Samacheer Class 6 Science - Changes Around Us

Blinking of an eye is a fast change because it occurs in a fraction of a second. Fast changes are those that happen very quickly in a short period of time, usually in seconds or minutes. In contrast, growth of a child is a slow change taking months or years, rusting is a slow chemical change taking days or weeks, and germination of a seed is a slow change taking several days. Blinking is an involuntary reflex action of our eyelids that protects our eyes and happens almost instantaneously.

Curdling of milk is a chemical change because new substances are formed during this process. When milk curdles, the milk proteins coagulate and separate into curds and whey due to the action of bacteria or acid, resulting in a completely different substance that cannot be easily reversed to its original form. In contrast, glowing of a bulb is a physical change involving light emission, lighting of a candle involves both physical melting of wax and chemical combustion, breaking of a coffee mug is a physical change where the shape changes but the material remains the same, and curdling of milk involves a chemical transformation of the milk's composition.

Trimming of hair is a man-made change because it is deliberately caused by human action using scissors or other tools. Man-made changes are those that occur due to human intervention and are intentionally brought about by people. In contrast, rotting of an egg is a natural change caused by decomposition, condensation of water vapour is a natural physical change caused by temperature decrease, and ripening of fruit is a natural biological change that occurs as part of the fruit's life cycle without human intervention.

Burning of kerosene is a chemical change because new substances are produced during combustion. When kerosene burns, it reacts with oxygen from the air to form carbon dioxide and water vapour, which are completely different substances from the original kerosene. This change is irreversible and cannot be undone. In comparison, inflating a balloon is a physical change where only the shape and size change, popping a balloon is also a physical change involving a change in shape, and fading of wall paint is a physical change where only the colour becomes lighter but the material remains the same.

Decaying of plants is associated with both slow change and chemical change. It is a slow change because the process of decay takes place over a long period of time, spanning days, weeks, or months as the plant material gradually breaks down. It is also a chemical change because during decay, the complex organic compounds in the plant tissues are broken down by microorganisms and bacteria into simpler substances like carbon dioxide, water, and other compounds. The original plant material is transformed into completely different substances, and this process cannot be reversed.

Yes, you can make a candle doll from candle wax by reshaping it into the desired form. This is a physical change because only the shape and form of the wax are altered, while the material itself remains candle wax. The chemical composition and properties of the wax do not change. Additionally, this change is reversible because the wax doll can be melted again and reshaped into any other form, including back into a candle. Physical changes involve only changes in appearance, shape, size, or state of matter without altering the fundamental nature of the substance.

A slow change is a change that takes place over a long period of time, such as hours, days, months, or even years. These changes occur gradually and are not immediately noticeable at any given moment, but become evident when observed over an extended duration. Examples of slow changes include the growth of a child, rusting of iron, decaying of plants, germination of seeds, and ripening of fruits. Slow changes are in contrast to fast changes, which occur very quickly in seconds or minutes.

Deforestation cannot be considered a desirable change because it is harmful to our environment and is not desired by us. Deforestation is classified as an undesirable change. When forests are cut down, it leads to loss of habitat for wildlife, reduces oxygen production, increases carbon dioxide in the atmosphere, causes soil erosion, disrupts the water cycle, and contributes to climate change. Deforestation also destroys the natural resources and biodiversity that forests provide. Therefore, it is a change that has negative consequences for both the environment and human society.

Germination of a seed is a slow change because it takes place over a long period of time, typically spanning several days or even weeks. During germination, the seed gradually develops into a young plant through a series of biological processes. The seed absorbs water, the embryo inside begins to grow, roots emerge and develop, and the shoot pushes upward through the soil. These processes happen gradually and are not immediately visible on a day-to-day basis, but become evident when observed over the course of days and weeks. Slow changes are those that occur over extended periods of time, in contrast to fast changes that happen in seconds or minutes.

When a candle is lit, several changes are observed. The wax melts due to the heat produced by the flame, converting the solid wax into a liquid state. The candle keeps burning as the melted wax is drawn up the wick by capillary action and reacts with oxygen in the air, producing heat, light, and gases. The size of the candle decreases as the wax is consumed during burning. The molten wax around the wick solidifies again as it cools, forming a pool of liquid wax that feeds the flame. These changes include both physical changes, such as the melting and solidification of wax, and chemical changes, such as the combustion of wax with oxygen to produce new substances like carbon dioxide and water vapour.

Changes which can be reversed to get back to the original state are known as reversible changes. These are physical changes where the substance can return to its previous form or state by altering conditions such as temperature, pressure, or other physical factors. For example, melting of ice into water is a reversible change because water can be frozen back into ice. Similarly, dissolving sugar in water is reversible because the water can be evaporated to get back the sugar crystals. Reversible changes do not create new substances; they only modify the physical properties or state of matter of the original material.

Water is known as the universal solvent because it has the ability to dissolve a wide range of substances. Water molecules are polar in nature, meaning they have positive and negative charges, which allows them to interact with and dissolve many different types of materials including salts, sugars, gases, and various minerals. This unique property makes water essential for many biological and chemical processes. In living organisms, water dissolves nutrients, gases, and waste products, enabling their transport throughout the body. In nature, water dissolves rocks and minerals, shaping landscapes over time. This remarkable dissolving ability of water is why it is called the universal solvent.

Physical changes alter the form or state of a substance without forming a new substance and are often reversible (for example melting or tearing), whereas chemical changes produce new substances with different properties and are usually not easily reversible and often involve energy change (for example burning or rusting).