The characteristic properties of acids and bases Revision Notes

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. This proton donation is the fundamental property that drives their chemical reactions. The presence of H⁺ ions makes solutions acidic and reactive towards certain substances such as metals, bases, and carbonates.

• Acids increase the concentration of H⁺ ions in aqueous solutions.
• This proton donation underlies their characteristic reactions.

When acids react with certain metals, a displacement reaction occurs where the metal atoms lose electrons to form positive ions, and hydrogen ions from the acid gain electrons to form hydrogen gas. This reaction produces a salt and hydrogen gas, which can be observed as bubbles.

For example, zinc reacts with hydrochloric acid as follows:

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) (bubbles) Here, zinc chloride is the salt formed, and hydrogen gas is released.

• Only metals above hydrogen in the reactivity series react with acids to produce hydrogen gas.
• The reaction is often vigorous and produces visible gas bubbles.

Acids react with bases in a neutralisation reaction to form water and a salt. Bases are substances that can accept H⁺ ions or provide hydroxide ions (OH⁻). When an acid and a base combine, the H⁺ ions from the acid react with OH⁻ ions from the base to form water, while the remaining ions form a salt.

For example, hydrochloric acid reacts with sodium hydroxide:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) This reaction is important in controlling pH and is widely used in industry and laboratories.

• Neutralisation reactions produce water and a salt.
• They reduce acidity by removing free H⁺ ions.

Acids react with carbonate compounds to produce carbon dioxide gas, water, and a salt. This reaction is often used as a test for carbonates, as the release of carbon dioxide gas causes bubbling or fizzing.

For example, hydrochloric acid reacts with calcium carbonate:

2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + CO₂(g) + H₂O(l) The carbon dioxide gas can be tested by bubbling it through limewater, which turns milky.

• Carbonates react with acids to release CO₂ gas.
• The reaction also produces water and a salt.

The reactions of acids with metals, bases, and carbonates illustrate their chemical properties and practical uses. The production of hydrogen gas from metals shows acid reactivity and safety considerations. Neutralisation reactions are fundamental in controlling pH in various contexts. The reaction with carbonates is useful for identifying carbonate compounds and understanding acid effects on minerals.

Understanding these reactions helps predict acid behaviour and apply this knowledge in laboratory and real-world situations.

• Acids donate protons causing characteristic reactions.
• Reactions produce salts and gases or water depending on the reactant.
• These reactions have practical applications in testing, manufacturing, and environmental science.

Indicators are substances that change colour when added to solutions of different pH values. They help detect whether a solution is acidic, neutral, or alkaline without needing complex equipment.

Indicators contain molecules that react chemically with hydrogen ions (H⁺) present in acids or hydroxide ions (OH⁻) in alkalis, causing a visible colour change. This reaction is reversible and depends on the concentration of these ions in the solution.

• Indicators provide a simple visual test for acidity or alkalinity.
• They work by changing colour due to chemical reactions with H⁺ or OH⁻ ions.
• Different indicators change colour at different pH ranges.

Litmus is a natural dye extracted from lichens and is commonly used as a pH indicator. It is available as paper strips or solution.

When litmus paper is dipped into an acidic solution, it turns red. This happens because the hydrogen ions in the acid interact with the dye molecules, altering their structure and thus their colour.

In neutral or alkaline solutions, litmus paper remains purple or turns blue, respectively, but the focus here is on the acidic effect.

• Litmus paper turns red in acidic solutions (pH < 7).
• The colour change is due to the interaction of H⁺ ions with the dye molecules.
• Litmus is a quick and easy test for the presence of acids.

Methyl orange is a synthetic indicator that changes colour over a narrow pH range, making it useful for detecting acidic conditions.

In acidic solutions, methyl orange turns red. In neutral or alkaline solutions, it appears yellow. This sharp colour change occurs because methyl orange molecules accept hydrogen ions in acidic conditions, changing their electronic structure and colour.

This property makes methyl orange particularly useful in titrations involving strong acids and weak bases.

• Methyl orange turns red in acidic solutions (pH < 3.1).
• It is yellow in neutral and alkaline solutions (pH > 4.4).
• The colour change is due to protonation of the indicator molecules.

By adding indicators like litmus or methyl orange to unknown solutions, you can determine if the solution is acidic based on the observed colour change.

For example, if litmus paper turns red when dipped into a solution, it confirms the solution is acidic. Similarly, if methyl orange turns red, the solution is acidic.

These tests are qualitative but provide quick and reliable identification of acids in laboratory and field settings.

• Add a few drops of indicator to the solution or dip indicator paper into it.
• Observe the colour change carefully and compare it to known colour charts.
• Confirm acidity if litmus turns red or methyl orange turns red.

Both litmus and methyl orange change colour in the presence of acids, but at different pH ranges and with different colours.

Litmus changes from purple to red in acidic conditions, while methyl orange changes from yellow to red.

Understanding these differences helps select the appropriate indicator for specific tests or experiments.

• Litmus: purple to red in acids.
• Methyl orange: yellow to red in acids.
• Choice of indicator depends on the required pH sensitivity and colour contrast.

Bases are substances that can neutralise acids, typically by reacting with hydrogen ions (H+) to form water and a salt. Chemically, bases are often oxides or hydroxides of metals. For example, copper(II) oxide (CuO) and magnesium hydroxide (Mg(OH)2) are bases. These compounds contain metal ions combined with oxygen or hydroxide ions.

Metal oxides are compounds formed from metal elements and oxygen. When these oxides react with acids, they form salt and water, demonstrating their basic nature. Similarly, metal hydroxides contain hydroxide ions (OH-) bonded to metal ions and also react with acids to neutralise them.

• Bases include metal oxides and metal hydroxides.
• They neutralise acids to form salt and water.
• Examples: copper(II) oxide (CuO), magnesium hydroxide (Mg(OH)2).

Alkalis are a special subset of bases that dissolve in water to produce hydroxide ions (OH-) in solution. This solubility in water distinguishes alkalis from other bases that may be insoluble or only sparingly soluble.

When an alkali dissolves, it increases the concentration of OH- ions in the solution, making the solution alkaline (basic). For example, sodium hydroxide (NaOH) and potassium hydroxide (KOH) are alkalis because they dissolve readily in water and release OH- ions.

The presence of hydroxide ions in aqueous solution is responsible for the characteristic properties of alkalis, such as their slippery feel and ability to turn red litmus paper blue.

• Alkalis are bases soluble in water.
• They produce hydroxide ions (OH-) in solution.
• Examples: sodium hydroxide (NaOH), potassium hydroxide (KOH).

Both bases and alkalis neutralise acids by reacting with hydrogen ions (H+) to form water and a salt. The general neutralisation reaction is:

H+ (aq) + OH- (aq) → H2O (l) (water) when hydroxide ions are involved.

For insoluble bases like metal oxides, the reaction with acids produces salt and water, for example: CuO (s) + 2HCl (aq) → CuCl2 (aq) + H2O (l). Here, copper(II) oxide reacts with hydrochloric acid to form copper(II) chloride and water. This shows the base neutralising the acid despite being insoluble in water.

• Bases neutralise acids to form salt and water.
• Alkalis provide OH- ions in solution to react with H+ ions.
• Example reaction: CuO + 2HCl → CuCl2 + H2O.

Common bases include metal oxides such as copper(II) oxide (CuO), zinc oxide (ZnO), and metal hydroxides like magnesium hydroxide (Mg(OH)2). These are often insoluble or only slightly soluble in water.

Common alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2) (sparingly soluble). These dissolve in water to give alkaline solutions.

To identify whether a substance is an alkali, test its solubility in water and check if the solution turns red litmus paper blue, indicating the presence of OH- ions.

• Bases: CuO, ZnO, Mg(OH)2 (mostly insoluble).
• Alkalis: NaOH, KOH, Ca(OH)2 (soluble or sparingly soluble).
• Alkalis turn red litmus paper blue due to OH- ions.

Bases are metal oxides or hydroxides that neutralise acids to form salts and water. Alkalis are bases that dissolve in water to produce hydroxide ions, making the solution alkaline.

Understanding the difference between bases and alkalis is important for predicting their behaviour in chemical reactions and their practical uses, such as in cleaning agents (alkalis) or antacids (some bases).

• Bases = metal oxides or hydroxides, may be insoluble.
• Alkalis = soluble bases producing OH- ions in water.
• Both neutralise acids to form salt and water.

Bases react with acids in a chemical process called neutralisation. During this reaction, the base accepts hydrogen ions (H⁺) from the acid, resulting in the formation of water (H₂O) and a salt. For example, when sodium hydroxide (a base) reacts with hydrochloric acid, the products are sodium chloride (a salt) and water:

NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l) This reaction demonstrates the key property of bases to neutralise acids, reducing their acidity and producing neutral compounds.

• Neutralisation involves acid H⁺ ions reacting with base OH⁻ ions to form water.
• A salt is formed from the remaining ions of the acid and base.
• This reaction is exothermic, releasing heat.

Bases that dissolve in water release hydroxide ions (OH⁻) into the solution. These ions are responsible for the basic (alkaline) nature of the solution. For example, sodium hydroxide dissociates in water as:

NaOH (s) → Na⁺ (aq) + OH⁻ (aq) The presence of OH⁻ ions increases the pH of the solution above 7, indicating alkalinity. The concentration of hydroxide ions determines the strength of the base in solution.

• Bases produce OH⁻ ions when dissolved in water.
• Higher OH⁻ concentration means stronger base and higher pH.
• Hydroxide ions react with H⁺ ions from acids during neutralisation.

Bases cause characteristic colour changes in chemical indicators, which help identify their presence. For example, red litmus paper turns blue in the presence of a base, while blue litmus paper remains blue. Methyl orange changes from red in acidic conditions to yellow in alkaline conditions. These colour changes occur because bases alter the concentration of hydrogen ions in solution, affecting the chemical structure of the indicator molecules.

• Red litmus paper turns blue in bases.
• Methyl orange turns yellow in alkaline solutions.
• Indicators provide a simple way to detect bases.

Bases often have a bitter taste and a slippery or soapy feel when touched. The slippery texture is due to the reaction of bases with the oils on skin, forming soap-like substances. For safety reasons, tasting or touching bases in a laboratory setting is not recommended. These physical properties can help distinguish bases from acids, which typically have a sour taste and do not feel slippery.

• Bases taste bitter (do not taste in lab).
• They feel slippery or soapy to touch.
• These properties arise from chemical interactions with skin oils.

Alkalis are a subset of bases that dissolve in water to produce hydroxide ions (OH⁻). For example, sodium hydroxide (NaOH) dissolves in water to form Na⁺ and OH⁻ ions. The presence of hydroxide ions makes the solution alkaline, meaning it has a pH greater than 7. Alkalis can neutralise acids by reacting with hydrogen ions (H⁺) to form water.

• Alkalis release OH⁻ ions in aqueous solution.
• They have pH values above 7, typically between 8 and 14.
• Common alkalis include sodium hydroxide and potassium hydroxide.

Indicators are substances that change colour depending on the pH of the solution they are in. They provide a visual way to identify whether a solution is acidic, neutral, or alkaline. Different indicators change colour at different pH ranges, making some more suitable for detecting alkalis than others.

• Indicators show colour changes due to chemical structure changes at different pH levels.
• They help estimate the pH without using electronic pH meters.
• Common indicators include litmus paper and methyl orange.

Litmus paper is a widely used indicator made from lichens. It is red in acidic conditions and blue in alkaline conditions. When a piece of red litmus paper is dipped into an alkaline solution, it turns blue. This colour change occurs because the alkaline solution causes the dye molecules in the litmus to accept electrons, altering their structure and colour.

• Red litmus paper turns blue in alkalis.
• Blue litmus paper remains blue in alkalis.
• This test quickly identifies whether a solution is alkaline.

Methyl orange is another indicator that changes colour over a specific pH range. In acidic solutions (pH below 3.1), methyl orange appears red. As the solution becomes alkaline (pH above 4.4), methyl orange changes to yellow. When methyl orange is added to an alkali, it shifts from red to yellow, signalling the presence of a basic environment.

• Methyl orange is red in acidic solutions and yellow in alkaline solutions.
• Colour change occurs between pH 3.1 and 4.4.
• It is useful for distinguishing strong acids from weak acids and alkalis.

To test whether a solution is alkaline, dip red litmus paper or add methyl orange indicator to the solution. If the red litmus paper turns blue, or methyl orange changes from red to yellow, the solution is alkaline. These colour changes are reversible; if the solution’s pH changes, the indicator colour will change accordingly. Using indicators together can help confirm the pH range more precisely.

• Use red litmus paper to detect alkalis by observing a blue colour change.
• Add methyl orange to solutions to see a red-to-yellow shift in alkalis.
• Combine indicators for more accurate pH estimation.

The pH scale is a numerical scale ranging from 0 to 14 that measures how acidic or alkaline a solution is. A pH of exactly 7 indicates a neutral solution, such as pure water. Solutions with pH values less than 7 are acidic, meaning they have a higher concentration of hydrogen ions (H⁺). Solutions with pH values greater than 7 are alkaline (or basic), meaning they have a higher concentration of hydroxide ions (OH⁻).

For example, lemon juice typically has a pH around 2, making it acidic, while household ammonia has a pH around 11, making it alkaline. Understanding the pH value helps predict chemical behaviour and reactions of the solution.

• pH scale ranges from 0 (strongly acidic) to 14 (strongly alkaline).
• pH 7 is neutral (pure water).
• pH < 7 indicates acidity.
• pH > 7 indicates alkalinity.

A universal indicator is a mixture of dyes that changes colour depending on the pH of the solution it is added to. When a few drops of universal indicator are added to a solution, the solution’s colour changes to a specific shade that corresponds to a pH value on a colour chart.

For instance, a red or orange colour indicates a strongly acidic solution (pH 1–3), green indicates neutral (pH 7), and purple or blue indicates a strongly alkaline solution (pH 11–14). This colour change provides a visual and quick way to estimate the pH without using electronic pH meters.

• Universal indicator contains multiple dyes sensitive to pH.
• Colour changes cover the full pH range from red (acidic) to purple (alkaline).
• Colour is compared to a standard pH colour chart to estimate pH.

To assess the nature of a solution, add a few drops of universal indicator to a small sample. Observe the resulting colour carefully and compare it to the universal indicator colour chart to estimate the pH value.

If the colour matches green, the solution is neutral. If it shifts towards red or orange, the solution is acidic. If it shifts towards blue or purple, the solution is alkaline.

For example, adding universal indicator to distilled water results in a green colour, confirming neutrality. Adding it to vinegar turns it orange, indicating acidity. Adding it to a dilute sodium hydroxide solution turns it purple, indicating alkalinity.

• Add 2–3 drops of universal indicator to the solution.
• Observe the colour change carefully.
• Compare the colour to the pH colour chart.
• Determine if the solution is acidic, neutral, or alkaline based on the colour.

Universal indicator provides an approximate pH value and is useful for quick assessments. However, the colour interpretation can be subjective, especially for colours between shades. Lighting conditions and the concentration of the solution can affect the colour intensity.

For more precise pH measurements, a pH meter can be used, but universal indicator remains a simple and effective tool in many laboratory and classroom settings.

Always use clean apparatus to avoid contamination, and handle acids and alkalis with care, wearing safety goggles and gloves if necessary.

• Colour interpretation can be subjective; use good lighting.
• Concentration affects colour intensity; very dilute solutions may give faint colours.
• Universal indicator gives approximate pH, not exact values.
• Use safety precautions when handling acids and alkalis.

1. Collect a small sample of the solution in a clean container.

2. Add 2–3 drops of universal indicator to the sample.

3. Observe the colour change carefully against a white background if possible to see the true colour clearly. 4. Compare the observed colour to the universal indicator colour chart to estimate the pH value. 5. Classify the solution as acidic (pH < 7), neutral (pH = 7), or alkaline (pH > 7) based on the colour and pH estimate.

• Use clean equipment to avoid contamination.
• Add universal indicator carefully to avoid dilution.
• Observe colour promptly before it fades.
• Use the colour chart for accurate pH estimation.
• Record the pH and classification clearly.