Alkalis | Chemistry for GCSE/IGCSE - Class 10 PDF Download

Neutralisation Reactions

• When acids are introduced to water, they generate positively charged hydrogen ions (H⁺), contributing to the solution's acidity.
• Conversely, when alkalis are added to water, they produce negative hydroxide ions (OH^–), rendering the solution alkaline.
• The pH scale serves as a numerical representation indicating the acidity or alkalinity level of a solution, reflecting the concentration of ions within the solution.
• Neutralization reactions occur when acids interact with alkalis.
• During a neutralization reaction, the hydrogen ions (H⁺) from acids combine with the hydroxide ions (OH^–) from alkalis, resulting in the formation of water.
• For instance, when hydrochloric acid undergoes neutralization, sodium chloride and water are generated.
  
• The simplified ionic equation for acid-alkali neutralizations, which results in a neutral solution due to water's pH of 7, is:
  H⁺  +  OH^– ⟶ H₂O

Hydrogen Ion Concentration & pH

The pH scale

• The pH scale is a tool used to determine the acidity or alkalinity of a solution.
• It ranges from 1 to 14, with values below 7 indicating acidity, above 7 indicating alkalinity, and 7 being neutral
• Acids have pH values below 7, while alkalis have pH values above 7.
• A lower pH signifies higher acidity, while a higher pH indicates higher alkalinity.
• Lemon juice typically has a pH of around 2, indicating strong acidity.
• Ammonia has a pH of about 11.5, showcasing its alkaline nature.
• The pH scale showing acidity, neutrality, and alkalinity:
  
• Acids contain hydrogen ions in solution.
• A stronger acid has a higher concentration of hydrogen ions, resulting in a lower pH.
• The concentration of hydroxide ions determines the pH level, with higher concentrations leading to higher pH values.
• pH serves as an indicator of the concentration of H+ ions in a solution, with an inverse relationship between pH and the concentration of H+ ions.
• Universal indicator helps determine pH levels.
  
• The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10
• Therefore an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4
• An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H+ ions than an acid with a pH of 4

Universal Indicator

• Universal indicator is a blend of different indicators used to measure pH levels.
• Indicators are substances that change color based on the pH of a solution.
• A drop of universal indicator is added to a solution, and its resulting color is compared to a color chart for pH determination.
• The pH scale, along with the Universal Indicator colors, helps determine the pH of a solution by matching colors.