Acids, bases, alkalis and indicators

Two colourless liquids may look alike but behave differently with an indicator. This lesson connects that visible evidence to the ions in their solutions. By the end, you should be able to distinguish an acid, a base and an alkali, compare pH readings, and separate an observation from the conclusion it supports.

You will use the idea of an ion: a charged particle. The state symbol (aq) means dissolved in water; it does not mean that a substance is a liquid on its own.

What is in the solution?

An acid produces hydrogen ions, H + (aq), in water. An alkali is a soluble base that produces hydroxide ions, OH-(aq), in water. For example, aqueous sodium hydroxide contains Na + (aq) and OH-(aq); it is the hydroxide ion that gives the solution its alkaline behaviour.

A base is a substance that reacts with an acid to form a salt and water. A base need not dissolve in water. Copper(II) oxide, CuO, is an insoluble base: it reacts with acid, but it is not an alkali. Sodium hydroxide dissolves in water, so it is both a base and an alkali. To justify the word alkali, you need the solubility and aqueous-ion ideas, not merely a reaction with acid.

A formula containing hydrogen does not, by itself, identify an acid. The definition concerns the ions produced in water. Do not infer acidity from the letter H in a formula or test an unfamiliar substance by taste or touch.

Both hydrogen and hydroxide ions are present in aqueous solutions. Their relative concentrations distinguish three cases:

Neutral therefore does not mean “contains no ions”. It describes the balance between these two ion concentrations. A neutral solution may also contain other ions.

From colour to a justified conclusion

An indicator changes colour over a pH range. Blue litmus turns red in an acid; red litmus turns blue in an alkali. Litmus distinguishes these conditions but cannot give an exact pH. Blue litmus staying blue is insufficient to distinguish an alkaline solution from a neutral one.

Universal indicator has several colours, which can be compared with its supplied chart. Red, orange and yellow indicate acidic conditions, green indicates neutral conditions, and blue or purple indicates alkaline conditions. Use the particular chart supplied with the indicator to estimate pH; a colour name alone does not justify decimal places.

A calibrated pH meter gives a numerical reading. At the same temperature, pH 3 is more acidic than pH 5, and pH 11 is more alkaline than pH 9. These comparisons do not require calculating an ion concentration or describing strong and weak acids.

Modelled example 1

What does an orange colour tell us?

Core

Problem

Universal indicator changes from green to orange in solution P. On the supplied chart, orange covers pH 3–4. State the observation, the classification and the ion relationship.

Study the worked solution

1. Record what was seen

   Method

   Universal indicator changes from green to orange.

   Reason

   This is an observation. “P is acidic” is the interpretation of that observation.

   Working

   Observation: green → orange.

2. Interpret the chart

   Method

   P is acidic, with an estimated pH of 3–4.

   Reason

   The chart puts orange below pH 7. It does not distinguish pH 3.1 from 3.8.

   Working

   Chart range: pH 3–4 → below 7 → acidic.

3. Connect to the ions

   Method

   The concentration of H + (aq) is greater than that of OH-(aq).

   Reason

   This is the particle-level meaning of an acidic solution. It does not mean hydroxide ions are absent.

   Working

   Hydrogen-ion concentration > hydroxide-ion concentration.

Guided practice 2

Read the evidence before naming the solution

About 4 min

Problem

A calibrated meter measures Q at pH 9 and R at pH 12 at room temperature. Classify Q and decide which solution is more alkaline.

Use the readings

Q is

AcidicNeutralAlkaline

More alkaline solution

QR

Check my attempt

Hints

Hint 1: Two comparisons

First compare each reading with 7. Then compare Q with R.

View solution step by step

1. Classify before ranking

   Method

   Both solutions are alkaline; R is more alkaline.

   Reason

   Both readings are above 7, and 12 is higher than 9.

   Working

   In each solution, the hydroxide-ion concentration exceeds the hydrogen-ion concentration.

Check a claim against its evidence

A learner writes: “Blue litmus stayed blue, so the liquid is pH 14.” The first clause records an observation. The conclusion goes beyond it: a neutral liquid would also leave blue litmus blue, and litmus does not measure a pH number. Use red litmus to distinguish neutral from alkaline conditions, or use universal indicator with its chart to estimate pH.

Try these without the worked examples:

1. Blue litmus turns red in S. What can you conclude, and what can you not conclude about its pH?
2. T gives a meter reading of pH 7. A learner says it contains no hydrogen ions. Correct the explanation.
3. Solid U is insoluble in water but reacts with an acid to form a salt and water. Is “base” or “alkali” justified?

Next, use the acid and base models to predict reactions and explain which observations distinguish their products.