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Majority of things that we see around us are said to be examples of matter. What is matter? Can you name some things that are matter around you. The whole concept of matter was coined at around 400 BC by Greek Philosophers Democritus and Aristotle. According to Democritus, if you cut matter into smaller and smaller pieces, you end up with its smallest bits which cannot be cut further. He called these ‘atomos’ which there after came to be known as ‘atoms’. Aristotle, at around 350 BC, modified the Democritus theory to state that matter was made up of four elements namely: fire, earth, air and water. His theory persisted for about 2000 years and was later dismissed and replaced by the modern day atomic theory proposed by English Scientist John Dalton. Atomic theory was as a result of experimentation into the nature of matter. It paved the way for deeper understanding of what happens during chemical reactions. This unit is about the various states of matter. So then, what is matter? What are the various states of matter? What happens when various forms of matter are subjected to different conditions?
Key topic competency: A learner should be able to relate properties of matter to daily life, physical and chemical phenomena.
Topic outline
3.1 Definition of matter
3.2 Changes of states of matter
3.3 Physical and chemical changes
3.4 Kinetic theory of matter
3.1 Definition of matter
All substances that are found in nature are made up of matter. Matter is anything that occupies volume and has mass.
Matter can be put into three different groups, that is:
• Solids - for example soil, chalk, salt, sugar, wood and metals.
• Liquids - for example water, kerosene, milk and spirit.
• Gases - for example air, biogas, oxygen and carbon dioxide. These three groups are commonly known as states of matter. Each of these states of matter has characteristic properties.
The following table gives characteristic properties of the three states of matter.
3.2 Changes of states of matter
Matter can be converted from one state to another.
When some solids are heated they change into liquids. This process is called melting. When the liquids are cooled they change back into the solid state. This process is known as freezing. When a piece of ice is heated, it changes into liquid water. The water slowly changes into vapour with continued heating. The process through which water changes state from liquid to gas is known as evaporation. When the vapour is cooled, it changes back into liquid form. This process is called condensation. It is also possible to change a solid directly into a gas. This is called sublimation. However, when a gas changes directly to a solid; the process is called deposition.
Questions
1. (a) What does ice form when it melts?
(b) Did temperature change during melting?
(c) Give a reason for your answer in 1(b) above.
2. (a) What does water change to when it boils?
(b) Did temperature change during boiling?
(c) Give a reason for your answer in 2(b) above.
3. Why is the thermometer used in this experiment?
Expected response
We can explain what happens in each region as follows:
• Region AB: The temperature rises steadily as the ice absorbs heat energy. The temperature rise stops at 0ºC.
• Region BC: The temperature remains constant (0ºC), until all the ice has melted. This is because the heat energy absorbed in this region is used to break the forces of attraction holding the solid particles together. Water changes its physical state from solid to liquid form at this point.
• Region CD: Temperature rises steadily as the liquid water absorbs heat energy. The temperature rise stops when the liquid water starts changing into vapour (evaporation).
• Region DE: The temperature remains constant as the liquid water changes into water vapour. Heat energy absorbed is used to break the forces of attraction holding water particles together. Water thus changes into vapour.
When some solids such as iodine are heated, they change from solid directly to gas without passing through the liquid state. This change is called sublimation. On cooling, the substances condense from gas to solid. The process is known as deposition.
Chemical changes
Investigating chemical changes
Apparatus and reagents
Test tubes, Bunsen burner, wooden splint, a piece of paper, magnesium ribbon, iron nails, water, pair of tongs.
Procedure
1. Light a wooden splint in a Bunsen burner flame.
2. Allow it to burn for some time.
3. Compare the product formed with the initial splint. Are they the same?
4. Burn a piece of paper to form ash.
5. Compare the product with the initial paper. Is it possible to get back the paper from the ash?
6. Using the pair of tongs, heat a piece of magnesium ribbon on a Bunsen burner flame. Allow the magnesium to burn. Caution! Burning magnesium produces intense light that can cause temporary loss of sight. Do not look directly at the light source.
7. Collect the product and compare it to the initial magnesium ribbon.
8. Place few iron nails in a test tube containing water.
9. Keep it in an open place for one week. What do you observe? 10. Explain your observation in (9) above.
The Facts
When wood is burnt, it changes to ash. We cannot get back wood from the ash. This reaction is thus irreversible. Similarly, when magnesium is burnt in air, it forms powder. It is not possible also to get back the magnesium ribon from the powder. The white powder formed is called magnesium oxide. It is as a result of combining magnesium with oxygen in air.
All these processes are well irreversible. They are therefore called parmanent changes. Parmanent changes are chemical changes. Some of the characteristics of chemical changes include:
1. New substances are formed.
2. It is difficult to change the new substance back into the original substance. (irreversible)
Self evaluation test
1. Cooking fat should be stored in a cool dry place. What is the importance of this precaution?
2. State whether the following are physical or chemical changes.
(i) Burning a match stick into ash. (ii) Freezing water to make an ice cube. (iii) Explosion of a bomb.
3. (i) Zinc oxide changes to _____________colour on heating and ________________________colour on cooling. It undergoes a__________ change. (ii) Iodine changes to _________________________ on heating and ________________________on cooling. It undergoes _______________ change.
4. State whether the following statements are true or false.
(a) Burning wood is a chemical change. (b) Drying a shirt in the sun is a chemical change.
(c) Dissolving sugar in tea is a physical change. (d) Cooking meat is a chemical change.
3.4 Kinetic theory of matter
Summary of Key topic concepts
Matter is anything that occupies volume and has mass.
• The three states of matter are: gas, liquid and solid.
• Matter can undergo physical or chemical change.
• Melting is the process by which a solid is converted to a liquid at constant temperature. The reverse of melting is freezing.
• Vaporisation is the process by which a liquid changes to vapour. The reverse of vaporisation is condensation.
• A physical change is one where no new substance is formed while a chemical change is when a new substance is formed.
• Kinetic theory of matter states that matter is composed of many small particles that are in constant motion.
• Brownian motion is the random movement of particles suspended in a fluid.
Revision Questions
1. State whether the following statements are true or false.
(a) The particles of a liquid are attracted to one another, but cannot move past each other.
(b) The atoms of a solid are very far apart and vibrate in fixed positions.
(c) Heating a gas completely stops all of its particles from motion.
(d) Air has mass.
2. When a thermometer is placed in warm water, the mercury inside moves up. This is mainly because ____________.
A. the mercury is thin.
B. the particles of mercury move faster and get a little further apart
C. hot mercury is lighter D. the glass of the thermometer gets hot.
3. Food colouring spreads out faster in hot water than it does in cold water. This is mainly because_____________.
A. the water molecules in hot water move more quickly.
B. the molecules in hot water are larger.
C. the food colouring molecules are small.
D. hot water is less dense.
4. To describe a gas, you would say_____________.
A. the particles are very attracted to each other.
B. the particles are not very attracted to each other.
C. the particles are very close together like a liquid.
D. the particles of a gas are farther apart than the particles in a liquid or solid.
6. Sometimes, especially on hot days, a metallic door, that opens and closes easily during cooler months, will be hard to open and close. Why is the door hard to open and close on hot days?
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