Atoms, Elements and Compounds
All substances are made of ATOMS
Some substances are made up of only one type of atom. We call these substances ELEMENTS.
ELEMENTS have various different properties; eg, Silver and gold as shiny solids, other elements such as oxygeon, nitrogen and chlorine are gases.
Almost all of the substances around us aren't pure elements, most are substances made of different atoms joined together to form COMPOUNDS. Some compounds are made from two types of atoms joined together; eg, water is made from hydrogen and oxygen. Other compounds consits of many different types of atom.
ATOMS are made up of a tiny central NUCLEUS with ELECTRONS around it.
TWO OR MORE ATOMS BONDED TOGETHER= MOLECULE
Sometimes atoms react together by transferring electrons to form chemical bonds. Other atoms may share electrons to form chemical bonds.
Key points; All substances are made of atoms. Elements only contain one type of atom. Different atoms can bond together by giving taking or sharing electrons to form compounds.
Limestone and It's Uses
Limestones is a rock mainly made from CALCIUM CARBONATE. Some types of limestone were formed from remains of small animals and plants that lived in the sea millions of years ago. Limestone is dug out of quarries around the world. Main use is building.
As well as buildings, we can process limestone to make other building materials. Powdered limestone can be heated with a mix of sand and sodium carbonate (soda) to make glass.
Powdered limestone can also be heated with powdered clay to make cement. When cement powder, water, sand and crushed rock are mixed a chemical reaction takes place; concrete is formed.
Calcium Carbonate = CaCO3. This tells us for every calcium atom (Ca) there is one carbon atom (C) and there oxygen atoms (O).
When limestone is heated it breaks down to form QUICKLIME (Calcium Oxide) CO2 is also produced. Breaking down a chemical by heating is called THERMAL DECOMPOSITION.
To make lots of quicklime the reaction is done in a LIME KILN. We fill the kiln with crushed limestone and heat it strongly using a supply of hot air. Quicklime comes out of the bottom, while waste gases leave the top.
We can show the thermal decomposition reaction by a word equation.
Calcium Carbonate ---Heat-----> Calcium Oxide (Quicklime) + Carbon Dioxide
Key Points; Limestone is made mainly of Calcium Carbonate.
Limestone is widely used in building
Limestone breaks down when we heat it strongly (thermal decomposition) to make quicklime and carbon dioxide.
- Some carbonates decompose when we heat them in a Bunsen Flame.
- There is the same number of each type of atom on each side of a balanced chemical equation.
- The mass of reactants is the same as the mass of the products in a chemical equation.
Quicklime and Slaked Lime
When we ADD WATER to QUICKLIME it reacts to produce SLAKED LIME. Slaked lime's chemical name is CALCIUM HYDROXIDE. This reaction gives a lot of heat.
Calcium Oxide(Cao) + Water (H20) ----> Calcium Hydroxide (Ca(OH)2)
We can dissolve a little calcium hydroxide in water. After Filtering, it produces a colourless solution; lime water. This can be used to test for CO2. When Carbon Dioxide is bubbled through clear lime water the solution turns cloudy. This is because CALCIUM CARBONATE is formed, which is insoluble in water.
Calcium hydroxide(Ca(OH)2) + Carbon Dioxide (CO2)------> Calcium Carbonate (CaCO3) + Water (H2O)
Mortar holds other building materials together- for example, stone blocks or bricks. It works because lime reacts with Co2 in the air producing Calcium Carbonate again.
Slaked Lime (Ca(OH)2)+ Carbon Dioxide (CO2)--------> Calcium Carbonate (CaCO3) + Water (H2O)
The amount of sand is important. Too little sand; mortar shrinks and dries. Too much sand; make it too weak.
Cement, Concrete and Glass
- Cement is made by heating limestone with clay in a kiln
- Concrete is made by mixing crushed rocks, cement and sand with water
- Glass is made by heating powdered limestone, sand and sodium carbonate together very strongly.