The Amazing World Of Solutes, Solvents, And Solutions

Hello my dear students! Welcome to today's science lesson. I am so happy to be here with you to explore one of the most fascinating topics in science — the amazing world of solutes, solvents, and solutions. Are you ready to discover some really interesting things about the world around you? Let's begin!

So students, let's start by thinking about some questions. Have you ever wondered why when you add sugar to your tea, it just disappears? Or have you noticed what happens when you add too much sugar to your tea and it stops dissolving? Let me tell you something interesting. You must have taken Oral Rehydration Solution, which we call ORS, at some point in your life when you were sick. You might have noticed that every sip of your homemade ORS tastes the same, whether you drink a little or a lot. It doesn't taste salty in one sip and sweet in another. Why do you think that happens? This is because when you add sugar and salt to water, they form a mixture in which the components are evenly distributed throughout. This is what we call a solution.

Now let me ask you something. Can you predict whether this mixture is uniform or not? What happens when you mix chalk powder with water — does it form a uniform mixture? When salt and sugar are mixed with water, a uniform mixture is formed. But when chalk powder or sand or sawdust is mixed with water, the components are not evenly distributed. Such mixtures are known as non-uniform mixtures. So students, this is our first important learning — mixtures can be of two types: uniform mixtures and non-uniform mixtures.

Now let's move on to understand what exactly is a solute, what is a solvent, and what is a solution. A uniform mixture, such as that of salt or sugar and water, is called a solution. Whenever a solid is mixed with a liquid to form a solution, the solid component is called the solute, and the liquid component is called the solvent. The solute dissolves in the solvent to form a solution. We can write this as: Solute plus Solvent gives us Solution.

Now here's an interesting point. When a solution is formed by mixing two liquids, it is not always clear which substance is dissolving the other. In such cases, the substance present in smaller amount is called the solute, while the one in larger amount is called the solvent.

Let me give you a wonderful Indian example here. You all love gulab jamun, don't you? The chashni or sugar syrup of gulab jamun is made of a large amount of sugar, which is solid, dissolved in a small amount of water, which is liquid. But even though there is more sugar than water, the water is still considered as the solvent and sugar as the solute! This is because in this case, water is the component in which the other substance dissolves. So students, remember — the solvent is the substance that does the dissolving, and the solute is the substance that gets dissolved.

Now let's think about another question. How much solute can a fixed amount of solvent dissolve? Let me explain this with a simple activity that you can try at home or in your school laboratory.

Let's do Activity 9.1. Take a clean glass tumbler and fill it half with water. Add one spoon of salt into it and stir well till it dissolves completely. Now gradually add a spoonful of salt into the glass tumbler and stir. Observe how many spoons of salt you can add before it stops dissolving completely. Record your observations in a table.

Now, what do you think will happen? You might have observed that, initially, the salt completely dissolves in the water, forming a solution. After adding a few more spoons of salt, a stage comes when the added salt does not dissolve completely and the undissolved salt settles at the bottom. This indicates that the water can no longer dissolve any more salt because it has reached its limit.

So students, this is very important. The solution in which more solute can be dissolved at a given temperature is called an unsaturated solution. However, when the solute stops dissolving and begins to settle at the bottom, the solution is called a saturated solution at that particular temperature. So remember — unsaturated solution means you can still add more solute, while saturated solution means no more solute can be dissolved at that temperature.

Now, what do we mean by concentration? The amount of solute present in a fixed quantity of solution or solvent is termed as its concentration. Depending upon the amount of solute present in a fixed quantity of solution, it can be called a dilute solution, which has less amount of solute, or a concentrated solution, which has more amount of solute. Dilute and concentrated are relative terms. So, in our activity, the solution obtained by dissolving one spoon of salt is dilute as compared to that obtained by dissolving two or more spoons of salt.

Now here's a question for you to think about. Which solution is more concentrated — 2 spoons of salt in 100 mL of water or 4 spoons of salt in 50 mL of water? Let's think about this carefully. In the first case, we have 2 spoons in 100 mL, which means 0.02 spoons per mL. In the second case, we have 4 spoons in 50 mL, which means 0.08 spoons per mL. So the second solution is more concentrated. This is because concentration is about how much solute is present in a fixed quantity of solvent.

Now, what is solubility? We can say that the maximum amount of solute that dissolves in a fixed quantity of the solvent is called its solubility. So students, solubility tells us how much of a substance can dissolve in a given amount of solvent at a particular temperature.

Now, does temperature affect the solubility of a solute? Let us find out!

Let's do Activity 9.2, which is a demonstration activity. Take about 50 mL of water in a glass beaker and measure its temperature using a laboratory thermometer, say 20 degrees Celsius. Add a spoonful of baking soda, which is sodium hydrogen carbonate, to the water and stir until it dissolves. Continue adding small amounts of baking soda while stirring, till some solid baking soda is left undissolved at the bottom of the beaker.

Now, heat the contents to 50 degrees Celsius while stirring. What happens to the undissolved baking soda? You will observe that it has dissolved! Continue adding more baking soda while stirring at this temperature until some solid baking soda remains undissolved. Again, heat the contents further to 70 degrees Celsius while continuing to stir. What do you observe? The undissolved baking soda dissolves again!

So students, what can we infer from this experiment? Water at 70 degrees Celsius dissolves more baking soda than water at 50 degrees Celsius. The amount of baking soda dissolved in water at 20 degrees Celsius is even lesser. It has been found that for most of the substances, the solubility increases with an increase in temperature. We can also say that a saturated solution at a particular temperature behaves as an unsaturated solution if the temperature is increased. This is a very important concept, so let me repeat it — when you heat a solution, it can dissolve more solute, which means a saturated solution can become unsaturated when heated!

Now, let me tell you something about our scientific heritage. Water has primarily been used as a solvent for the preparation of medicinal formulations in Ayurveda, Siddha, and other traditional systems of medicine in India. Additionally, drug formulations have been prepared using hydro-alcoholic extracts of the herbs. The Indian systems of medicine have also referred to the use of oils, ghee, milk, and other substances as solvents for drug formulations, to help achieve the therapeutic benefits of the drug.

Now, let me introduce you to a great Indian scientist. Have you heard of Asima Chatterjee? She is renowned for her work in developing anti-epileptic and anti-malarial drugs. She used solvents and solutions extensively to extract and isolate important compounds from medicinal plants. She earned a Doctorate of Science, becoming the second Indian woman to do so after Janaki Ammal. She became the first woman to receive the Shanti Swarup Bhatnagar Award in the field of chemical science and was also honoured with the Padma Bhushan. So students, let her be an inspiration to you!

Now, let's talk about gases. Do gases also dissolve in water? Yes, they do! Many gases, including oxygen, dissolve in water. Oxygen dissolves in water only to a small extent. Even though present in minute quantities, it is this dissolved oxygen that sustains all aquatic life, including plants, fishes, and other organisms. So the next time you see fish in water, remember that they are breathing the oxygen that is dissolved in the water!

Is the mixture of gases in water a uniform or non-uniform mixture? It is a uniform mixture because the gases dissolve evenly in water.

Now, does temperature affect the solubility of gases in liquids also? If so, how? It has been observed that the solubility of gases generally decreases as temperature increases. More oxygen can dissolve in cold water, ensuring sufficient oxygen for aquatic life. On the other hand, when water warms up, the solubility of oxygen decreases. This is why cold water bodies have more aquatic life than warm water bodies. This is also why when you heat water, you can see tiny bubbles forming — these are the gases that were dissolved in the water escaping!

Now students, let's move on to another interesting topic. Why do objects float or sink in water? You must have observed that some objects float while others sink in water. You may have noticed that, while washing rice, husk particles present in the rice float on the surface of water while rice sinks to the bottom of the container. Why does this happen? If you add oil to water, it floats on water. Generally, it is believed that objects that float in a liquid are lighter and others that sink are heavier than the liquid.

A wooden stick and an iron rod may be of the same size, yet the iron rod feels much heavier. When we say that iron is heavier than wood, we are referring to a special property known as density, which describes the heaviness of an object. However, the density of a substance is not the only factor that decides whether it will float or sink in a particular liquid.

Now let's explore what density really means. Imagine a crowded bus where many people are packed together — this is an example of high density. Whereas, the same bus with only a few people is an example of low density. Similarly, a forest where trees grow close to each other is called a dense forest, but if the trees are far apart, it is considered less dense.

So students, how do scientists define density? We have learnt that matter is anything that possesses mass and occupies space, which we call volume. Density is defined as the mass present in a unit volume of that substance. We can express this mathematically using the formula:

Density equals Mass divided by Volume.

The density of a substance is independent of its shape or size. However, it is dependent on temperature and pressure. Pressure primarily affects the density of gases, while its effect on solids and liquids is negligible.

Now, what are the units of density? The units in which density is expressed will depend upon the units of mass and volume taken. As you have learnt, the SI units of mass and volume are kilogram, abbreviated as kg, and cubic metre, abbreviated as m³, respectively. Therefore, the SI unit of density is kilogram per cubic metre, abbreviated as kg/m³. In case of liquids, other units of density are also used for convenience, such as gram per millilitre, abbreviated as g/mL, and gram per cubic centimetre, abbreviated as g/cm³.

Let me give you a conversion factor for density. 1 kg/m³ equals 1000 g/m³ equals 1000 g/1000 L equals 1 g/L equals 1 g/1000 mL equals 1 g/1000 cm³.

The mass of 1 mL of water is close to 1 gram at room temperature. For the measurement of the mass of water, we generally consider the volume in mL and its mass in g. Hence, 10 mL of water would be approximately 10 g. Similarly, 100 mL of water would be approximately 100 g.

Now, suppose the mass of an aluminium block is 27 g and its volume is 10 cm³. Its density would be 2.7 g/cm³. From this, it can be said that aluminium is 2.7 times denser than water. We express this fact by saying that the relative density of aluminium with respect to water is 2.7. It is a number without any units. The relative density of any substance equals the density of that substance divided by the density of water at that temperature.

Now, here's a question for you. Have you noticed that some packets of ghee or oil are labelled with a volume of 1 litre but a weight of only say 910 grams? What does this tell us about the density of the oil? Is it less or more than that of water? Since 1 litre of oil weighs less than 1 kilogram, the density of oil is less than that of water, which is 1 g/mL or 1000 kg/m³. That's why oil floats on water!

Now let's learn how to determine the density of an object by measuring its mass and volume.

How to measure mass? You learnt the term mass in your previous classes. Mass is the quantity of matter present in any object. The instrument used to measure the mass of an object is known as a balance. You must have seen various types of balances being used by shopkeepers. Here, we are using a digital weighing balance to measure the mass.

Let me tell you how to measure mass through Activity 9.3. Switch ON the digital weighing balance. Observe the initial reading on the digital weighing balance display. It should show a zero reading. If not, then we must bring it to zero by pressing the tare or reset button. Place a dry and clean watch glass or butter paper on the pan. Note the reading on the digital weighing balance. Reset the digital weighing balance reading to zero by pressing the tare or reset button. Now, carefully place the solid object, such as a stone, on the watch glass. Note the reading displayed on the balance, which gives the mass of the stone, say 16.400 g.

The mass of a liquid may be measured by replacing the watch glass with a beaker and pouring the desired amount of liquid into it.

Now, here's an important note. As mentioned in your previous chapters, the words mass and weight are often used interchangeably in everyday language. But they have different meanings in science, which can sometimes cause confusion. Mass is the quantity of matter present in an object or a substance. Its units are gram and kilogram. On the other hand, weight is the force by which the Earth attracts an object or a substance towards itself, and it is measured in newtons. Most balances actually measure weight, but their scales are marked in mass units, so they show values in grams or kilograms.

Now let's learn how to measure volume. A tetra pack says it contains 200 mL buttermilk, which we call chach. What does that mean? You learnt that volume is the space occupied by an object. You also know that the SI unit of volume is cubic metres, written as m³. It is the volume of a cube whose each side is one metre in length. Volume of smaller objects is conveniently expressed in a decimetre cube, which is dm³, or centimetre cube, which is cm³. One centimetre cube is also written as one cc. Volume of liquids is expressed in litres, which is equivalent to 1 dm³. A commonly used submultiple of a litre is millilitre, which is mL, and is equivalent to 1 cm³.

One of the common apparatuses used to measure the volume of liquids is a measuring cylinder. It is a narrow transparent cylindrical container with one side open and the other side closed. There are markings on the transparent body of the cylinder that indicate the volume of liquid in the measuring cylinder. We can use it to measure the desired amount of a liquid.

Measuring cylinders are available in different sizes to measure volume — 5 mL, 10 mL, 25 mL, 50 mL, 100 mL, 250 mL, and so on. How accurately can these measuring cylinders measure? Let's find out!

In Activity 9.4, we need to observe and calculate. Take a measuring cylinder and observe it carefully. Note down the following: What is the maximum volume it can measure? Now look at the measuring cylinder carefully. The cylinder is marked as 100 mL; therefore, it can measure volume up to 100 mL. What is the smallest volume it can measure? Look at the measuring cylinder again. How much is the volume difference indicated between the two bigger marks, for example between 10 mL and 20 mL? How many smaller divisions are there between the two bigger marks? How much volume does one small division indicate?

For the measuring cylinder shown in your textbook, the volume difference indicated between 10 mL and 20 mL, or between 40 mL and 50 mL, is 10 mL. The number of divisions between these marks is 10. So, one small division can read 10 divided by 10, which equals 1 mL. That is, the smallest value that this measuring cylinder can read is 1 mL.

The smallest volume that a measuring cylinder can measure depends on the capacity of the measuring cylinder. Usually it is 0.1 mL in smaller measuring cylinders with a capacity of 10 mL or 25 mL, it is 1 mL in a 100 mL measuring cylinder, 2 mL in a 250 mL measuring cylinder, and 5 mL in a 500 mL measuring cylinder.

Suppose we want to take 70 mL of water. If we use a 50 mL measuring cylinder, it would not be possible to measure 70 mL of water in one step. First, we have to measure 50 mL water and then 20 mL. Measuring volume in more than one step is not convenient. On the other hand, if a 250 mL or 500 mL measuring cylinder is used, the measurement can be done in one step but the accuracy would be reduced as the smallest volume that these measuring cylinders can measure is greater than that of a 100 mL measuring cylinder. Hence, a 100 mL measuring cylinder is the best choice for this measurement.

Now let's do Activity 9.5 to measure 50 mL of water. Place a clean, dry measuring cylinder on a flat surface. Pour water slowly into the measuring cylinder up to the required mark. If required, adjust the level of water in the measuring cylinder by adding or removing a small amount of water using a dropper. On careful observation, you will notice that the water inside the measuring cylinder forms a curved surface. This curved surface is called the meniscus. Read the mark on the measuring cylinder that coincides with the bottom of the meniscus for water or other colourless liquids. Make sure that the eyes are at level with the bottom of the meniscus while noting the readings. Once it reaches the required level — that is, 50 mL — transfer this water to the required container.

In case of coloured liquids, the mark on the measuring cylinder should coincide with the top of the meniscus!

Now let's learn about determining the volume of solid objects with regular shapes. In Activity 9.6, we can calculate. Collect various objects with cuboid shapes, such as a notebook, a shoe box, or a dice. Measure the length, width, and height of the objects using a scale. Suppose the length of the notebook is 25 cm, the width is 18 cm, and the height is 2 cm. Calculate the volume by using the following formula: Volume equals length times width times height. So volume equals 25 cm times 18 cm times 2 cm, which equals 900 cm³. Record this in your notebook.

Now, what about objects with irregular shapes? Imagine you have an object, like a stone, that does not have a regular shape. To calculate its density, the main challenge is to find its volume. Let us learn how the volume of a solid with an irregular shape can be determined.

In Activity 9.7, let us measure. Collect various objects from your surroundings, such as a stone, metal keys, and so on. Fill a measuring cylinder with water up to any desired volume, say 50 mL, and record the initial volume taken in a table. Tie the object, say a stone, with a thread and slowly lower it into the measuring cylinder. What do you notice? The water level rises! Record the final volume after the level rises, say 55 mL. Subtract the initial volume from the final volume after the object is put into the measuring cylinder. This is the volume of the object. Record your observations in the table.

The values of volume are obtained in units of mL, which can be written in the equivalent unit cm³ for solids. Since 1 mL equals 1 cm³, this is easy to convert.

Now, let's calculate density. Density can be calculated using the following formula: Density equals Mass divided by Volume. For example, if the mass of a stone is 16.400 g and its volume is 5 cm³, then density equals 16.400 g divided by 5 cm³, which equals 3.28 g/cm³.

Now, let me tell you something fascinating about our planet Earth. Did you know that our planet Earth is composed of several layers, such as crust, upper mantle, lower mantle, outer core, and inner core, each with its particular range of density? The outermost layer, called the crust, is the lightest, and the density of the different layers increases as we move towards the centre. As one moves deeper into the Earth, both the pressure and the temperature rise significantly, making the materials heavier and more compact.

Now, here's something interesting from our heritage. In ancient times, before large ships were invented, people used bamboo and wooden logs to travel across rivers and seas. Bamboo was used because it is light, hollow, and floats easily on water. People tied bamboo poles together to make rafts and small boats for fishing, trading, and crossing water bodies. Wooden logs, especially from strong trees, were either hollowed out to make boats or used as rafts. These simple boats, made from locally available materials, were important for moving around and connecting different places. Even today, similar traditional boats made of bamboo or wood are used in some regions — not just for transport, but also as tourist attractions.

Now let's discuss the effect of temperature on density. Generally, the density of a substance decreases with heating and increases with cooling. This can be explained on the basis of what you have learnt in the chapter on particulate nature of matter. As temperature increases, the particles of a substance, whether solid, liquid, or gas, tend to move away and spread. This results in an increase in volume, but there is no change in mass. Since density equals mass divided by volume, upon heating, the volume increases and the density decreases. This explains why hot air moves up as it is less dense than the cool air around it. The hot air balloon works on the same principle!

Now, what about the effect of pressure on density? Pressure affects density differently depending on the state of matter. For gases, increasing pressure causes the particles to move closer together. As a result, the volume of the gas decreases and its density increases. In the case of liquids, pressure has a small effect because they are nearly incompressible. The particles in solids are very close to each other. So, how is the density of solids affected when pressure is applied? Solids are even less affected by pressure than liquids, and changes in their density are usually negligible.

Now, here's something really interesting that you might have wondered about. Why does ice float on water? Ice floats on water because it is lighter than liquid water. Water has a special property that its density is highest at 4 degrees Celsius. As the temperature drops and water turns into ice at 0 degrees Celsius, it undergoes a change in structure — the particles arrange themselves in a way that takes up more space. This process is called expansion. Because the same amount of water now occupies a larger volume, its density decreases. As a result, ice becomes lighter than liquid water and floats on its surface.

This is very important for animals living in lakes and oceans. Because ice floats, it forms a layer on top, keeping the water underneath warm enough for fish and other creatures to survive, even in extremely cold weather. Isn't that wonderful?

Now, here's a thinking exercise for you. Take a glass tumbler and fill it with tap water. Carefully place a raw whole egg into the water and observe what happens. You will notice that the egg sinks to the bottom. What change can you make to this setup to make the egg float in water instead of sinking? In this chapter, you have learnt the concept of density and how it explains partially why some objects float while others sink. Think about it — if you add salt to water, what happens to its density? Can you make the egg float by making the water denser? This is something you can try at home!

Now students, let's go through the key points we have learned in this chapter. Let me summarize everything for you.

A solution is said to be formed when two or more substances mix to form a uniform mixture. In the solution formed by dissolving a solid in a liquid, the solid component is known as a solute and the liquid component is known as a solvent. In a solution formed by mixing two liquids, the component present in less quantity is known as solute and the other component is called solvent.

A solution in which the maximum amount of solute has been dissolved, and no more of it can be dissolved at that temperature is called a saturated solution. A solution in which more solute can be dissolved at a given temperature is called an unsaturated solution.

Solubility is the maximum amount of solute that can be dissolved in a fixed quantity, which is 100 mL, of a solution or a solvent at a particular temperature.

Generally, in liquids, the solubility of solids increases and that of gases decreases with an increase in temperature.

The amount of matter present in an object is known as its mass. The space occupied by an object or a substance is known as its volume. Devices used to measure mass and volume are a weighing balance and a measuring cylinder, respectively.

The mass per unit volume of a substance is known as its density, which is calculated as Density equals Mass divided by Volume. Generally, density decreases with an increase in temperature, and pressure affects density differently depending on the state of matter.

Now let's solve the exercises together. Let's start with the first question in "Keep the curiosity alive".

Question 1: State whether the statements given below are True or False. Correct the false statements.

(i) Oxygen gas is more soluble in hot water rather than in cold water. This is FALSE. Actually, oxygen gas is more soluble in cold water than in hot water. This is because the solubility of gases decreases with increase in temperature.

(ii) A mixture of sand and water is a solution. This is FALSE. A mixture of sand and water is a non-uniform mixture, not a solution, because sand does not dissolve in water.

(iii) The amount of space occupied by any object is called its mass. This is FALSE. The amount of space occupied by any object is called its volume, not mass. Mass is the quantity of matter present in an object.

(iv) An unsaturated solution has more solute dissolved than a saturated solution. This is FALSE. An unsaturated solution has less solute dissolved than a saturated solution. In a saturated solution, the maximum amount of solute is dissolved at that temperature.

(v) The presence of different gases in the atmosphere is also a uniform mixture. This is TRUE. The atmosphere is a uniform mixture of various gases like nitrogen, oxygen, carbon dioxide, and others.

Now let's look at Question 2: Fill in the blanks.

(i) The volume of a solid can be measured by the method of displacement, where the solid is immersed in water and the rise in water level is measured.

(ii) The maximum amount of solute dissolved in solvent at a particular temperature is called solubility at that temperature.

(iii) Generally, the density decreases with increase in temperature.

(iv) The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a saturated solution of glucose.

Now Question 3: You pour oil into a glass containing some water. The oil floats on top. What does this tell you?

(i) Oil is denser than water (ii) Water is denser than oil (iii) Oil and water have the same density (iv) Oil dissolves in water

The correct answer is (ii) Water is denser than oil. Since oil floats on water, it means oil is less dense than water.

Now Question 4: A stone sculpture weighs 225 g and has a volume of 90 cm³. Calculate its density and predict whether it will float or sink in water.

Let's calculate. Density equals mass divided by volume. So density equals 225 g divided by 90 cm³, which equals 2.5 g/cm³. Now, the density of water is 1 g/cm³. Since the density of the stone, which is 2.5 g/cm³, is greater than the density of water, the stone will sink in water.

Now Question 5: Which one of the following is the most appropriate statement, and why are the other statements not appropriate?

(i) A saturated solution can still dissolve more solute at a given temperature. (ii) An unsaturated solution has dissolved the maximum amount of solute possible at a given temperature. (iii) No more solute can be dissolved into the saturated solution at that temperature. (iv) A saturated solution forms only at high temperatures.

The most appropriate statement is (iii) No more solute can be dissolved into the saturated solution at that temperature. This is the correct definition of a saturated solution.

Statement (i) is incorrect because a saturated solution cannot dissolve more solute at a given temperature. Statement (ii) is incorrect because an unsaturated solution has NOT dissolved the maximum amount of solute possible — that's what makes it unsaturated. Statement (iv) is incorrect because a saturated solution can form at any temperature, not just at high temperatures.

Now Question 6: You have a bottle with a volume of 2 litres. You pour 500 mL of water into it. How much more water can the bottle hold?

First, we need to convert both to the same unit. 2 litres equals 2000 mL. You have already poured 500 mL. So the bottle can hold 2000 mL minus 500 mL, which equals 1500 mL, or 1.5 litres more.

Now Question 7: An object has a mass of 400 g and a volume of 40 cm³. What is its density?

Density equals mass divided by volume. So density equals 400 g divided by 40 cm³, which equals 10 g/cm³.

Now Question 8: Analyse the figures. Why does the unpeeled orange float, while the peeled one sinks? Explain.

This is a great question! An unpeeled orange has air trapped in its peel, which makes it less dense overall. The peel contains air pockets, which reduce the average density of the whole orange. When you peel the orange, you remove this air-containing peel, making the orange denser. Since the density of the peeled orange becomes greater than that of water, it sinks. The unpeeled orange floats because its overall density is less than that of water due to the air trapped in the peel.

Now Question 9: Object A has a mass of 200 g and a volume of 40 cm³. Object B has a mass of 240 g and a volume of 60 cm³. Which object is denser?

Let's calculate the density of both objects. Density of object A equals 200 g divided by 40 cm³, which equals 5 g/cm³. Density of object B equals 240 g divided by 60 cm³, which equals 4 g/cm³. Since 5 g/cm³ is greater than 4 g/cm³, object A is denser than object B.

Now Question 10: Reema has a piece of modeling clay that weighs 120 g. She first moulds it into a compact cube that has a volume of 60 cm³. Later, she flattens it into a thin sheet. Predict what will happen to its density.

This is a very interesting question! When Reema moulds the clay into a compact cube, its volume is 60 cm³ and mass is 120 g, so density equals 120 g divided by 60 cm³, which equals 2 g/cm³. When she flattens it into a thin sheet, the mass remains the same at 120 g, but the volume changes. However, here's the key point — the density of a substance does not change with its shape or size! Whether you mould it into a cube or flatten it into a sheet, the material itself remains the same, so its density remains 2 g/cm³. The density is a property of the material, not its shape. So students, remember — the density of a substance is independent of its shape or size!

Now Question 11: A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?

We know that density equals mass divided by volume. So volume equals mass divided by density. Volume equals 600 g divided by 7.9 g/cm³, which equals approximately 75.95 cm³.

Now Question 12: You are provided with an experimental setup. On keeping the test tube in a beaker containing hot water, around 70 degrees Celsius, the water level in the glass tube rises. How does it affect the density?

When the test tube is placed in hot water, the air inside the test tube gets heated. As we learned, when gases are heated, they expand and become less dense. The heated air inside the test tube becomes less dense, which causes it to rise up the glass tube, pushing the water up. So the density of the air inside the test tube decreases when heated.

Now students, let me tell you about some interesting research projects and activities you can do.

Research project on Dead Sea: Why is there no aquatic life in the Dead Sea? Try to find out if there are any other similar water bodies. The Dead Sea is extremely salty — much saltier than regular seawater. This high salinity means that the water is very dense, and most organisms cannot survive in such an environment. The high salt content also means that the water is very buoyant, which is why people float so easily in the Dead Sea!

Investigate how well common salt dissolves in different solvents, such as water, vinegar, and oil. Compare the solubility of salt in each solvent and record your observations. You will find that salt dissolves very well in water, somewhat in vinegar, but hardly at all in oil. This is because water is a polar solvent, while oil is non-polar, and salt is ionic.

Now, debate in class — Is water truly the most versatile solvent? Think about all the things that dissolve in water and all the things that don't. Is there any other solvent that can dissolve as many different substances?

Now let me tell you something beautiful about our heritage. In Ningel village in Manipur's Thoubal district, salt is still produced using traditional methods. The village has a few salt wells, one of which is lined with a 100-year-old tree trunk placed into the ground to draw up salty water. A few families, mostly women, continue this sacred practice by collecting the salt solution and boiling it in large metal pans over firewood kilns. Once the water evaporates and salt crystals form, they are shaped into round salt cakes using banana leaves and handmade tools. These cakes are then wrapped in a traditional cloth to protect them. The salt cakes are believed to have some medicinal value too. Salt in Ningel is more than just food — it is history, culture, belief, and a beautiful example of India's living heritage.

Students, we have covered the entire chapter thoroughly. Let me give you a complete summary of everything we have learned today.

We started by understanding what mixtures are — uniform and non-uniform mixtures. We learned that a uniform mixture is called a solution. We learned about solute, which is the substance that gets dissolved, and solvent, which is the substance that does the dissolving. We understood the difference between unsaturated and saturated solutions. We learned about concentration and solubility, and how temperature affects solubility — solubility of solids increases with temperature, while solubility of gases decreases with temperature.

We then moved on to understanding floatation and sinking, and learned about the concept of density. We learned that density is mass per unit volume, and its formula is density equals mass divided by volume. We learned about the units of density, including kg/m³, g/cm³, and g/mL. We learned about relative density and how to calculate it.

We learned how to measure mass using a balance and how to measure volume using a measuring cylinder. We learned about the meniscus and how to read it correctly. We learned how to calculate the volume of regular-shaped objects using length, width, and height, and how to calculate the volume of irregular-shaped objects using the displacement method.

We learned about the effect of temperature on density — density generally decreases with increase in temperature. We learned about the effect of pressure on density, which affects gases more than liquids and solids. We learned why ice floats on water, which is because water has the special property of being most dense at 4 degrees Celsius, and ice is less dense than liquid water.

We solved many numerical problems related to density, volume, and mass. We learned that the density of a substance is independent of its shape or size. We learned about the Dead Sea and why it has no aquatic life. We learned about traditional salt-making in Manipur.

This has been a comprehensive lesson, and you should now be able to answer any question from this chapter. Remember, science is all around us, and every day you experience solutions, solubility, density, and floating and sinking in your daily life. Keep observing, keep questioning, and keep learning!

Thank you so much for your attention, students. I hope you enjoyed this lesson as much as I enjoyed teaching it. Until next time, keep exploring the amazing world of science!