Welcome to Acids, Bases, and Salts!
Have you ever wondered why lemon juice tastes sour, why soap feels slippery between your fingers, or how a simple antacid tablet settles an upset stomach? All of these everyday experiences are governed by the chemistry of acids, bases, and salts.
In this chapter, we explore how chemical substances transform and interact. This fits right into our broader theme of Chemical change and quantities. By understanding how acids and bases behave, you will be able to explain real-world phenomena, design scientific investigations (Criterion B), analyse experimental data (Criterion C), and evaluate how chemistry impacts society and the environment (Criterion D). Don't worry if chemistry sometimes feels abstract—we will break everything down into clear, easy-to-follow steps!
Note: For broader patterns in chemical reactions, refer to "Types of chemical reactions", and for quantitative measurements, see "Quantities: the mole and stoichiometry".
---1. Understanding Acids and Bases
What is an Acid?
An acid is a substance that produces hydrogen ions (\(\text{H}^{+}\)) when dissolved in water (an aqueous solution). These free hydrogen ions are responsible for the characteristic properties of acids.
Common properties of acids include:
• They have a distinct sour taste (like the citric acid in lemons or ethanoic acid in vinegar). Never taste chemicals in a laboratory!
• They have a pH value of less than 7 (\(\text{pH} < 7\)).
• They turn blue litmus paper red.
• They conduct electricity in aqueous solutions because they release mobile ions.
What is a Base? What is an Alkali?
A base is a substance that reacts with an acid to neutralise it, forming a salt and water. Bases are typically metal oxides, metal hydroxides, or metal carbonates.
An alkali is simply a base that dissolves in water. When an alkali dissolves, it produces hydroxide ions (\(\text{OH}^{-}\)).
• Analogy: Think of bases as "vehicles" and alkalis as "submarines." All submarines are vehicles, but only the ones designed to go into the water count as submarines! Similarly, all alkalis are bases, but not all bases are alkalis.
Common properties of alkalis include:
• They feel slippery or soapy to the touch.
• They have a bitter taste.
• They have a pH value greater than 7 (\(\text{pH} > 7\)).
• They turn red litmus paper blue.
• They conduct electricity in aqueous solution due to mobile ions.
Quick Review: Acids release \(\text{H}^{+}\) ions in water; alkalis release \(\text{OH}^{-}\) ions in water.
---2. The pH Scale and Indicators
The pH Scale
The pH scale is a numerical scale running typically from 0 to 14 that measures how acidic or alkaline a solution is based on the concentration of hydrogen ions (\(\text{H}^{+}\)):
• Acidic: \(\text{pH} < 7\) (The lower the pH, the higher the concentration of \(\text{H}^{+}\) ions and the stronger the acid).
• Neutral: \(\text{pH} = 7\) (Pure water at standard conditions, where \([\text{H}^{+}] = [\text{OH}^{-}]\)).
• Alkaline: \(\text{pH} > 7\) (The higher the pH, the lower the concentration of \(\text{H}^{+}\) ions, the higher the concentration of \(\text{OH}^{-}\) ions, and the stronger the alkali).
Measuring pH with Indicators
An indicator is a chemical substance that changes colour depending on the pH of the solution it is added to.
• Litmus Paper: Turns red in acidic solutions and blue in alkaline solutions. (Memory trick: Blue for Base!).
• Universal Indicator: A mixture of dyes that shows a smooth gradient of colours across the entire pH scale (red for strong acids, yellow/orange for weak acids, green for neutral, blue for weak alkalis, and purple for strong alkalis).
• pH Meters and Probes: Electronic sensors that provide a digital, quantitative measurement of pH to one or two decimal places, eliminating human error in judging colours.
Did you know? Many plants contain natural indicators! Red cabbage extract turns pink/red in lemon juice (acid) and green/yellow in baking soda solution (alkali).
Key Takeaway: pH tells us the acidity or alkalinity of a system. Indicators provide qualitative colour changes, while digital pH probes give precise quantitative data.
---3. Characteristic Chemical Reactions of Acids
Acids undergo predictable patterns of transformation. Learning the general word equations below will help you solve problems in unfamiliar scenarios with confidence.
Reaction 1: Acid + Metal
When an acid reacts with a sufficiently reactive metal, it produces a salt and hydrogen gas:
\(\text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen gas}\)
• Example: Hydrochloric acid + Magnesium \(\rightarrow\) Magnesium chloride + Hydrogen
• Chemical equation: \(2\text{HCl}\text{ (aq)} + \text{Mg}\text{ (s)} \rightarrow \text{MgCl}_{2}\text{ (aq)} + \text{H}_{2}\text{ (g)}\)
• Test for Hydrogen gas: Collect the gas in a test tube and hold a burning wooden splint near the mouth. It will extinguish with a characteristic squeaky pop.
Reaction 2: Acid + Base / Alkali (Neutralisation)
When an acid reacts with a metal oxide or metal hydroxide, they neutralise each other to form a salt and water:
\(\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}\)
• Example: Sulfuric acid + Copper(II) oxide \(\rightarrow\) Copper(II) sulfate + Water
• Chemical equation: \(\text{H}_{2}\text{SO}_{4}\text{ (aq)} + \text{CuO}\text{ (s)} \rightarrow \text{CuSO}_{4}\text{ (aq)} + \text{H}_{2}\text{O}\text{ (l)}\)
• Ionic basis of neutralisation: When an acid meets an alkali in solution, the hydrogen ions react with hydroxide ions to form water molecules:
\(\text{H}^{+}\text{ (aq)} + \text{OH}^{-}\text{ (aq)} \rightarrow \text{H}_{2}\text{O}\text{ (l)}\)
Reaction 3: Acid + Metal Carbonate
When an acid reacts with a carbonate, it forms a salt, water, and carbon dioxide gas:
\(\text{Acid} + \text{Metal Carbonate} \rightarrow \text{Salt} + \text{Water} + \text{Carbon dioxide}\)
• Example: Nitric acid + Calcium carbonate \(\rightarrow\) Calcium nitrate + Water + Carbon dioxide
• Chemical equation: \(2\text{HNO}_{3}\text{ (aq)} + \text{CaCO}_{3}\text{ (s)} \rightarrow \text{Ca(NO}_{3}\text{)}_{2}\text{ (aq)} + \text{H}_{2}\text{O}\text{ (l)} + \text{CO}_{2}\text{ (g)}\)
• Test for Carbon Dioxide gas: Bubble the gas through clear limewater (aqueous calcium hydroxide). The limewater turns milky/cloudy due to the formation of a fine white precipitate.
Summary of Salt Naming:
• Hydrochloric acid forms chloride salts.
• Sulfuric acid forms sulfate salts.
• Nitric acid forms nitrate salts.
4. What is a Salt and How are Salts Prepared?
A salt is an ionic compound formed when the hydrogen ion (\(\text{H}^{+}\)) of an acid is replaced by a metal ion or an ammonium ion (\(\text{NH}_{4}^{+}\)). Everyday table salt (\(\text{NaCl}\)) is just one example of a vast family of chemical salts.
Preparing a Soluble Salt (Insoluble Base Method)
To produce pure crystals of a soluble salt (such as copper(II) sulfate), scientists typically react an acid with an excess of an insoluble base or carbonate. Follow this step-by-step method:
1. Heating and Reacting: Gently warm a measured volume of dilute acid in a beaker. Add the insoluble metal oxide powder gradually, stirring continuously, until no more dissolves (the base is in excess). This ensures all the acid has reacted.
2. Filtration: Filter the mixture using filter paper and a funnel to remove the unreacted excess solid base. The clear liquid collected is the pure salt solution (filtrate).
3. Evaporation: Heat the salt solution in an evaporating basin over a water bath until you reach the crystallisation point (crystals start forming on a cool glass rod dipped into the liquid).
4. Crystallisation: Allow the remaining concentrated solution to cool slowly at room temperature. Large, regular crystals will form as solubility decreases.
5. Drying: Filter out the crystals and pat them dry carefully between two sheets of filter paper.
Common Mistake to Avoid: Do not boil the solution until completely dry! Boiling drives off water too quickly, leaving a powdery, anhydrous residue rather than well-formed crystal structures.
---5. Scientific Inquiry & Investigation Skills (Criteria B & C)
In MYP Sciences eAssessments, half of your examination marks come from inquiry and data-processing tasks. Acid-base chemistry provides an ideal context to demonstrate these skills.
Formulating Hypotheses and Controlling Variables
Imagine investigating how the concentration of acid affects the rate of carbon dioxide produced when reacting with marble chips (\(\text{CaCO}_{3}\)):
• Independent Variable: The factor you deliberately alter (e.g., concentration of hydrochloric acid in \(\text{mol}\cdot\text{dm}^{-3}\)).
• Dependent Variable: The factor you measure (e.g., volume of \(\text{CO}_{2}\) collected in \(\text{cm}^{3}\) after 60 seconds).
• Controlled Variables: Factors kept strictly constant to ensure a valid and fair test (e.g., mass and surface area of marble chips, total volume of liquid, reaction temperature).
• Hypothesis: "If the concentration of hydrochloric acid increases, then the volume of carbon dioxide produced per unit time will increase, because a greater concentration provides more reacting particles per unit volume, increasing the frequency of successful collisions."
Data Processing and Evaluation
• Transforming Data: Always calculate the mean (average) of repeated trials, excluding clear anomalies. Ensure calculated values preserve appropriate decimal places.
• Graphing Conventions: Plot the independent variable on the horizontal axis (\(x\)-axis) and the dependent variable on the vertical axis (\(y\)-axis), complete with labels and units.
• Evaluating Methods: Identify sources of systematic and random error. For example, if gas escaped before attaching the rubber bung, an improvement would be using a divided flask or suspended test tube to mix reactants only after the system is fully sealed.
6. Real-World Applications and Global Contexts (Criterion D)
Scientific knowledge is not confined to the lab. Understanding acids and bases allows societies to innovate, manage environments, and improve human health.
1. Environmental Impact: Acid Deposition and Soil Chemistry
• The Issue: Industrial emissions of sulfur dioxide and nitrogen oxides dissolve in atmospheric water to create acid rain. This lowers the pH of lakes and soil, leaching essential nutrients and releasing toxic ions that harm aquatic life and plant roots.
• The Scientific Solution: Farmers and environmental managers apply crushed limestone or lime (calcium carbonate/hydroxide), which acts as a base to neutralise overly acidic soils and restore aquatic ecosystems.
• Implications: While liming remediates the local pH, it is expensive and addresses only the symptom. Long-term sustainability requires international regulations and technological innovation (such as industrial scrubbers) to curb emissions at the source.
2. Human Health: Regulating Stomach Acidity
• The Issue: The human stomach secretes hydrochloric acid (\(\text{pH} \approx 1.5 - 2.0\)) to activate digestive enzymes and destroy pathogens. Excess acid production can cause heartburn and tissue irritation.
• The Scientific Solution: Antacid medications contain mild, non-toxic bases (such as magnesium hydroxide or calcium carbonate) that undergo neutralisation reactions with excess stomach acid, forming neutral salts and raising the pH to a comfortable level.
• Reflecting on Science: Understanding chemical quantities ensures that antacids neutralise sufficient acid without making the stomach excessively alkaline, which would disrupt healthy digestion.
Chapter Review & Summary Checklist
Before moving on, verify that you can:
• Define acids as sources of \(\text{H}^{+}\) and alkalis as soluble bases that produce \(\text{OH}^{-}\).
• Interpret the pH scale from 0 to 14 and describe how indicators and digital meters distinguish acids, neutrals, and alkalis.
• Predict the products and write equations for reactions of acids with metals, bases, and carbonates.
• Outline the practical laboratory method for preparing pure, dry crystals of a soluble salt using an insoluble base.
• Formulate testable hypotheses, identify variables, and design valid experimental procedures to investigate acid-base reactions.
• Discuss the environmental and social implications of acid-base chemistry in contexts like soil management and healthcare.