Applications and oxide behaviour

A useful chemical choice needs two things: evidence about the problem and a reaction that can address it. Use the acid–base reactions from the previous lesson to choose a soil treatment, then use paired reaction tests to classify oxides.

Treat excess soil acidity using evidence

Soil pH affects plant growth and the availability of mineral nutrients. Different crops have suitable pH ranges, so the aim is the crop’s required range—not automatically pH 7, and not the highest possible pH.

Calcium hydroxide is a base used to treat excess soil acidity. It neutralises acid and raises soil pH. A treatment decision should therefore start with the measured soil pH and the crop’s target range. Adding more base after the soil reaches that range can make the soil too alkaline; “more treatment” is not necessarily better.

Modelled example 1

Choose a treatment for a stated problem

Core

Problem

A crop grows best in soil at pH 6–7. A soil sample measures pH 4.5. Choose between calcium hydroxide and an acidic treatment, and explain the change required.

Study the worked solution

1. Compare with the crop's range

   Method

   The soil is too acidic for the stated crop.

   Reason

   Its measured pH, 4.5, is below the required range of 6–7.

   Working

   4.5 is below the lower limit of 6.

2. Choose a reaction that addresses the problem

   Method

   Use calcium hydroxide to neutralise excess acidity and raise pH towards the required range.

   Reason

   Calcium hydroxide is a base. An acidic treatment would move the pH in the wrong direction.

   Working

   excess acid + calcium hydroxide → salt + water

Guided practice 2

Does every field need calcium hydroxide?

About 4 min

Problem

The same crop requires pH 6–7. Field A measures pH 5; field B measures pH 6.5. A learner proposes adding calcium hydroxide to both because it helps crops grow. Decide where the evidence supports treating excess acidity.

Compare each field with the target

Evidence supports treating excess acidity in

A onlyB onlyBoth fields

Check my attempt

Hints

Hint 1: Use the crop's target

A reading below 7 is acidic, but that does not make it unsuitable for every crop.

View solution step by step

1. Match the treatment to the need

   Method

   Treat A’s excess acidity; B already meets the stated range.

   Reason

   The purpose is to reach suitable conditions, not to make all soil alkaline.

   Working

   A: pH 5 is below 6–7. B: pH 6.5 is within 6–7.

Classify an oxide: pattern first, evidence next

An oxide is a compound of oxygen with another element. Many metal oxides are basic, while many non-metal oxides are acidic. These patterns help make a prediction, but there are exceptions. Use the substance’s known behaviour or supplied reaction evidence to check the prediction.

A basic oxide neutralises an acid, as copper(II) oxide did in the previous lesson. An acidic oxide reacts with an alkali. For example, carbon dioxide reacts with calcium hydroxide in limewater; the insoluble calcium carbonate makes the liquid milky:

This connects an acidic oxide’s behaviour with a gas test you already know. Carbon dioxide need not contain hydrogen to be an acidic oxide: that classification describes its reactions, rather than making it an aqueous acid under the earlier definition.

Amphoteric means that an oxide reacts with both acids and alkalis. Aluminium oxide is a metal oxide, but its reaction with alkali prevents a basic-only classification. You need this distinction; you do not need complex-ion formulae or advanced amphoteric reaction mechanisms here.

A neutral oxide is not classified as acidic or basic in these tests. “Neutral” does not mean chemically unreactive in every situation, and it does not assign a pH reading to a dry solid or a gas. Carbon monoxide is a non-metal oxide that is neutral, so “all non-metal oxides are acidic” is false.

Common misconception 3

Which observation overturns the shortcut?

Find and correct the mistake

Learner claim

Oxide X reacts with hydrochloric acid and aqueous sodium hydroxide. A learner calls X basic because it reacts with acid. Correct the classification and identify the decisive extra evidence.

Use both reaction tests

Classification of X

AcidicBasicAmphotericNeutral

Evidence that rules out the learner's basic-only classification

Reaction with hydrochloric acidReaction with aqueous sodium hydroxide

Check my attempt

View solution step by step

1. Keep both observations

   Method

   X is amphoteric because it reacts with both acid and alkali.

   Reason

   The acid reaction alone cannot distinguish basic from amphoteric; the alkali reaction supplies the missing evidence.

   Working

   Reacts with acid AND alkali → amphoteric.

Combine evidence without overclaiming

Use these supplied results for unfamiliar oxides Y and Z. Assume the tests were performed under suitable conditions and the observations are reliable.

1. Classify Y and Z. For each, use both observations in your explanation.
2. An oxide W reacts with acid, but nobody has tested it with alkali. Explain why “W is definitely basic” is not justified, and name the missing test.
3. Another crop needs soil at pH 5–6. Its soil measures pH 5.5. Explain why adding calcium hydroxide merely to reach pH 7 is not justified.
4. A white solid produces a gas with dilute hydrochloric acid; that gas turns limewater milky. A learner calls the solid a basic oxide because it reacts with acid. Identify the more appropriate reactant class and explain why acid reaction alone was insufficient.

You can now connect a measurement to an ion model, predict products from the reactant class, and use reaction evidence to justify a material choice or oxide classification. Use the course’s topic check to find which of those steps needs more practice.