Processing phosphate rock to make fertilisers Revision Notes | GCSE AQA Higher Chemistry
Phosphate rock is insoluble, so it is processed with acids to make soluble fertilisers that plant roots can absorb. Free notes and 14 questions answered.
Updated 19 Mar 2026 · Revise Right Team
Processing phosphate rock to make fertilisers is part of The Haber process and the use of NPK fertilisers (chemistry only) in GCSE AQA Higher Chemistry.
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GCSE AQA Higher
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What this lesson covers
Phosphate rock is insoluble, so it is processed with acids to make soluble fertilisers that plant roots can absorb. Free notes and 14 questions answered.
Recall the names of salts produced when phosphate rock is treated with nitric acid, sulfuric acid and phosphoric acid
Explain why phosphate rock cannot be used directly as a fertiliser
Describe how phosphate rock is processed to make soluble fertiliser salts
Compare the different acids used to treat phosphate rock and their products
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Use this lesson to stay inside The Haber process and the use of NPK fertilisers (chemistry only) while connecting the detail back to the wider Topic 10: Using resources topic.
Topic 10: Using resources
The Haber process and the use of NPK fertilisers (chemistry only)
GCSE AQA Higher Chemistry
Revision notes
Read the core explanations from Processing phosphate rock to make fertilisers before testing yourself from memory.
1. The Problem with Raw Phosphate Rock
Plants need phosphorus to grow, but there's a catch. We can dig phosphate rock straight out of the ground, but plants can't use it directly. Why not? The phosphorus is locked up in calcium phosphate - a compound that doesn't dissolve in water.
Think of it like trying to feed someone sugar cubes that are sealed in plastic. The sugar is there, but it's not accessible. Plant roots can only absorb nutrients that are dissolved in water around them.
2. The Solution - Acid Treatment
The solution is clever chemistry. We treat phosphate rock with acids to break down the insoluble calcium phosphate and convert it into soluble salts. Three different acids can do this job, each producing different fertiliser products.
The key principle is this: acids react with the calcium phosphate to form new compounds that dissolve easily in water. Once dissolved, plant roots can absorb the phosphorus they need.
Worked example: Processing Phosphate Rock with Nitric Acid
What happens when phosphate rock reacts with nitric acid?
Calcium phosphate + Nitric acid — Phosphate rock contains Ca₃(PO₄)₂
Calcium nitrate + Phosphoric acid — The acid breaks down the phosphate structure
Ca₃(PO₄)₂ + 6HNO₃ → 3Ca(NO₃)₂ + 2H₃PO₄ — Both products are water-soluble
Calcium nitrate and phosphoric acid — These provide both nitrogen and phosphorus
Answer: Calcium nitrate and phosphoric acid are produced - both soluble fertiliser compounds
3. The Three Acid Treatments
Each acid produces different fertiliser salts. You need to memorise these three reactions because they're exam favourites. Notice how each treatment gives us different combinations of nutrients.
Here's the pattern: the acid always breaks apart the calcium phosphate structure. The calcium combines with the acid to form a new salt, while phosphoric acid is often produced as a by-product.
4. The Complete Picture - NPK Fertilisers
The processed phosphate rock provides the 'P' in NPK fertilisers. But plants need all three nutrients - nitrogen (N), phosphorus (P) and potassium (K). That's where potassium chloride and potassium sulphate come in.
These potassium compounds are also mined from the ground, but unlike phosphate rock, they're already soluble. No acid treatment needed - they can go straight into fertiliser mixtures.
The beauty of this system is that we can mix and match. Nitric acid treatment gives us nitrogen and phosphorus. Add some potassium chloride, and we have a complete NPK fertiliser with all three essential nutrients.
5. Exam Technique - Getting Full Marks
Exam questions on this topic follow predictable patterns. You'll be asked to name the salts produced, explain why processing is necessary, or write balanced equations. Here's how to nail every mark.
Common trap: Students often forget that phosphoric acid is produced in two of the three reactions. It's not just a reactant - it's also a valuable fertiliser product that provides phosphorus to plants.
Exam precision note: In Processing phosphate rock to make fertilisers - Production and uses of NPK fertilisers - The Haber process and the use of NPK fertilisers (chemistry only), always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Phosphate rock is itself obtained by mining, just like potassium chloride and potassium sulfate. The reason phosphate rock needs extra processing is not where it comes from, but the fact that calcium phosphate is insoluble. Treating the rock with acid converts the phosphorus into forms that are much more available to plants. This makes the distinction between mined raw materials and soluble fertiliser products explicit.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-19.
Calcium phosphate is Ca₃(PO₄)₂. It is the compound in phosphate rock that locks up the phosphorus, and because it is insoluble in water it must be treated with nitric, sulfuric or phosphoric acid before use as a fertiliser.
What is phosphate rock?
Phosphate rock is a naturally occurring mineral containing calcium phosphate, Ca₃(PO₄)₂, which is obtained by mining. It is insoluble in water, so it only provides the P in NPK fertilisers after it has been treated with acid.
How are NPK fertilisers made from raw materials?
NPK fertilisers are made by combining processed phosphate rock, which provides the phosphorus, with nitrogen compounds and potassium compounds. Phosphate rock is treated with acid to make soluble salts, while potassium chloride and potassium sulfate are mined and are already soluble, so they go straight into fertiliser mixtures.
Why do fertilisers need to be soluble?
Fertilisers need to be soluble because plant roots can only absorb nutrients that are dissolved in water around them. Insoluble calcium phosphate in phosphate rock is like sugar sealed in plastic: the phosphorus is there but not accessible until acid converts it into soluble salts.
Why can't phosphate rock be used directly as a fertiliser?
Phosphate rock cannot be used directly as a fertiliser because its phosphorus is locked up in calcium phosphate, Ca₃(PO₄)₂, which is insoluble in water. Plant roots can only absorb nutrients that are dissolved in water, so the rock is treated with acids to make soluble salts.
What salts are made when phosphate rock is treated with acid?
Phosphate rock treated with nitric acid gives calcium nitrate and phosphoric acid, with sulfuric acid it gives calcium sulphate and phosphoric acid, and with phosphoric acid it gives calcium dihydrogen phosphate. Each product is soluble, so plant roots can absorb the phosphorus.
What is the equation for phosphate rock and nitric acid?
The equation for phosphate rock reacting with nitric acid is Ca₃(PO₄)₂ + 6HNO₃ → 3Ca(NO₃)₂ + 2H₃PO₄. The products, calcium nitrate and phosphoric acid, are both water-soluble and provide both nitrogen and phosphorus.
Where does the potassium in NPK fertilisers come from?
The potassium in NPK fertilisers comes from potassium chloride and potassium sulphate, which are mined from the ground. Unlike phosphate rock, they are already soluble, so no acid treatment is needed before they go into fertiliser mixtures.
Phosphate rock cannot be used directly as a fertiliser because it is what?
Phosphate rock cannot be used directly as a fertiliser because it is insoluble in water. The phosphorus is locked up in calcium phosphate, Ca₃(PO₄)₂, and plant roots can only absorb nutrients dissolved in the water around them, so the rock is treated with acid first.
What is made when phosphate rock reacts with sulfuric acid?
Phosphate rock reacts with sulfuric acid to produce calcium sulphate and phosphoric acid. This treatment provides phosphorus and sulfur nutrients, because the acid breaks apart the insoluble calcium phosphate into soluble compounds.
What is made when phosphate rock reacts with phosphoric acid?
Phosphate rock reacts with phosphoric acid to produce calcium dihydrogen phosphate. This salt provides concentrated phosphorus nutrients and, unlike calcium phosphate, it dissolves in water so plants can absorb it.
Does potassium chloride need to be treated before use as a fertiliser?
Potassium chloride does not need acid treatment before use as a fertiliser because it is already soluble. Like phosphate rock it is mined from the ground, but phosphate rock needs processing only because calcium phosphate is insoluble.
Which acid treatment of phosphate rock gives nitrogen and phosphorus?
Treating phosphate rock with nitric acid gives both nitrogen and phosphorus, because it produces calcium nitrate and phosphoric acid. Adding potassium chloride to these then gives a complete NPK fertiliser with all three essential nutrients.
Is phosphoric acid a product of treating phosphate rock?
Phosphoric acid is produced in two of the three phosphate rock reactions, those with nitric acid and sulfuric acid. It is not just a reactant but also a valuable fertiliser product that provides phosphorus to plants, which is a common point students forget.