Every day we see so many changes happening around us. If we look carefully, we can notice these changes everywhere in our daily life.
When we take ice cubes from the freezer, they slowly melt and become water. This shows us a change in state from solid to liquid. In our garden, we can see how a small bud slowly grows and turns into a beautiful flower – this is a change that shows growth. Sometimes when we take out a cold water bottle from the fridge, after some time it becomes warm. This is a change in temperature.
Even fruits change over time. Bananas start getting more brown spots and develop a stronger smell as they become ripe. All these changes affect different things like size, shape, smell, color, or other properties of objects.
We can notice these changes using our five senses – we can see changes with our eyes, smell changes with our nose, feel changes by touching, hear changes with our ears, and sometimes even taste changes. These senses help us understand what type of change is happening around us.
A Substance May Change in Appearance but Remain the Same!
Sometimes things look different but they are actually the same substance inside. This is very imp to understand.
When we fold a piece of paper, its shape changes completely. But if we unfold it, we get back the same paper. The paper didn’t become something new – it just changed its shape temporarily.
Similarly, when we blow air into a balloon, it becomes bigger and changes its size. But when we let the air out, the balloon goes back to its original size. The balloon material is still the same.
If we take a piece of chalk and crush it into powder, it looks very different. But the powder is still chalk – it’s just in smaller pieces now. We haven’t made any new substance.
Water is another good example. It can be ice (solid), liquid water, or steam (gas). Even though these three forms look and feel very different, they are all still water. No new substance is formed when water changes its state.
Properties of these changes:
- They change physical properties like shape, size, or state
- No new substance is formed
- The original substance remains the same
- These are called physical changes
A Substance May Change in Appearance and Not Remain the Same!
Some changes are different – they actually create new substances that were not there before.
When we blow air into normal tap water, we see some bubbles, but no new substance is formed. However, when we blow our breath into lime water, something interesting happens. The lime water becomes milky white. This milky appearance shows that a new substance has been formed.
The new substance formed in lime water is called calcium carbonate, which doesn’t dissolve in water. This happens because the carbon dioxide in our exhaled breath reacts with lime water. The reaction also produces a small amount of water.
The chemical reaction is: Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
This milky lime water is actually used as a test to check if carbon dioxide gas is present. Scientists use this test in laboratories.
Another example is mixing vinegar with baking soda. This creates lots of bubbles and produces carbon dioxide gas. When this gas is passed through lime water, it turns milky, proving that a chemical change has happened.
The reaction is: Vinegar + Baking soda → Carbon dioxide + Other substances
But if we just mix baking soda with plain water, no bubbles are formed. This shows it’s only a physical change.
Changes that form new substances are called chemical changes. These changes involve chemical reactions where the original substances react to form completely new substances.
Some Other Processes Involving Chemical Changes
Rusting
When iron objects are left outside in moist air, they develop a brown coating called rust. This rust is actually iron oxide, which is a completely new substance different from iron.
Rusting happens when iron reacts with oxygen and water in the air. Since a new substance (rust) is formed, rusting is a chemical change. This is why rusted iron becomes weak and can break easily.
Rusting causes lot of damage to iron objects like cars, bridges, and tools. To prevent this, we paint iron objects or use other protective coatings.
Combustion
Combustion means burning, and it’s one of the most common chemical changes we see. When something burns, it reacts with oxygen and produces heat and light.
When we burn a magnesium ribbon, it produces a bright white light and forms magnesium oxide – a new white powder. This shows that burning is a chemical change.
The reaction is: Magnesium + Oxygen → Magnesium oxide + Heat + Light
Things that can burn are called combustible substances. Examples include wood, paper, cotton, kerosene, and many other materials.
Oxygen is very imp for combustion. If we cover a burning candle with a glass, it stops burning because the oxygen inside gets used up.
When things burn, they often produce carbon dioxide gas. We can test this by passing the gas through lime water – it will turn milky.
Fire safety tip: If clothes catch fire, wrap the person in a blanket to cut off air supply. But never use synthetic blankets because they can melt and stick to skin.
Fireflies produce light through a special chemical change called bioluminescence. This process produces light without heat in living organisms.
Is the Presence of Air Enough for Combustion?
Just having air is not enough for burning. Combustion needs three things:
- Combustible substance (fuel)
- Oxygen (from air)
- Heat (to reach ignition temperature)
Paper is combustible, but it doesn’t burn just by being in air. We need to heat it with a matchstick or focused sunlight to make it burn.
Every substance has an ignition temperature – this is the minimum temperature it needs to catch fire. A matchstick produces enough heat to reach paper’s ignition temperature, so paper burns immediately.
These three requirements – fuel, oxygen, and heat – form what we call the fire triangle. All three must be present for combustion to happen.
Can Physical and Chemical Changes Occur in the Same Process?
Sometimes both types of changes happen together in the same process. A burning candle is a perfect example of this.
Physical changes in a burning candle:
- Wax melts due to heat from the flame
- Melted wax moves up through the wick
- Wax turns into vapor
Chemical changes in a burning candle:
- Wax vapor burns to produce flame
- New substances like carbon dioxide are formed
- Heat and light are produced
So in one candle, we see both physical changes (melting and evaporation) and chemical changes (burning) happening at the same time.
The famous scientist Michael Faraday studied candles to explain these physical and chemical processes. His lectures called “Chemical History of a Candle” explained how both types of changes work together.
Are Changes Permanent?
Some changes can be undone, while others cannot be reversed.
Reversible changes:
- Melting ice can be frozen back into ice
- Boiling water can be condensed back into liquid water
- These changes can go back to their original form
Irreversible changes:
- Chopping vegetables cannot restore their original shape
- Making popcorn from corn cannot turn back into corn
- The original substance or form is lost forever
Understanding whether a change is reversible or irreversible helps us predict what will happen to materials.
Are All Changes Desirable?
Not all changes are good for us. We need to understand which changes help us and which ones cause problems.
Desirable changes are useful and beneficial:
- Milk curdling into curd (helps in digestion)
- Fruits ripening (makes them sweet and nutritious)
- Cooking food (makes it safe and tasty)
Undesirable changes cause harm or problems:
- Rusting of iron (damages metal objects)
- Food spoiling during storage (causes waste and health problems)
Some changes can be both desirable and undesirable depending on the situation. Food decomposition is bad when we’re storing food, but it’s good when we want to make compost for plants.
Human activities sometimes cause undesirable changes. Burning fuels increases carbon dioxide in air, which affects our environment. Drying paint releases harmful substances that pollute the atmosphere.
Some Slow Natural Changes
Nature also causes changes, but they happen very slowly over long periods of time.
Weathering of Rocks
Weathering is the process that breaks big rocks into smaller pieces and eventually forms soil. This happens in two ways:
Physical weathering:
- Temperature changes make rocks expand and contract, causing cracks
- Tree roots grow into cracks and break rocks apart
- Water freezing in cracks expands and splits rocks
Chemical weathering:
- Water reacts with chemicals in rocks
- Iron in rocks reacts with air or water to form red iron oxide
- This changes the composition of rocks
Weathering involves both physical and chemical changes. It’s a very slow process that takes thousands of years. The small pieces (sediments) collect at the bottom of mountains.
Erosion
Erosion moves rock pieces, soil, and sediments from one place to another using wind or water.
Fine sand gets carried by rivers and settles in riverbeds or lakes. Landslides are examples of sudden erosion. River rocks become smoother because water constantly rubs against them.
When wind or water slows down, sediments settle down in oceans or lakes. Over thousands of years, these sediments become hard and form new rocks.
Erosion is mostly a physical change and is usually irreversible.
Questions and Answers
1. Which of the following statements are the characteristics of a physical change?
Let’s check each option:
- (i) The state may or may not change: True – physical changes can involve state changes like melting
- (ii) A substance with different properties is formed: False – no new substance forms in physical changes
- (iii) No new substance is formed: True – this is the main feature of physical changes
- (iv) The substance undergoes a chemical reaction: False – chemical reactions form new substances
Answer: (c) (i) and (iii)
2. Are the changes of water to ice and water to steam, physical or chemical? Explain.
Both water to ice (freezing) and water to steam (evaporation) are physical changes.
When water freezes, it changes from liquid to solid state, but it’s still water. When water evaporates, it changes from liquid to gas state, but it remains water.
No chemical reaction happens during these changes – only the physical state changes. The substance (water) stays the same.
Answer: Both are physical changes because they only change the state without forming new substances.
3. Is curdling of milk a physical or chemical change? Justify your statement.
Curdling of milk is a chemical change.
When milk curdles, bacteria or acids cause chemical reactions that form new substances like lactic acid and other compounds. The composition of milk changes completely to form curd.
This change is irreversible – we cannot turn curd back into milk. The properties of milk change permanently during curdling.
Answer: Curdling is a chemical change because it forms new substances through reactions and cannot be reversed.
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