# How do we know that a matter is acidic or basic without knowing their $pH$ value? [duplicate]

There are too many examples for acidic matters as seen below. $$HCI \tag {1}$$ $$HNO_3 \tag {2}$$ $$H_2SO_4 \tag {3}$$

And the examples for basic matters

$$NH_3 \tag {4}$$ $$NaOH \tag {5}$$

According to this, how do we differenciate between acidic and basic matters without knowing their $pH$ value?

Regards

• @Karl Can you please show it? That method seems very very good. – Cargobob Nov 7 '17 at 20:44
• @86BCP2432T What is it? – Cargobob Nov 7 '17 at 20:45
• People seem to not understand that this community is not intended only for those with basic/advanced knowledge in Chemistry. There might be users here that are in high school or other majors who need basic chemistry information... @Maxime , you need to know that acids and bases are categorized as such only in comparison to another substance. When this 'comparing' substance is water, it is easy to tell because it will generate H+ if it's an acid and OH- if it's a base. However, if you mix sulphuric acid and nitric acid, the latter will react as a base, because H2SO4 is a stronger acid than HNO3 – Raul Luciano Nov 7 '17 at 22:47
• @Raul Luciano I get what you are saying, and some of the responses could definitely be less terse, but the site arguably isn't meant for questions like this. We aren't trying to replace Google or chemistry textbooks; information that could found from a simple search of either of these isn't well suited to the model of the site. – Tyberius Nov 7 '17 at 23:37
• I agree @Tyberius , however, comments like "basic chemistry" add absolutely nothing to the discussion. If someone thinks the answer to the question is too broad that can't be presented in the answer box then the post should be flagged and addressed to a reliable source (textbook or website). – Raul Luciano Nov 7 '17 at 23:54

pH is simply a measure of H+/H3O+ concentration in a solution. I will use the two terms interchangeably below.

The acidity or basicity of a compound has to do with its chemical behaviour.

There are three definitions of acidity.

The simplest definition is the Arrhenius definition:

• An acid dissociates in water to form H+.
• A base ionizes in water to form OH-.

This definition is limited to aqueous solutions, and so is usually ignored in favor of the Brønsted-Lowry definition, where:

• An acid is a hydrogen ion donor.
• A base is a hydrogen ion acceptor.

This definition is more generally, because it can be used to describe acid-base reactions that occur in solvents other than water.

There is a third definition, Lewis:

• A Lewis acid is an electron pair acceptor.
• A Lewis base is an electron pair donor.

The Lewis definition is the broadest definition of acidity. However, most entry-level courses stick with the Brønsted-Lowry definition since it does an excellent job of describing what we typically consider to be acid-base chemistry.

To summarize, we recognize acids and bases from their chemical structure.

## Recognizing Acids

Acids generally come in one of three forms:

Binary Acids These have the form HX, like HCl, or HF.

Oxyacids The have an oxygen-containing polyatomic anion. Examples include H2SO4 or HNO3.

Organic Acids You can't tell if an organic (carbon-containing) compound is an acid based on its formula. However, organic acids are often written RCOOH. The COOH represents the carboxylic acid functional group, which is what makes the compound an acid. For example, acetic acid is often written CH3COOH.

## Recognizing Bases

Metal Hydroxides Soluble metal hydroxides are all bases (since when the dissociate in water they generate OH-).

Nitrogen-containing Bases Nitrogen-containing compounds similar to ammonia (NH3) are also bases.
$$\ce {NH3 + H2O <=> NH4+ + OH-}$$