Acidic and alkaline solutions
Acids
acidSubstance producing more hydrogen ions than hydroxide ions when dissolved in water. form acidicHaving a pH lower than 7. solutionMixture formed by a solute and a solvent. in water. Acids produce hydrogen ionElectrically charged particle, formed when an atom or molecule gains or loses electrons., H+ in aqueous solution. For example:
HCl(aq) 鈫 H+(aq) + Cl-(aq)
Acidic solutions have pHScale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline. values less than 7.
Alkalis
alkaliSubstance producing more hydroxide ions than hydrogen ions when dissolved in water. form alkalineHaving a pH greater than 7. solutions in water. Alkalis produce hydroxide ions, OH- in aqueous solution. For example:
NaOH(aq) 鈫 Na+(aq) + OH-(aq)
Alkaline solutions have pH values greater than 7.
Neutral solutions
A neutralWhen a substance is neither acidic nor alkaline, and has a pH of 7. solution is neither acidic, nor alkaline. A neutral solution has a pH value of 7.
Learn more on the pH scale in this podcast.
Listen to the full series on 大象传媒 Sounds.
Indicators and the pH scale
The pH scale measures the acidity or alkalinity of a solution. The pH of a solution can be measured using a pH probe, or estimated using universal indicatorA chemical solution that produces many different colour changes corresponding to different pH levels. and a colour chart.
Universal indicator is one example of an acid-alkali indicatorA substance that has different colours, depending upon the pH of the solution it is in.. Indicators show whether a solution is acidic, neutral (pH 7) or alkaline. The table shows the colours for litmus paperA type of indicator that can be red or blue. Red litmus turns blue in alkalis, while blue litmus turns red in acids..
Indicator | Acidic | Neutral | Alkaline |
Red litmus paper | Stays red | Stays red | Turns blue |
Blue litmus paper | Turns red | Stays blue | Stays blue |
Indicator | Red litmus paper |
---|---|
Acidic | Stays red |
Neutral | Stays red |
Alkaline | Turns blue |
Indicator | Blue litmus paper |
---|---|
Acidic | Turns red |
Neutral | Stays blue |
Alkaline | Stays blue |