Acids and Alkalis

Today, we were being taught on a new chapter, acids and alkalis. There are also many concepts and formulae that have to be learnt and memorised. Hence, we have to pay close attention to what Mdm Riza is teaching us as the concepts learnt may be tested in our examinations. We had also done quite a number of experiments in today's Science lesson.

I used to think that acids and alkalis are both the same. But now I think that they are DIFFERENT from one another in terms of their definitions and properties. An acid is a substance which is diluted in water to form solution and hydrogen ions are produced. The higher the amount of hydrogen ions, the stronger the acid is. The acid properties are present only when acids are dissolved in water. As for alkalis, they produce hydroxide ions when they are being dissolved in water. The stronger the alkalis, the higher the amount of hydroxide ions present. The three main differences between acids and alkalis are:

• Acids have a sour taste but alkalis have a bitter taste
• Acids turn moist blue litmus paper red, while alkalis turn moist red litmus paper blue
• Acids have a pH value lesser than 7. However, alkalis have a pH value more than 7

I used to think that acids and alkalis cannot be dissolved in water. But now I think that both of them are actually soluble in water. I also used to think that both acids and alkalis will react when being mixed with reactive metals and metal carbonates. But I was wrong. Now I think that only acids will then react with reactive metals (form salt and hydrogen gas) and metal carbonates (form salt, carbon dioxide gas and water).

I used to think that only litmus paper test that can help us differentiate between acids and alkalis. But now I think that there are also many other indicators that will tell us which are acids and which are alkalis. Other examples of indicators are:

• Universal indicator - turns into different colours, where each colour corresponds to a pH value, when it is added to different substances
• Phenolphthalein - use best on indicating alkalis as it changes to pink when alkalis are added to it. Stays colourless when being mixed with acidic/neutral substances
• Methyl orange - changes into yellow when being mixed with alkalis but turns red when acids are mixed into it
• Rose petals (made from plants)
• Red cabbage (made from plants) - blue (neutral), red (acidic), yellow (alkaline)

Lastly, I used to think that the hydrogen gas and carbon dioxide gas produced when acids react with reactive metals and metal carbonates respectively, are colourless and thus we cannot know if the substances reacting are acid or alkalis. But now I think that both gases can actually be tested. Hydrogen gas can be tested with a lighted splint as when they get in contact with one another, the splint will extinguish with a 'POP' sound. As for carbon dioxide gas, it can be tested by bubble the gas through limewater. When both the gas and limewater come in contact with one another, white precipitate will be observed in the limewater.

(Example of litmus paper test)

(Example of the pH scale)

Sources: Google Images