Changes Around Us Physical and Chemical Class 7 Notes Science Chapter 5

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Class 7 Science Chapter 5 Changes Around Us Physical and Chemical Science Notes

Class 7 Changes Around Us Physical and Chemical Science Notes

Class 7 Science Chapter 5 Notes – Changes Around Us Physical and Chemical Science Notes Class 7

→ Changes: We observe many changes around us like melting of ice, growing of bud into a flower, warming of cold water when kept outside, rotting of banana, etc. During these phenomenon, shape, size, state or smell changes.

→ Physical Properties: Properties such as shape, size and state of a substance are called physical properties. We can observe these changes with the help of our sense of sight, smell, touch, hearing, and taste.

→ Physical Change: A change in which only physical properties of any substance change and no new substance is formed is called a
physical change.

→ Characteristics of Physical Change:

• The physical changes do not form a new substance or substances.
• The physical change does not change the composition of a substance.
• For example, freezing of water into ice, stretching of rubber band or spring and attraction of iron filing by a magnet. Chemical

→ Chemical Change: A change in which one or more new substances with different chemical properties are formed is called a chemical change. These new substances are different from the original substances in their chemical properties.

→ Chemical Reaction: The process through which new substances are formed is known as chemical reaction.

→ Chemical Equation: The chemical reaction involved in the change can be represented in short form as a chemical equation. For example:

• When carbon dioxide gas is passed through freshly prepared lime water (calcium hydroxide), lime water turns milky due to formation of calcium carbonate (insoluble substance).
  This reaction can be written as:
  
• When we add a pinch of baking soda to vinegar or lemon juice, a new substance carbon dioxide is evolved. This reaction can be written as:
  

→ Characteristics of Chemical Change:

• New substances are formed which have different chemical properties from original substances.
• Most of the chemical changes are irreversible,
• The chemical changes always result in energy changes.

→ Rusting: If a piece of iron is exposed to oxygen in the air, water or water vapour for some time, it acquires a film of a new brown coloured substance. This substance is called rust. The process by which rust is formed on an iron object is called rusting.
The process of rusting can be represented by the following equation:

→ Combustion: A chemical reaction in which a substance reacts with oxygen and produces heat and/or light is called combustion.
For example, when the magnesium ribbon is burnt, a new substance, magnesium oxide is formed. This burning of magnesium ribbon is a chemical change. Heat and light are also produced during this Burning of reaction. magnesium ribbon.
This reaction can be written as:

→ Combustible Substances: Those substances that undergo combustion reactions are called combustible substances. For example, wood, paper, kerosene, cotton, etc.

→ Bioluminescence: Some insects like fireflies emit light in late evenings, which is produced by a chemical change. This type of light production (without heat) in living organisms is called bioluminescence.

→ Ignition Temperature: The minimum temperature at which a substance catches fire is known as its ignition temperature. A substance cannot catch fire or burn as long as its temperature is lower than its ignition temperature. Conditions for combustion to occur:

• A combustible substance
• Oxygen
• Heat that allows the fuel to reach its ignition temperature.

→ Burning of Candle: Burning of a candle involves both physical and chemical change.

→ Reversible Changes: Those changes that can be reversed or returned to the original object or substance with which we started are called reversible changes. For example:

• Melted ice can be frozen back to ice again by keeping in refrigerator.
• Boiling of water is also an example of reversible change as we get water back from vapour by condensation (cooling).

→ Irreversible Changes: Those changes that cannot be reversed or cannot returned to the original object or substance are called irreversible changes. For example, chopped vegetables or fruits cannot get back to original shape.

→ Desirable Changes: Useful changes are called desirable changes. For example, making icecream from milk, ripening of fruits, cooking of food, drying of clothes, etc.

→ Undesirable Changes: Those changes that we do not prefer to occur as they are harmful and destroy the objects are called undesirable changes. For example, rotting of food, rusting of bicycle, etc.

→ Some Slow Natural Changes:

• Weathering of Rocks: This is the natural process by which rocks are broken down into smaller pieces over time due to exposure to environmental factors like wind, water, temperature changes, and biological activities. Rocks undergo physical and chemical changes due to weathering to form soil.
• Erosion: Rock pebbles, soil, and sediments break down and move from one location to another to form sand. This takes place by natural forces like wind and flowing water. Erosion caused by flowing water and wind is a physical change.

Class 7 Science Chapter 5 Notes Changes Around Us Physical and Chemical

→ A physical change is one in which a substance or object undergoes a change in its physical properties and no new substance is formed.

→ A chemical change is one in which one or more new substances are formed. It involves a chemical reaction and can be represented by a chemical equation.

→ Combustion, cooking, and rusting are examples of chemical changes.

→ Substances that undergo combustion are combustible substances. Heat and/or light are given out during combustion.

→ The lowest temperature at which a substance can catch fire is called its ignition temperature.

→ Some changes can be reversed and some cannot.

→ Some changes are desirable and some are not.

→ Rocks undergo physical and chemical changes due to weathering to form soil.

→ Erosion caused by flowing water and wind is a physical change.

A Substance May Change in Appearance but Remain the Same!

A. Creating some objects with paper

• Take a few sheets of paper and fold them to create new objects (Figure).
• Do you get the same paper back when you unfold these objects?

Fig. Objects made from paper

B. Playing with a balloon

• Take a balloon and inflate it. Now, loosen your grip and let the air escape out.
• Do you get the uninflated balloon back?
• Take another balloon; inflate it and grip the opening tightly. Now, prick it with a pin.

Caution—Be careful while using a pin.
What happens? Will you be able to get the uninflated balloon back?

C. Crushing a piece of chalk

• Crush a small piece of chalk into powder.
• Can you get the chalk piece back from the powder?

Is there any similarity in the changes listed in A, B, and C?
During all these changes, materials, such as the paper, the rubber sheet of the balloon, and the chalk, remained the same, though their appearances (shape or size) may have changed. You may also recall from the Grade 6 Science textbook Curiosity that water can exist in different states (solid, liquid, and gas), and can change from one state to another.

In all these cases, although we observe a change in the appearance, no new substance is formed. Such changes in which only physical properties like shape, size, and state of substances change are called physical changes.
Let us explore a different type of change.

A Substance May Change in Appearance and Not Remain the Same!

Let us explore

• Take two glass tumblers or small transparent bottles. Mark them A and B.
• Fill one-fourth of glass tumbler A with tap water and one-fourth of glass tumbler B with lime water.
• Now, blow air (exhale) into each glass tumbler, one at a time, using separate straws (Figure) and observe them.

Caution  — Do not suck the water or lime water while doing this.

Fig. Blowing air in (a) tap water; (b) lime water

In glass tumbler A, blowing air into water creates bubbles only, and there is no change in the appearance of the water. In glass tumbler B, blowing air into lime water creates bubbles, and turns the lime water milky (or cloudy). If we leave it for some time, a white substance settles at the bottom of the glass tumbler, indicating something new has formed. Such changes, in which one or more new substances are formed, are called chemical changes. New substances are formed through a process called chemical reaction. In this case, carbon dioxide from the air you breathe out (exhale) reacts with lime water, and forms a new white-coloured substance (calcium carbonate) that is insoluble in water. Therefore, the liquid in the bottle appears milky. Along with this a small amount of water is also formed. This formation of a new substance indicates a chemical change. The chemical reaction involved in this change can be represented in short form as a chemical equation.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water (Lime water) (Insoluble substance)
The turning of lime water milky is also used as a test for carbon dioxide. Let us explore this with another activity using some substances from our kitchen.

Let us experiment
Take a teaspoonful of vinegar or lemon juice in a test tube.
Add a pinch of baking soda (sodium hydrogen carbonate) to it.

• What do you observe?
• You would hear a fizzing bubbling sound and see the gas bubbles forming.
• Pass this gas through freshly prepared lime water kept in another test tube, as shown in Fig. (a).
• What do you observe?
• The lime water turns milky. What do you infer about the gas formed by mixing vinegar and baking soda?

This indicates that the gas formed is carbon dioxide.
This activity can also be performed using two small used bottles instead of test tubes and a flexible straw, as shown in Fig. (b).

Fig. Reaction of vinegar and baking soda

Since a new substance, carbon dioxide, is formed in this reaction, we say that a chemical change has occurred here as well. It can be represented as—
Vinegar + Baking soda → Carbon dioxide + Other substances
Repeat the above activity using baking soda and water. Do you observe any bubble formation? Is this a physical or a chemical change?

Some Other Processes Involving Chemical Changes

Rusting
In the rusting of iron, which you studied in the chapter ‘The World of Metals and Non-metals’, a new brown-coloured substance called rust is formed. Thus, rusting is also a chemical change because it involves the formation of a new substance, iron oxide (Figure).

Fig. Rusted iron nails

Combustion
Let us recall the burning of a magnesium ribbon (Figure) learnt in the chapter ‘The World of Metals and Non-metals’. Can you predict if this is a physical or a chemical change?
When the magnesium ribbon is burnt, a new substance, magnesium oxide, is formed. So, the burning of magnesium ribbon also involves a chemical change. We observed that heat and light are also produced along with the formation of a new substance in this reaction.

Fig. Burning magnesium ribbon

The burning of magnesium ribbon can be represented as—
Magnesium (Ribbon) + Oxygen (Air) → Magnesium oxide (White powder) + Heat + Light

A chemical reaction in which a substance reacts with oxygen and produces heat and/or light is called combustion. Substances that undergo combustion reactions are called combustible substances. For example, wood, paper, cotton, kerosene, etc., are combustible substances.

Fig. Candle (a) burning (b) covered with a glass tumbler

Let us find out whether the presence of oxygen is essential for combustion.

Let us investigate

• Place two identical candles on two separate petri dishes and light them.
• Cover one of these with a glass tumbler, as shown in Figure.
• What happens to the candle flames in the two cases?

We observe that the candle that was not covered (Fig. (a)) continues to burn, whereas the candle that was covered with a glass tumbler (Fig. (b)) stops burning after some time. Why does this happen?

Since the candle covered by the glass tumbler does not have a continuous supply of air, the flame gets extinguished soon. The component of air that supports combustion is oxygen. This can be confirmed by the presence of carbon dioxide in the glass tumbler inverted on the candle. How can you test the presence of carbon dioxide gas?
You can test the carbon dioxide gas by adding a small amount of lime water in the petri dish. You will notice that it turns milky. This carbon dioxide was formed by the carbon from the wax and the oxygen from the air. In other words, oxygen is required for combustion.

Science And Society
If a person’s clothes catch fire, what is the best way to extinguish the fire?
Wrap a blanket or cloth around the person. This cuts off the supply of air, and the fire gets extinguished.
Caution—Synthetic blanket or cloth should never be used to put out a fire, as these can melt and stick to the skin.

Fascinating Facts
Nature’s wonders: You might have seen some insects emitting light in a garden or a field in late evenings. These insects are called fireflies, and their light is produced by a chemical change. This type of light production (without heat) in living organisms is called bioluminescence.

Fireflies

Is the Presence of Air Enough for Combustion?
We learnt above that combustible substances and oxygen are necessary for combustion. We also know that paper is a combustible substance, but we can keep it in the air for any length of time without it catching fire. What else is needed to start combustion?
Let us learn about this.

Let us investigate
Caution  — Perform this activity under the supervision of your teacher or an adult.

• Hold a piece of paper with a pair of tongs and bring a lighted matchstick to it. It quickly catches fire. Do we say that we need a fire to start the burning process?
• Take another piece of paper. Using a magnifying glass, focus the sun rays to make the smallest and brightest spot on the paper, as shown in the Fig. (a). Hold it there for some time.

What do you observe?
We observe that the paper starts to emit smoke, and then catches fire (Fig. (b)). Thus, we find that a substance can burn even without fire. How do we
explain this change?

Focusing sun rays on the paper heats it. The temperature of the paper increases with time. After some time, the paper becomes so hot that it starts burning. This minimum temperature at which a substance catches fire is called its ignition temperature. Since the temperature of the lighted matchstick was already higher than the ignition temperature of the paper, it caught fire almost immediately.

Fig. (a): Focusing the sunrays using a magnifying glass
Fig. (b): Paper catching fire

Fig. Fire triangle
So, we can conclude that for the combustion process to occur, there are three requirements (Figure) —
(i) A combustible substance, also called ‘fuel’
(ii) Oxygen
(iii) Heat that allows the fuel to reach its ignition temperature.

Can Physical and Chemical Changes Occur in the Same Process?

What changes take place when a candle is lit? Let us explore!
Look at the Fig. Analyse what students are discussing about the burning candle. What do you think?

Fig. What changes occur when a candle burns?

The wax of the candle melts, is carried up the wick, and evaporates due to the heat of the flame. The vapour of wax burns to produce a flame. The melting of wax, its solidification, and evaporation are physical changes. The burning of vapour on the other hand, is a chemical change. This shows that the burning of a candle involves both physical and chemical changes.

Know A Scientist
The candle you just observed has long been an object of curiosity for several scientists including Michael Faraday, who made significant contributions to several areas of science. In the nineteenth century, Faraday delivered a series of lectures called Chemical History of a Candle. Faraday believed that the candle was the perfect object to introduce scientific study. Through it, he discussed differences between various physical and chemical processes like melting, vapourisation, and combustion.

Are Changes Permanent?
Once something has undergone a change, can we get it back in its original form?

Let us think
Think again about all the changes that we have discussed or talked about so far. In which of these can we get back the object or substance in the form we started with? Record your observations.

Returning to the original object or substance with which we started shows that the changes we brought can be reversed. For example, when ice melts, it can be refrozen into ice. Similarly, when water evaporates, it can be condensed back into liquid water. However, some changes cannot be reversed since we cannot get the original object or substance back after the change. For example, chopped vegetables cannot return to their original size and shape, and making popcorn cannot go back to its original form. Thus, changes around us could be grouped into those that can be reversed and those that cannot be reversed.

Are All Changes Desirable?
Many useful changes happen in our daily life. For example, the changing of milk into curd, ripening of fruits, cutting of fruits, and cooking of food. All these are desirable changes. Can you think of some other desirable changes happening around you?

On the other hand, some changes may be undesirable, such as the rusting of iron or the decay of food during its storage. A change that is undesirable in some situations may be desirable in other situations. For example, decomposition of food can be very useful in converting food waste into compost.

Some changes occurring over the years due to human activity can have a long-term environmental impact. For example, the increased consumption of fuels in cars, trains, aeroplanes, etc., is increasing the amount of carbon dioxide in the atmosphere. Drying of paint on walls, doors, furniture, etc., releases many substances through evaporation, causing atmospheric pollution.

Some Slow Natural Changes

Weathering of rocks
Have you seen heaps of sand, soil, and stones lying at the base of mountains as shown in Fig. 5.10a? These are called sediments. How have these formed? These are formed by physical changes that break up large rocks into smaller pieces. Temperature changes due to climatic conditions, growing roots of trees, and even freezing of water within cracks in the rocks can cause them to break.

Water or chemicals present in water, can also react with the rocks and cause chemical changes in their composition. An example of such a chemical change is shown in Fig. (b). Here, an originally black-coloured rock called basalt, which contains iron, has chemically changed to produce a red-coloured layer. The red colour is a result of the iron oxide produced when the rock surface was exposed for a long time to water or air containing water vapour. These physical and chemical changes in rocks are collectively called weathering, which eventually leads to the formation of soil.

Fig. (a) Sediments at the base of a cliff (b) red sediment layer

Erosion

Have you noticed fine sand collecting on riverbeds or in lakes? This sand is formed when rock pebbles, soil, and sediments are broken down and moved from one location to another by natural forces like wind and flowing water. This process is called erosion.

Erosion during a landslide is an example of a physical change. River rocks and pebbles often appear smoother due to the constant erosion caused by the flowing water. When the speed of the water or wind decreases, such as in an ocean or a lake, the material transported during erosion settles down at the bottom. These sediments harden over time and become new rocks. Most of these changes take place over thousands of years and cannot be reversed.

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