Term 2 Learning Points 2

Hi all,

Now I will be telling you guys about Acids and Alkalis.

So what are acids and alkaline?

Acids are substances that when dissolved in water, produces hydrogen ions as the only ions that are positive. They are also substance that are under pH of 7

Alkalis on the other hand are metal hydroxide that are soluble in water and produces hydroxide ions. They are also substances that are over the pH of 7.

This doesn't ends the definition of Acids and Alkalis.

Acids and Alkalis are further split into few parts: Strong & weak acids, Strong & weak alkalis.

Strong Acids

Strong acids ionises completely in water to produce Hydrogen ions. The water then contains a very high concentration of ions which will mean the solution is a good conductor of electricity. Some examples will be HydroChloric acid (HCl), Nitric Acid (HNO3).

Weak Acids

Weak acids do not ionise completely in water to produce hydrogen ions. Therefore, the solution will contain low concentration of ions which tells us that the solution is a poor conductor of electricity. Examples of weak acids will be ethanoic acid (CH3COOH) and citric acid (C6H8O7).

Strong Alkalis

Strong alkalis, similar to strong acids, ionises completely in water to form ions. However, instead of hydrogen ions, they form hydroxide ions. An example of strong alkalis will be sodium hydroxide (NaOH).

Weak Alkalis

Similar to the weak acids, weak alkalis do not ionise completely in water to produce hydroxide ions. Examples of weak alkalis will be ammonia (NH3) and pyridine (C5H5N)


So here, I guess some of you may have heard of bases. So what are bases... Bases in simpler terms are substances that form salt and water ONLY when reacted with acids. These substances are the oxides or hydroxides of metal, while on the other hand, alkalis are the hydroxide of metals. However, ALL alkalis are bases BUT not ALL bases are alkalis. So why is this so?

Bases are also known as substances that are able to accept H+ ions. However, that defies the OH- rule of alkalis. But if the base contains OH- ions, the base will also be referred as an alkali. With this, I will give an example of a base that is not an alkali, ammonia (NH3).

Enough said, I will now move on to the next point of acids and bases, how do they react to other substances.

NOTE that the salt I write below is not table salt, sodium chloride.

Acids + bases -> salt + water

Acids + metals -> salt + hydrogen

Acids + carbonates -> salt + water + carbon dioxide

Lastly, before I end off, some of you may ask, how do we differentiate between acids and alkalis and how do we test for them. We can test for acids and alkalis using litmus paper. Litmus paper contains 2 colors, blue and red. The blue paper is treated with a base and the red paper is treated with an acid. Therefore, when used to test for an acid, the blue paper will turn red while the red paper remains red. Vice versa, when used to test for alkalis, the blue paper remains blue while the red paper turns blue. We can also use methyl orange and phenolphthalein to test for strong acids and strong alkalis respectively.

In conclusion, this ends off the topic on acids and bases. Just a quick summary of what I have written. Acids are substances that are lower than pH 7 while bases are higher than pH 7. Alkalis are bases but not all bases are alkalis. Acids will also react with substances to form products.

Reflection:

I feel that this topic is very fun and interesting as most of the time when we are learning this topic, we stayed in the Science Laboratory and we had the first-hand experience of seeing the substances reacting and form products. We also got to taste some acids and alkalis, and feel them. I remember we tasted HCl and it was very sour as well as very spicy. Acids and bases is therefore a very exciting and interesting topic.

Thanks,
Aloysius Chan