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Chemistry in Building and Agriculture

Builders use chemistry to turn limestone into lime, cement, concrete and glass, and to add steel where concrete is weak. Farmers use chemistry to give plants nitrogen, phosphorus and potassium. In both jobs the right amount matters.

🎬 Step-by-step story

  1. Look at a house and a field. Both are made with chemistry. Let us see how.
  2. Limestone is heated to make quicklime. Water turns it into slaked lime. Air then turns it back into stone. This is how old lime mortar hardens.
  3. Concrete is strong when squeezed but cracks when pulled. A steel bar at the bottom takes the pulling, so the beam does not break.
  4. Sand, soda ash and limestone are melted together. When the melt cools, it becomes clear glass.
  5. Now the field. A plant needs three main foods: N for green leaves, P for roots, K for a strong stem.
  6. Move the fertiliser slider. The crop grows fast at first, then it stops growing. The extra fertiliser washes away and harms water.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Does lime mortar harden by drying?

Not only. It slowly takes CO2 from the air and turns back into calcium carbonate, which is hard like stone. Watch the last block in the lime cycle.

Why is the steel bar at the bottom of the beam?

Under load the bottom stretches. Concrete cracks there, so the steel must be there to take the pull.

Why is glass clear when sand is not?

Sand is many tiny grains that scatter light. Melted and cooled glass is one smooth solid without grains, so light passes through.

Which part of the plant does each nutrient help?

N helps leaves, P helps roots, K helps the stem and health. The labels are placed beside each part.

Why does the crop stop growing at high fertiliser doses?

The plant can only use so much. Move the slider past 60 and watch the red bar: the extra is wasted.

Where does the wasted fertiliser go?

It washes into streams and ponds, feeds algae and lowers oxygen for fish.

Lime and mortar: the lime cycle

Limestone is calcium carbonate, CaCO3. When it is heated to about 900 °C it loses carbon dioxide and becomes quicklime (calcium oxide, CaO).

Quicklime reacts with water and gets very hot. The product is slaked lime (calcium hydroxide, Ca(OH)2). Mixed with sand and water it is lime mortar, which holds bricks together.

Slowly the slaked lime takes CO2 from the air and becomes calcium carbonate again. That is why lime mortar hardens. The cycle is: stone, heat, water, air, stone.

Slaked lime is also used for whitewash and to reduce acidity in soil.

Cement and concrete, and why steel is added

Cement is made by heating limestone and clay to about 1450 °C, then grinding the result with a little gypsum. It hardens by reacting with water. The full story is in the lesson Cement.

Concrete is cement + sand + stones + water. It is very good at carrying a squeeze (compression) but weak when it is pulled (tension).

In a beam the top is squeezed and the bottom is pulled. So builders put steel bars near the bottom. Steel is strong when pulled. Concrete with steel inside is reinforced concrete (RCC). Steel and concrete expand almost equally when heated, so they do not tear apart.

Glass and other building materials

Ordinary window glass is soda-lime glass. It is made from sand (silica, SiO2), soda ash (sodium carbonate) and limestone, melted at about 1500 °C. When the melt cools quickly, the atoms do not line up in a pattern, so it is a clear, hard, brittle solid.

Other common materials: bricks (clay baked in a kiln), steel (iron with a little carbon), plaster of Paris (heated gypsum), and paint (colour in a binder). Each is chosen for strength, weight, cost and how long it lasts.

Plant food: the N, P and K fertilisers

Plants take water and CO2 from the air and water, but they also need minerals from soil. Crops use up these minerals, so farmers add fertilisers.

A bag marked NPK 10:26:26 has 10% N, 26% P2O5 and 26% K2O by mass. Ammonia for nitrogen fertilisers is made by the Haber process (see Haber process).

Using fertiliser wisely, and soil pH

Crop yield rises with more fertiliser, but only up to a point. After that, extra fertiliser gives no extra crop. It runs off with rain into ponds and rivers. This feeds algae, which grow too much and use up the oxygen. This is eutrophication, and fish die.

Good practice: test the soil, add only what is missing, split the dose, and use compost and crop rotation with beans (they add nitrogen). If soil is too acidic, farmers add lime (CaO or Ca(OH)2) to raise the pH.

Pesticides kill pests, but they can harm useful insects and stay in food, so they must be used in small, safe amounts.

Try it

In the 3D, move the fertiliser slider from 0 to 100. Predict first: will 100 give twice the crop of 50? Check the crop percentage and the red bar. At home, look at a fertiliser or plant-food label and find the three numbers. Whitewash a small stone with lime water and see it turn white as it dries and takes CO2 (ask an adult for help; lime can burn skin).

Key formulas and definitions

Worked examples

1. What are the three steps of the lime cycle?

Limestone is heated to quicklime. Quicklime plus water gives slaked lime. Slaked lime takes CO2 from air and becomes limestone again.

2. A 50 kg bag of urea contains 46% nitrogen. How much nitrogen is in it?

50 × 46 ÷ 100 = 23 kg of nitrogen.

3. Why is steel put at the bottom of a concrete beam, not the top?

When a beam carries a load, the bottom is pulled and the top is squeezed. Concrete is weak when pulled, so the steel goes where the pulling happens, at the bottom.

4. A bag says NPK 10:26:26. How much P2O5 is in 20 kg of this fertiliser?

20 × 26 ÷ 100 = 5.2 kg of P2O5.

5. 100 kg of pure limestone is heated until all of it becomes quicklime. The molar masses are CaCO3 100, CaO 56. How much quicklime is made?

100 g of CaCO3 gives 56 g of CaO, so 100 kg gives 56 kg of quicklime. The other 44 kg leaves as CO2 gas.

6. A field needs 69 kg of nitrogen per hectare. How much urea (46% N) must be spread on 2 hectares?

For 1 hectare: 69 ÷ 0.46 = 150 kg urea. For 2 hectares: 300 kg of urea.

Common mistakes

Practice quiz

1. Heating limestone gives:
2. Which nutrient makes leaves green?
3. Why does concrete need steel bars?
4. Soda-lime glass is made from sand, limestone and:
5. Too much fertiliser washed into a pond causes:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What does NPK mean on a fertiliser bag?

It shows the percent of nitrogen (N), phosphorus (as P2O5) and potassium (as K2O) in the bag, in that order.

Why is glass transparent?

Glass has no crystals or grain boundaries to scatter light, and its electrons do not absorb visible light, so light passes through.

How does lime help acidic soil?

Lime is a base. It reacts with the acids in soil and raises the pH, so crops can take up nutrients better.

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