Comprehensive Study Guide on Acids, Bases, and Salt Preparations

Year 7 Chemistry Foundation Review

Litmus Paper Colors: * Colour of litmus in acid: Red. * Colour of litmus in alkali: Blue.
Evaporation Apparatus for Salt Solutions: * To evaporate salt from a solution, the following equipment is used in the setup: * Evaporating basin (contains the solution). * Gauze (placed on top of the tripod). * Tripod (supports the evaporating basin and gauze). * Bunsen burner (heat source). * Heatproof mat (protects the bench surface).
Atomic Composition of Common Compounds: 1. $CuSO_4$ (Copper(II) sulfate): Contains $1$ atom of copper, $1$ atom of sulfur, and $4$ atoms of oxygen. 2. $HNO_3$ (Nitric acid): Contains $1$ atom of hydrogen, $1$ atom of nitrogen, and $3$ atoms of oxygen. 3. $Na_2CO_3$ (Sodium carbonate): Contains $2$ atoms of sodium, $1$ atom of carbon, and $3$ atoms of oxygen.
Separation Techniques: * Filtration: Used to remove an insoluble solid mixed into a liquid. * Evaporation: Used to remove a soluble salt from a solution.
Bunsen Burner Flame Types: * Roaring Flame: Occurs when the air hole is open. * Safety Flame: Occurs when the air hole is closed.

Syllabus Requirements: Acids, Bases, and Salt Preparations

Learning Objectives: * 2.37: Describe the reactions of hydrochloric acid ( $HCl$ ), sulfuric acid ( $H_2SO_4$ ), and nitric acid ( $HNO_3$ ) with metals, bases, and metal carbonates to form salts (noting that reactions between nitric acid and metals are typically excluded at this level). * 2.38: Understand that metal oxides, metal hydroxides, and ammonia ( $NH_3$ ) can act as bases. Alkalis are specifically bases that are soluble in water. * 2.42 (Practical): Prepare a sample of pure, dry hydrated copper(II) sulfate crystals starting from copper(II) oxide ( $CuO$ ).

The pH Scale and Indicators

The pH Scale: Provides a numerical value (0–14) representing the acidity or alkalinity of a substance. * 0–3: Strong acid. * 4–6: Weak acid. * 7: Neutral. * 8–10: Weak alkali. * 11–14: Strong alkali.
Universal Indicator: A chemical that changes color to determine pH experimentally: * Red/Orange: Acidic (low pH). * Green: Neutral (pH 7). * Blue/Purple: Alkaline (high pH).
Common Indicators: * Litmus. * Universal indicator. * Methyl orange.

Defining Acids, Bases, Alkalis, and Salts

Acids: * Substances that react with water to release hydrogen ions ( $H^+$ ). * pH value is below 7. * Examples: * Hydrochloric acid: $HCl$ * Sulfuric acid: $H_2SO_4$ * Nitric acid: $HNO_3$ * Phosphoric acid: $H_3PO_4$ * Ethanoic acid: $CH_3COOH$
Bases: * Substances that react with acids to form a salt and water (and sometimes carbon dioxide). * Common bases include metal oxides (e.g., Calcium oxide, $CaO$ ) and metal carbonates (e.g., Sodium carbonate, $Na_2CO_3$ ). * Examples: * Magnesium oxide: $MgO$ * Copper(II) oxide: $CuO$ * Zinc oxide: $ZnO$ * Calcium carbonate: $CaCO_3$ * Iron(III) oxide: $Fe_2O_3$
Alkalis: * A special type of base that is soluble in water and releases hydroxide ions ( $OH^-$ ). * pH value is above 7. * Usually metal hydroxides. * Examples: * Lithium hydroxide: $LiOH$ * Sodium hydroxide: $NaOH$ * Potassium hydroxide: $KOH$ * Ammonia: $NH_3$ * Barium hydroxide: $Ba(OH)_2$ * Calcium hydroxide: $Ca(OH)_2$ * Rubidium hydroxide: $RbOH$
Salts: * Ionic substances formed when an acid is neutralized by a base. * The name of the salt depends on the acid used: * Hydrochloric acid forms Chlorides ( $Cl^-$ ). * Sulfuric acid forms Sulfates ( $SO_4^{2-}$ ). * Nitric acid forms Nitrates ( $NO_3^-$ ). * Phosphoric acid forms Phosphates ( $PO_4^{3-}$ ). * Ethanoic acid forms Ethanoates. * Citric acid forms Citrates. * Examples: * Sodium chloride: $NaCl$ * Potassium nitrate: $KNO_3$ * Sodium sulfate: $Na_2SO_4$ * Calcium sulfate: $CaSO_4$ * Magnesium chloride: $MgCl_2$ * Copper(II) sulfate: $CuSO_4$

General Reactions of Acids

1. Acid + Alkali (Neutralization)

General Equation: $\text{Acid} + \text{Alkali} \rightarrow \text{Salt} + \text{Water}$
Example Word Equation: $\text{Hydrochloric acid} + \text{Sodium hydroxide} \rightarrow \text{Sodium chloride} + \text{Water}$
Example Symbol Equation: $HCl_{(aq)} + NaOH_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)}$
Observation: Both reactants are colourless. There is no visible change unless a Universal Indicator is added. The indicator changes from red (acid) to green (neutral salt solution).

2. Acid + Metal

General Equation: $\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen}$
Example Word Equation: $\text{Hydrochloric acid} + \text{Magnesium} \rightarrow \text{Magnesium chloride} + \text{Hydrogen}$
Example Symbol Equation: $2HCl + Mg \rightarrow MgCl_2 + H_2$
Observation: Fizzing/effervescence occurs as hydrogen gas is produced. The reaction between a colourless acid and shiny metal can be confirmed by a "squeaky pop" test with a lighted splint.

3. Acid + Base (Metal Oxide)

General Equation: $\text{Acid} + \text{Base (Metal oxide)} \rightarrow \text{Salt} + \text{Water}$
Example Word Equation: $\text{Hydrochloric acid} + \text{Copper(II) oxide} \rightarrow \text{Copper(II) chloride} + \text{Water}$
Example Symbol Equation: $2HCl + CuO \rightarrow CuCl_2 + H_2O$
Observation: Dark solid powder (copper oxide) reacts with colourless acid to form a pale blue clear solution.

4. Acid + Carbonate

General Equation: $\text{Acid} + \text{Metal Carbonate} \rightarrow \text{Salt} + \text{Carbon dioxide} + \text{Water}$
Example Word Equation: $\text{Hydrochloric acid} + \text{Calcium carbonate} \rightarrow \text{Calcium chloride} + \text{Carbon dioxide} + \text{Water}$
Example Symbol Equation: $2HCl + CaCO_3 \rightarrow CaCl_2 + CO_2 + H_2O$
Observation: Effervescence occurs; the gas produced turns limewater cloudy, confirming the presence of carbon dioxide.

Practical: Preparation of Pure, Dry Hydrated Copper(II) Sulfate Crystals

Aims: To produce pure, solid crystals from an insoluble base ( $CuO$ ) and an acid ( $H_2SO_4$ ).
Reactants: * 3 spatulas of Copper(II) oxide ( $CuO$ ) - a black powder. * $25\,cm^3$ of Sulfuric acid ( $H_2SO_4$ ).
Procedure: 1. Tip the solid copper oxide into a $100\,cm^3$ beaker. 2. Measure $25\,cm^3$ sulfuric acid using a measuring cylinder. 3. Add warm acid to the beaker and stir for 5 minutes. 4. Add the base in excess to ensure all the acid has reacted/been neutralized. 5. Filter the mixture through filter paper in a filter funnel into an evaporating dish to remove the excess insoluble base. 6. Heat the blue filtrate strongly in an evaporating basin over a Bunsen burner to remove some water and concentrate the solution (creating a saturated solution). 7. Transfer the solution to a crystallising dish and leave it to cool on a windowsil.
Observations: * Initial appearance: Colourless liquid ( $H_2SO_4$ ) and black powder ( $CuO$ ). * After mixing: The liquid turns into a blue clear solution. * Filtering: Residue is a wet black powder sludge; filtrate is a blue clear liquid. * Crystals: As the hot saturated solution cools, solubility decreases, causing the solute to form crystals.
Equations: * Word: $\text{Copper oxide} + \text{Sulphuric acid} \rightarrow \text{Copper sulphate} + \text{Water}$ * Symbol: $CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$

Practical: Preparation of Copper(II) Chloride from a Carbonate

Aim: Produce pure solid copper chloride from copper carbonate and hydrochloric acid.
Procedure Order: 1. Add the solid carbonate ( $CuCO_3$ ) to the acid solution ( $HCl$ ) and stir. 2. Continue adding small quantities of copper carbonate until there is no more fizzing (effervescence) and solid can be seen in the tube (representing excess). 3. Filter the mixture into an evaporating dish. 4. Heat over a Bunsen burner flame until crystals form on a cold glass rod dipped into the solution. 5. Transfer into a crystallising dish and leave to cool.
Observations: * Clear $HCl$ reacting with green copper carbonate powder turned into a teal solution. * Once filtered, it became a light blue solution with green sludge remaining as residue.
Efficiency Checks: * Effervescence is caused by the release of $CO_2$ . * Dipping a glass rod helps identify the point where the solution is saturated and crystals begin to form.

Practical: Preparation of Hydrated Zinc Sulfate

Method: * Step 1: Add an excess of zinc ( $Zn$ ) to $25\,cm^3$ of dilute sulfuric acid ( $H_2SO_4$ ) and warm. * Step 2: Filter to remove excess zinc. * Step 3: Partly evaporate the filtrate and leave it to cool.
Key Reasoning: * Excess zinc ensures all acid is used up. * Completion of reaction is indicated by the cessation of fizzing (no gas given off). * Partial Evaporation: Only some liquid is removed to assist crystallization. Total evaporation would remove the water needed for the hydrated structure, preventing crystal formation. * Drying: Crystals should be dried with gentle heat or filter paper. Strong heating would remove the water of crystallization.
Equations: * Word: $\text{Zinc} + \text{Sulphuric acid} \rightarrow \text{Zinc sulphate} + \text{Hydrogen}$ * Symbol (Anhydrous): $Zn_{(s)} + H_2SO_{4(aq)} \rightarrow ZnSO_{4(aq)} + H_{2(g)}$ * Symbol (Hydrated): $Zn_{(s)} + H_2SO_{4(aq)} + 7H_2O_{(l)} \rightarrow ZnSO_4 \cdot 7H_2O_{(s)} + H_{2(g)}$

Study Guide and Review Notes

Naming Salt Logic: * The first part of the salt name comes from the metal in the base, carbonate, or metal reactent. * The second part (the "surname") comes from the acid: * Hydrochloric acid $\rightarrow$ chloride. * Sulfuric acid $\rightarrow$ sulfate. * Nitric acid $\rightarrow$ nitrate.
Critical Concepts for Assessment: * Warming the Acid: Speeds up the reaction (e.g., in copper sulfate preparation). * Excess Reactents: Used to ensure all acid is neutralized and a saturated solution is formed. * Apparatus Identification: Be able to name beakers, measuring cylinders, filter funnels, and evaporating basins. * Methods to Dry Crystals: Filter them out or pat them dry with filter paper. * Ways to Start Crystal Growth: One method is to heat the solution gently to concentrate it. * Word and Chemical Equations: Be prepared to write them for various combinations of acids and bases/metals.