Chapter-6 Physical and Chemical Changes

Explanation:

A protective zinc layer is deposited on Iron or zinc to protect them from rusting. This process of applying a zinc layer is called galvanisation. One of the methods to carry this out is hot dip galvanising, in which a section of steel is dipped into a bath of molten zinc.

Explanation:

The process in which there is a formation of iron oxides which is called rust, over the surface of iron articles or objects is known as rusting. In an environment containing moisture, the redox reaction between Iron and oxygen is the leading cause behind the formation of rust. 

Explanation:

The process of forming crystals, the organised structure made from atoms and molecules, is termed crystallisation. In crystallisation, no new substance gets formed; hence, it is an example of physical change. Solid-liquid separation techniques are included in this process. 

Explanation:

The process in which a layer of iron oxide is formed on the surface of Iron is called rusting. In this process, a reddish-brown substance termed rust is formed over the surface of Iron. So, it means a new sense gets created, and Iron's chemical composition gets changed; hence rusting of Iron is included in the chemical change.

Explanation:

Properties of stainless steel include high tensile strength. Staining stainless steel doesn't take place, so it is rust-free. It is made for increased durability and is low maintenance. Stainless steel is also environmentally friendly and temperature resistant.

Explanation:

Manure forms when the wastes of plants and animals are dumped in an open pit. This is a chemical change as its chemical composition gets changed, and also, a new substance that is manure gets formed from the old substance that is waste of animals and plants. 

Explanation:

In a physical change, only physical properties like size, shape etc., get changed, and no new substance is formed. This change can be reversed back also. While in chemical transformations, the chemical composition of the meaning gets altered, and an unknown substance gets formed. A chemical change is permanent because it cannot be reversed back to its original form.

Explanation:

Chemical change is a type of change in which the chemical composition of the substance changes and one or more new substances are formed. In this type of change, the process cannot be reversed, unlike physical change, which means we cannot return to the original form. The Aging of human beings is an example of chemical change.

Explanation:

The presence of moisture that is water, and the company of air is oxygen are the two most important conditions required for rusting Iron. When an iron article is in the mood for a long time, corrosion occurs as it forms iron oxides. 

Explanation:

Stainless steel has anti-corrosion properties meaning it doesn't rust and is made for high durability and temperature resistance. It is formed by mixing carbon and metals like nickel and chromium with Iron. Hence, it is made stainless to fulfil all the above-written purposes. 

Explanation:

Souring of leaves is a chemical change because of the milk change's chemical composition or chemical properties. The milk, which had its original substance lost its specifications and changed into a new chemical substance. This newly formed substance cannot be reversed back to its original form and is termed chemical change. 

Explanation:

When a metal surface is kept accessible in air or water, a chemical reaction causes the metal to break down and form rust. Rust is a reddish-brown substance that loosens the metal and makes it weak and can eventually cause it to disintegrate.

Explanation:

Copper sulphate pentahydrate is known as blue vitriol as its crystals are blue. Blue vitriol is produced when copper metal or copper oxide reacts with sulphuric acid, typically found in blue crystals. It is soluble in water, and its solution is acidic.

Explanation:

Galvanisation and Painting are the two methods to fend off rusting. In the galvanisation process, a zinc layer is deposited on the surface of the Iron, which stops Iron from forming its oxides. While in painting, the direct interaction of Iron with moisture or air is prevented. 

Explanation:

A chemical reaction releases carbon dioxide (CO2) gas when baking soda is treated with vinegar. The following equation represents the above statement: 

NaHCO3 + CH3COOH → NaCH3COO + H2O + CO2

The reaction between baking soda and vinegar is an example of an acid-base response. 

Explanation:

Crystallisation is the best and the most common technique for obtaining sugar from a sugar solution. In this process, the sugar solution is heated to evaporate to form a saturated solution. This saturated solution contains more sugar than can be dissolved in the liquid at that temperature. Then, the sugar crystals get formed by allowing the sugar solution to cool down. 

Explanation:

Iron is a reactive metal with which chemical reactions can occur with oxygen and moisture to form rust (iron oxide). Once the rust has started on the iron surface, it can continue to grow and spread, causing the metal to become weaker and eventually deteriorate. This is why it is essential to dry iron pans and other iron objects thoroughly after they have been washed or come into contact with water.

Explanation:

Rusting occurs faster during the rainy season because it is a time when the humidity levels are higher, and because of that there is a high level of moisture in the environment subsequently. When metal is exposed to water or steam, a chemical reaction with oxygen takes place in the air to form rust. This reaction is accelerated in high humidity because the moisture in the air supports the transfer of oxygen to the metal surface, allowing rust to develop more quickly.

Explanation:

Physical properties are attributes of a substance that can be seen or measured without changing the identity of the essence. They are properties that can be determined without any chemical reactions or changes occurring to the substance. They can be reversed back to their original form, and no new meaning is formed. 

Colour, shape, density etc., are examples of physical properties.

Explanation:

A physical change is a change in a substance's physical state, appearance, or form without changing its chemical composition. In other words, the substance remains the same before and after the physical change occurs. There is no formation of any new substance after the reaction, and this change can be reversed back to its previous form. 

The stretching of a rubber band is an example of physical change.

Explanation:

Chemical change is when one or more substances are transformed into new substances with different chemical properties and compositions. A chemical change is also called a chemical reaction. Unlike physical changes, chemical changes result in the formation of new substances which are chemically different from the previous substances. It is an irreversible reaction. The growth of a plant is included in a chemical reaction or change.

Explanation:

When an apple is sliced, its cells are damaged, and enzymes within the cells are exposed to air. A natural chemical reaction occurs when fruit or vegetable enzymes react with oxygen in the air. The reaction is supported by exposure to air and higher temperatures, so an apple slice not consumed immediately will turn brown over time.

Explanation:

Rusting occurs when Iron or steel is exposed to air and moisture over time. The formation of rust weakens the Iron or steel and can cause it to become brittle and break down over time. The chemical reaction that takes place during rusting- 

4 Fe + 3 O2 + 6 H2O = 4 Fe(OH)3

Prevention of rusting can be achieved through the use of protective coatings or paints.

Explanation:

Tearing a paper into pieces is a physical change because it does not change the chemical composition of the paper. The paper is still made up of the same materials before and after being torn. During the tearing process, the paper is physically broken into smaller pieces, but the cellulose fibres that make up the paper remain unchanged. The tearing action may break some of the fibres, but they are still cellulose and have the same chemical properties.

Explanation:

The formation of magnesium hydroxide (Mg(OH)2) forms when magnesium oxide reacts with water. Heat is evolved during this reaction which makes it an exothermic reaction. 

MgO + H2O = Mg(OH)2

In this reaction, the magnesium oxide reacts with water to produce magnesium hydroxide. Magnesium hydroxide is a white, insoluble solid that precipitates out of the solution.

Explanation:

During the spoiling process, the food changes the chemical composition of its taste, texture, colour, and smell due to the formation of new compounds such as acids, alcohol, and gasses. These changes are a result of the chemical breakdown of the food components. Unlike physical change, it cannot be reversed back to its original ingredients, which is why it is termed a chemical change.

Explanation:

The curd's chemical composition changes, meaning it involves the conversion of lactose into lactic acid, and heat generation during the reaction also takes place. Also, there is a change in texture, taste, and smell, all resulting from a complex series of biochemical reactions that transform the milk proteins into curd. Moreover, it cannot be reversed back to its original form, milk, so it is a chemical change. 

Explanation:

When the firework is lit, it ignites a reaction that produces a large amount of heat and gas. It is an exothermic reaction. The heat causes the solid chemicals in the firework to vaporise and form a gas, which rapidly expands and causes the firework to explode. During this process, the chemicals in the firework undergo a chemical reaction, which results in the formation of new compounds. Fireworks cannot be reversed back to their original form because it is a chemical change.

Explanation:

 It does not include a change in the chemical composition of the blade. Instead, it is a change in the physical state of the metal that makes up the blade, like the colour change, because the blade's surface forms a thin oxide layer, which can be reversed by cooling or cleaning the blade. Therefore, the change in colour on heating is considered a physical change rather than a chemical change.

Explanation:

Paint only provides a temporary solution against rusting but does not prevent the issue. With time, the paint can wear off or become damaged; this allows moisture and oxygen to come into contact with the metal surface of the ship. When this happens, the process of rusting can begin.

Explanation:

Painting creates a physical barrier that prevents moisture and oxygen from contacting the iron surface. When moisture and oxygen come into contact with Iron, a chemical reaction occurs that forms iron oxide or rust. Painting is a temporary stoppage of rusting as it doesn't allow direct contact of air or moisture with the iron surface. 

Explanation:

Ice melting is a physical change because it does not involve any change in the substance's chemical composition. Ice is simply the solid form of water, and when it melts, it turns into liquid water. The melting process is a physical change because the water molecules do not change chemically. Also, it can get reversed back to its previous form by freezing the water, which is an example of physical change.

Explanation:

A reaction forms a white precipitate of calcium carbonate when carbon dioxide is passed through lime water. The presence of carbon dioxide in a sample can be checked by using this reaction. The following chemical equation can represent the reaction between carbon dioxide and lime water -

CO2 + Ca(OH)2 = CaCO3 + H2O

CaCO3 is calcium carbonate. It is insoluble in water and appears as a cloudy white solid. 

Explanation:

Coastal areas have higher air moisture and salt content levels. This moisture, in combination with salt in the air from the sea, facilitates the flow of electrons between the Iron and oxygen, speeding up the rusting process. While in a desert area, the moisture content is less and hence the rusting rate is slow.

Explanation:

When a rubber band is stretched, only its physical properties or physical attributes, that is, size, change and come back to their original form when we stop stretching it. There is no change in the chemical properties of the rubber; it is still made with the same material. This change is reversible, and no new substance gets formed, which is why it is termed a physical change.

Explanation:

Ozone is a molecule containing three atoms of oxygen that is O3. It acts as a natural shield against ultraviolet radiation (UV) because it absorbs UV radiation in the stratosphere, preventing most of it from reaching the Earth's surface. Skin damage, eye damage, and an increased risk of skin cancer are some effects of UV rays on humans. Ozone absorbs UV radiation in the stratosphere, acting as a protective barrier between the harmful radiation and the Earth's surface, which helps in protecting human life.

Explanation:

When magnesium ribbon is burnt, magnesium gets oxygen and reacts with it to form Magnesium Oxide. It is a bright, white light and a white powder. It releases significant heat and light energy and can ignite nearby materials; hence, this reaction is highly exothermic. The chemical equation for the reaction is:

2Mg(s) + O2(g) = 2MgO(s)

Explanation:

The reaction between baking soda and lemon juice is an acid-base reaction. Baking soda is a base, and lemon juice is an acid. When the two are mixed, they react to form carbon dioxide gas, water, and salt. Bubbles formed are a sign of the release of carbon dioxide gas in the reaction. The chemical equation for the reaction is:

NaHCO3 (s) + H+ (aq) = Na+ (aq) + CO2 (g) + H2O (l)

Explanation:

Chemical changes are responsible for many of the processes and phenomena we encounter regularly and are essential in our daily lives. These processes or phenomenons include- 

Digestion: Chemical changes occur in our bodies during digestion as food is broken down into nutrients that cannot be reversed back to its original form.

Cooking:  During the process of cooking, chemical changes take place. Heat causes chemical reactions to occur between the ingredients, resulting in the transformation of raw food into a variety of delicious dishes.

Materials science: Chemical reactions create new materials with improved properties.

Explanation:

Besides forming new products, several other changes may accompany a chemical change. These changes may include the evolution of gas that is in some chemical reactions, gas is produced as a product. Temperature change also can accompany a chemical reaction as they may either absorb or release heat energy, resulting in a temperature change.