Compounds and mixtures
In this topic you will:
- make a compound and a mixture
- learn about the difference between a compound and a mixture
- give examples of mixtures.
Key words
- composition
- evaporating basin
- filings
- mineral
- mixture
- natural emissions
- pipe-clay triangle
- pure
Compounds and mixtures
When atoms of elements are bonded tightly to form a compound, the properties of the compound are completely different from the properties of the elements that it is made from.
For example, iron is a metal. It is hard, grey, strong, conducts heat and electricity and is magnetic.
Sulfur is a non-metal. It is yellow, brittle, does not conduct heat or electricity and is not magnetic.
When these two elements are heated, they combine together to form the compound iron sulfide. Iron sulfide is not magnetic and does not conduct heat or electricity.
Think Like a Scientist
Investigation: Using Iron and Sulfur
In this task, you will create both a mixture and a compound using iron and sulfur. This will help you understand the difference between physical mixtures and chemical changes.
You will need: safety glasses, beaker, stirrer, boiling tube, test tube holder, Bunsen burner, powdered sulfur, iron filings, mineral wool
Safety: Wear safety glasses. Do not touch your face or eyes while handling iron filings. These may have sharp edges and can irritate your skin. Work in a well-ventilated area.
Part 1 – Making a Mixture
1. Place some iron filings in a beaker.
2. Add a small amount of powdered sulfur.
3. Stir well to combine the substances evenly.
4. You have now created a physical mixture of iron and sulfur. Try moving a magnet near the beaker to observe how the iron responds.
Part 2 – Making a Compound
1. Place the iron and sulfur mixture into a boiling tube.
2. Heat the tube gently using a Bunsen burner while holding it with a test tube holder.
3. As the mixture starts to glow, stop heating and let the tube cool.
4. You have now formed a new substance: iron sulfide. Try placing a magnet near the tube to see if it behaves differently than the mixture.
Show Answer
The mixture appears as a combination of grey and yellow particles — you can still see separate bits of iron and sulfur.
Show Answer
Iron sulfide is a dark, uniform material. It looks different from either element and no longer shows separate iron or sulfur particles.
Show Answer
No, the iron has chemically reacted with the sulfur to form iron sulfide. The iron is no longer magnetic and cannot be separated using a magnet.
Air is a mixture
When you mix iron and sulfur together, you make a mixture of two elements.
In science, the word pure is used to describe something that only contains a single substance. Pure water contains only water, with no other substances mixed with it.
A mixture is not pure. It is made up of different kinds of particle that are mixed together. The mixture may be of elements, compounds or both. There are solids, liquids and gases that are mixtures.
For example, air is a mixture of several different elements and compounds. Air contains nitrogen, oxygen, carbon dioxide, water vapour and small quantities of some other gases.
The composition of air varies because the amount of water vapour changes all the time, depending on the weather.
The amount of carbon dioxide and other gases also change. This can be a result of natural emissions, such as when animals and plants produce carbon dioxide when they respire. Plants also absorb carbon dioxide when they make food, so this removes carbon dioxide from the air. The changes in the composition of air can also be as a result of human activity increasing the amount of carbon dioxide that is given out as a result of burning fossil fuels. Other forms of pollution also change the composition of the air.
The composition of air has changed over millions of years; at one time there was much less oxygen in the atmosphere.
Questions
Show Answer
Nitrogen
Show Answer
3
Show Answer
Carbon dioxide
Mineral water is a mixture
The label on a bottle of mineral water lists many minerals. There is more than just water in the bottle. The bottle contains a mixture of water and other substances.
The minerals are dissolved in the water. The mineral water is a solution. A litre of water may have about of minerals dissolved in it.
Mineral Water Label – Typical Mineral Content
| Mineral | Amount (mg/l) |
|---|---|
| Calcium | 55 |
| Magnesium | 19 |
| Potassium | 2 |
| Sodium | 24 |
| Bicarbonate | 248 |
| Chloride | 37 |
| Sulphate | 13 |
| Nitrate | < 0.1 |
| Iron | 0 |
| Aluminium | 0 |
| Dry Residue at 180°C | 280 |
| pH at Source | 7.4 |
Question
List the three most abundant minerals in this bottle of mineral water.
Show Answer
Dry residue, bicarbonate, calcium
Think Like a Scientist
Investigation: Is Water Really a Mixture?
In this investigation, you will test whether drinking water contains only water or if other substances are present as well.
You will need: Evaporating basin, tripod, pipe-clay triangle, Bunsen burner, tongs, safety glasses
Safety: Wear safety glasses. The solution may spit when boiling — take care during heating. Use tongs to handle the hot evaporating basin. Do not touch it with your hands. Work in a well-ventilated space and read all safety notes before starting.
Steps:
1. Pour a small amount of drinking water into the evaporating basin.
2. Heat the basin using a Bunsen burner until the water starts to boil.
3. Reduce the heat and continue heating gently. Watch as the water evaporates.
4. When about half the water is gone, or the solution starts to spit, turn off the heat.
5. Leave the basin in a safe place to cool. Depending on the temperature, complete evaporation may take a day or two.
6. Observe what remains in the basin.
Show Answer
Heating gives water particles more energy, so they move faster and escape from the liquid into the air as gas — this is evaporation.
Show Answer
Some solid material or residue may remain — such as minerals or salts dissolved in the original water.
Show Answer
The residue was originally dissolved in the water — it came from minerals or other substances found in the drinking water source.
Show Answer
It was a mixture. Pure water would evaporate completely, but the presence of residue shows other substances were dissolved in it.
Show Answer
To protect your eyes from hot liquid that might spit or splash while boiling — a risk during evaporation.