈ -186°C). ## Step 2: Identify the component at -186°C During fractional distillation of liquid air, components with lower boiling points vaporize first. The boiling point of argon is approximately -186°C, so the component collected at -186°C is argon (Ar). # Answer: The component collected at -186°C is argon (Ar). ### Question 17(a) # Brief Explanations: Magnesium ribbon, when exposed to air, forms a layer of magnesium oxide (MgO) on its surface due to oxidation: $ 2\text{Mg} + \text{O}_2 ightarrow 2\text{MgO} $. This MgO layer is less reactive than pure magnesium and can prevent or slow down the reaction of magnesium with oxygen during the burning experiment. Cleaning the magnesium ribbon with steel wool removes this oxide layer, exposing the fresh, reactive magnesium metal, ensuring that the reaction (burning in oxygen) proceeds completely and accurately (so that the mass change can be measured correctly for stoichiometric calculations). # Answer: It is necessary to clean magnesium ribbon to remove the layer of magnesium oxide (MgO) that forms on its surface due to oxidation in air. This oxide layer is less reactive and would prevent or slow down the reaction of magnesium with oxygen during burning, so removing it exposes the fresh, reactive magnesium for a complete reaction.

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Explanation:

Question 11

Step 1: Identify the nature of solutions P and R

Step 2: Reaction between equal moles of strong acid and strong base

Answer:

Question 12(a)