Changes Around Us
Class 6 | Science — Chemistry | Unit 3
Class 6 Unit 3 Science
Unit 3. Changes Around Us
Learning Objectives
- To recognize and enlist a few changes that happens in our day-to-day life
- To classify the observed changes as, slow or fast, reversible or irreversible physical and chemical changes desirable or undesirable, natural or human made
- To explain the process of dissolution
- To distinguish between a solvent and a solute
Exercise Observe and Fill
Observe the pictures in the previous page and fill in the gaps.
| Initial stage | Changing stage |
|---|
| Seed | Sapling |
| Night |
| Rock | |
| raw fruit | |
What is common in all the above pairs?
Introduction
What is a change?
The process in which something becomes different from what it was earlier? It is the observable difference between initial state and the final state of any substance. Change is the law of nature. In our day - to - day life we see many changes around us. Weather changes periodically (daily or seasonally), season change periodically. A paper burns readily while it takes a few days for an iron nail to rust. It takes a few hours for milk to turn into curd but vegetables get softened in a few minutes when cooked.
The what happens during a change? The above said changes are accompanied by change in the properties like shape, colour, temperature, position and composition. Some changes can be observed while some are not possible to notice. Can you list some of the changes that you have observed in your daily life?
Activity 1 What happens when you blow air into a balloon? Is there any change in size? Is there any change in shape?
Is there any other change?
3.1. Classification of Changes
There are different types of changes that occur around us. Some changes take place very quickly while others take hours, days or even years. Some changes are temporary while some are permanent. Some changes produce new substances while others do not. Some changes are natural while others are made by human beings. Some changes are desirable to us but some changes are not desirable. We shall now try to classify changes on the basis of certain similarities and differences.
- slow and fast changes
- reversible and irreversible changes
- physical and chemical changes
- desirable and undesirable changes
- natural and man - made changes
3.1.1. Slow and Fast changes
Activity 2 Look at the pictures and discuss about the duration for the changes to take place.
Slow changes
Changes which take place over a long period of time (hours or days or months or years) are known as slow changes.
Examples: growth of nail and hair, change of seasons, germination of seed.
Fast Changes
Changes which take place within a short period of time (seconds or minutes) are known as fast changes.
Examples: Bursting of balloon, breaking of glass, bursting of fire crackers, burning of paper.
3.1.2. Reversible and Irreversible changes
Reversible change
Changes which can be reversed (to get back the original state) are known as reversible changes.
Examples: Touch me not plant (Responding to touch), stretching of rubber band, melting of ice.
Activity 3 Try to make a boat and an aeroplane one by one using the same piece of paper.
This means the change of shape discussed here is reversible.
Irreversible change
Changes which cannot be reversed or to get back the original state are known as irreversible changes.
Activity 4 What kind of changes are they?
a) Burning of a candle.
b) Piercing a balloon with a pin.
Examples: Change of milk into curd, digestion of food, making idly from batter.
3.1.3. Physical and Chemical Changes
Physical changes
Activity 5
Take an apple and cut it into two halves. Cut one half into pieces and share it with your friends. Is there any change in the composition of the apple while cutting? No, only the shape and size have changed. This can be called a physical change. Leave the other half on the table for some time. You can see brown patches formed on the cut surface because of the reaction between some substances in the apple and the air around it. This is a chemical change.
Physical changes are the temporary changes in which there is change in the physical appearance or physical state of the substance but not in its chemical composition. Here no new substance is formed.
Example: Melting of ice, the solution of salt or sugar, stretching of rubber band.
Let us now understand the physical changes that take place in water. You already know that water exists in three states as solid, liquid and gas. Change of state takes place either by heating or cooling. By heating energy is supplied and by cooling energy is taken away. These are the reasons for the changes. Let us name a few processes connected with the changes in states of water.
| Change | Process |
|---|
| ice into water on heating | melting |
| water into steam on heating | vapourisation |
| steam into water on cooling | condensation |
| water into ice on cooling | freezing |
More to Know
The change of state from solid to gas directly is called sublimation. Example: Camphor
Let us understand one more physical change
Dissolution
The spreading of the solid particles (broken into individual molecules) among the liquid molecules is called as dissolution. Solvent is a substance that dissolves the solute. Solute is a substance that is dissolved in a solvent to make a solution. When solute is dissolved in a solvent it forms a solution.
Do you know
1. Water is known as the universal solvent. It dissolves a wide range of substance.
Activity 6 Take half a cup of water, add one spoon full of sugar and stir well.
a. What do you observe?
b. What happened to the sugar?
c. Where is it gone?
d. The solute in the above solution is
e. The solvent in the above solution is
f. Have you seen a glass of water and a glass of sugar solution looking alike?
Chemical changes
Chemical changes are the permanent changes in which there is a change in the chemical composition and new substance is formed.
Examples: Burning of wood, popping of popcorn, blackening of silver ornaments, and Rusting of iron.
| Physical Change | Chemical Change |
|---|
| No new substance formed in this change. | New substance formed in this change. |
| No change in the chemical composition. | There a is change in the chemical composition. |
| It is a temporary change. | It is a permanent change. |
| It is reversible. | It is irreversible. |
Activity 7 Look at the pictures and write whether they are physical or chemical changes.
3.1.4. Desirable and Undesirable Changes
Desirable changes
Activity 8 Look at the pictures and write whether they are desirable or undesirable changes.
The changes which are useful, not harmful to our environment and desired by us are known as desirable changes.
Examples: Ripening of fruit, growth of plants, cooking of food, milk changing to curd.
Undesirable changes
The changes which are harmful to our environment and not desired by us are known as undesirable changes.
Examples: Deforestation, decaying of fruit, rusting of iron.
3.1.5. Natural and human made changes
Activity 9 Identify the type of changes Natural or Human made
Natural changes
Changes which take place in nature on their own and are beyond the control of human beings are known as Natural changes.
Examples: Rotation of the earth, Changing phases of the Moon, Rain.
Human made or artificial changes
the changes which are brought about by human beings are known as human made or artificial changes. They will not happen on their own.
Examples: Cooking, Deforestation, Cultivating crops, construction of buildings.
ICT Corner
Changes Around Us
Through this activity you will be able to understand reversible & irreversible changes.
Step 1: Use the given URL in the browser. 'Reversible and irreversible change's page will open. Use the arrow marks on both sides of the substance to choose another substance to test.
Step 2: Click and drag the substance into the beaker, observe whether it dissolves or not. Click the Dissolving OR Reversing button to switch between the both activities.
Step 3: In the Reversing activity, with some substances you can choose either to cool or to Heat them. With other substances you can choose either to Heat or to filter them by clicking the respective buttons.
Step 4: Click on the Reset button to clear.
Reversible and irreversible change's URL: http://www.bbc.co.uk/schools/scienceclips/ages/10_11/rev_irrev_changes_fs.shtml
*Pictures are indicative only
Evaluation
Exercise I. Choose the appropriate answer
1. When ice melts to form water, change occurs in its
a. position b. colour c. physical state d. composition
2. Drying of wet clothes in air is an example of
a. Chemical change b. Undesirable change c. irreversible change d. physical change
3. Formation of curd from milk is
a. a reversible change b. a fast change c. an irreversible change d. an undesirable change
4. Out of the following an example of a desirable change is
a. rusting b. change of seasons c. earthquake d. flooding
5. Air pollution leading to acid rain is a
a. reversible change b. fast change c. natural change d. human made change
Points to remember
- Everything in this world undergoes changes.
- Changes occur in position, shape, size, state, colour, temperature, composition etc.
- Fast change – short period of time.
- Slow change – long period of time.
- Reversible change – can get back to its original state.
- Irreversible change – cannot get back to its original state.
- Desirable change - changes that are useful and harmless to our environment.
- Undesirable change - changes that are harmful to our environment.
- Natural change - changes that take place in nature on their own.
- Human made change - changes that are brought about by human beings.
- A solute when dissolved in a solvent makes a solution.
- The process of dissolving the solute in solvent is called dissolution.
Exercise II. Fill in the blanks
1. Magnet attracts iron needle. This is change. (a reversible or an irreversible)
2. Boiling of egg results in change. (a reversible or an irreversible)
3. Changes that are harmful to us are (desirable or undesirable)
4. Plants convert carbon-di-oxide and water into starch. This is an example of change. (natural or human made)
5. Bursting of fire crackers is a change whereas germination of seeds is a change. (slow or fast)
Exercise III. True or False. If False, give the correct statement 1. Growing of teeth in an infant is slow change. 2. Burning of match stick is a reversible change. 3. Change of new moon to full moon is human made. 4. Digestion of food is a physical change. 5. In a solution of salt in water, water is the solute Exercise IV. Analogy
1. Curdling of milk: irreversible change: Formation of clouds: change
2. Photosynthesis: change: burning of coal: Human – made change
3. Dissolution of glucose: reversible change: Digestion of food: change
4. Cooking of food: desirable change: decaying of food: change
5. Burning of matchstick: change: Rotation of the Earth: Slow change
Exercise V. Circle the odd one out. Give reason for your choice
1. Growth of a child, Blinking of eye, Rusting, Germination of a seed
2. Glowing of a bulb, lighting of a Candle, breaking of a coffee mug, curdling of milk
3. Rotting of an egg, condensation of water vapour, trimming of hair, Ripening of fruit
4. Inflating a balloon, popping a balloon, fading of wall paint, burning of kerosene
Exercise VI. Give very short answer
1. What kind of a change is associated with decaying of a plants?
2. You are given some candle wax. Can you make a candle doll from it? What kind of change is this?
3. Define a slow change.
4. What happens when cane sugar is strongly heated? Mention any two changes in it.
5. What is a solution?
Exercise VII. Give short answer
1. What happen when paper is burnt? Explain.
2. Can deforestation be considered a desirable change? Explain.
3. What type of changes is associated with germination of a seed? Explain
Exercise VIII. Answer in detail
1. Give one example for each case that happens around you.
a. Slow and fast change
b. Reversible and irreversible change
c. Physical and chemical change
d. Natural and man-made change
e. Desirable and undesirable change
Exercise IX. Question based on Higher Order Thinking Skills
1. When a candle is lit the following changes are observed.
a. Wax melts.
b. Candle keeps burning.
c. The size of the candle decreases.
d. The molten wax solidifies
e. Which of the changes can be reversed? Justify your answer.
📝 Summary
- Change is the observable difference between initial state and final state of any substance, and it is the law of nature that we see in our day-to-day life.
- Changes can be classified as slow changes (taking hours, days, months or years like growth of nail and hair) or fast changes (taking seconds or minutes like bursting of balloon or breaking of glass).
- Reversible changes can be reversed to get back the original state (like stretching of rubber band or melting of ice), while irreversible changes cannot be reversed (like burning of candle or change of milk into curd).
- Physical changes involve only change in shape, size or state without any change in composition of the substance (like cutting an apple into pieces).
- Changes occur around us with variations in properties like shape, colour, temperature, position and composition, and can be classified as desirable or undesirable, and natural or human-made.