NCERT Exemplar Problems class 9 Science in PDF form to download. These exemplar contains good quality of questions to improve your thinking skills about surroundings and science. To prepare quality question papers, teachers used to include some of the questions from these books to test the mental ability of the students. Even in competitive exams, the difficulty level of the questions are close to exemplar books. Students must go through these books side by side doing NCERT syllabus.
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Chapter 1: Matter in Our Surroundings
- Physical Nature of Matter
- Matter is made up of particles
- How small are these particles of matters?
- Characteristics of Particles of Matter
- Particles of matter have space between them
- Particles of matter are continuously moving, that is, they possess what we call the kinetic energy. As the temperature rises, particles move faster. So, we can say that with increase in temperature the kinetic energy of the particles also increases.
- Particles of matter have force acting between them. This force keeps the particles together. The strength of this force of attraction varies from one kind of matter to another.
- States of Matter
- Matter around us exists in three different states– solid, liquid and gas. These states of matter arise due to the variation in the characteristics of the particles of matter.
- Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid.
- Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but can be called fluid.
- Gases are highly compressible as compared to solids and liquids. The liquefied petroleum gas (LPG) cylinder that we get in our home for cooking or the oxygen supplied to hospitals in cylinders is compressed gas. Compressed natural gas (CNG) is used as fuel these days in vehicles. Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily. Gases have high speed of particles and large space between them, gases show the property of diffusing very fast into other gases.
- Can Matter Change its State?
- Applying pressure and reducing temperature can liquefy gases. Solid carbon dioxide (CO2) is on of the example. It is stored under high pressure. Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice.
- Evaporation
- At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic energy. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.
- The rate of evaporation increases with– • an increase of surface area: We know that evaporation is a surface phenomenon. If the surface area is increased, the rate of evaporation increases. For example, while putting clothes for drying up we spread them out. • an increase of temperature: With the increase of temperature, more number of particles get enough kinetic energy to go into the vapour state. • a decrease in humidity: Humidity is the amount of water vapour present in air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water in air is already high, the rate of evaporation decreases. • an increase in wind speed: It is a common observation that clothes dry faster on a windy day. With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding.
- Why should we wear cotton clothes in summer?
- During summer, we perspire more because of the mechanism of our body which keeps us cool. During evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.
- Why do we see water droplets on the outer surface of a glass containing ice-cold water?
- The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, in the form of water droplets.
Chapter 2: Is Matter Around Us Pure
- What is a Mixture?
- Mixtures are constituted by more than one kind of pure form of matter, known as a substance. A substance cannot be separated into other kinds of matter by any physical process.
- Types of mixtures
- Depending upon the nature of the components that form a mixture, One can have different types of mixtures.
- What is a Solution?
- A solution is a homogeneous mixture of two or more substances. A solution has a solvent and a solute as its components. The component of the solution that dissolves the other component in it (usually the component present in larger amount) is called the solvent. The component of the solution that is dissolved in the solvent (usually present in lesser quantity) is called the solute.
- What are the properties of a solution?
- A solution is a homogeneous mixture.
- The particles of a solution are smaller than 1 nm (10^9 metre) in diameter. So, they cannot be seen by naked eyes.
- Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
- The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.
- What do you mean by saturated and unsaturated solutions?
- At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving, is said to be a saturated solution. In other words, when no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution. The amount of the solute present in the saturated solution at this temperature is called its solubility.
- If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.
- What is a suspension?
- Non-homogeneous systems, in which solids are dispersed in liquids, are called suspensions. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye.
- What are the properties of a Suspension?
- Suspension is a heterogeneous mixture.
- The particles of a suspension can be seen by the naked eye.
- The particles of a suspension scatter a beam of light passing through it and make its path visible.
- The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. They can be separated from the mixture by the process of filtration.
- What is a colloidal solution?
- The mixture in which the particles are uniformly spread throughout the solution, is called colloidal. Due to the relatively smaller size of particles, as compared to that of a suspension, the mixture appears to be homogeneous. But actually, a colloidal solution is a heterogeneous mixture, for example, milk.
- Because of the small size of colloidal particles, we cannot see them with naked eyes. But, these particles can easily scatter a beam of visible light. This scattering of a beam of light is called the Tyndall effect after the name of the scientist who discovered this effect.
- Properties of a colloid
- A colloid is a heterogeneous mixture.
- The size of particles of a colloid is too small to be individually seen by naked eyes.
- Colloids are big enough to scatter a beam of light passing through it and make its path visible.
- They do not settle down when left undisturbed, that is, a colloid is quite stable.
- They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation (perform activity 2.5), can be used to separate the colloidal particles.
- What is meant by dispersion medium and dispersed phase?
- The components of a colloidal solution are the dispersed phase and the dispersion medium. The solute-like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the dispersed phase is suspended is known as the dispersing medium. Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase.
- Methods of separating the components of a mixture
- Heterogeneous mixtures can be separated into their respective constituents by simple physical methods like handpicking, sieving, filtration that we use in our day-to-day life. Sometimes special techniques have to be used for the separation of the components of a mixture.
- Ink is a mixture of a dye in water. Thus, we can separate the volatile component (solvent) from its non-volatile solute by the method of evaporation.
- Sometimes the solid particles in a liquid are very small and pass through a filter paper. Such mixtures are separated by centrifugation. The principle is that the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.
- The principle is that immiscible liquids separate out in layers depending on their densities.
- Ammonium chloride changes directly from solid to gaseous state on heating. So, to separate such mixtures that contain a sublimable volatile component from a non-sublimable impurity (salt in this case), the sublimation process is used
- A dye is a mixture of two or more colours. The coloured component that is more soluble in water, rises faster and in this way the colours get separated. This process of separation of components of a mixture is known as chromatography. Kroma in Greek means colour. This technique was first used for separation of colours, so this name was given. Chromatography is the technique used for separation of those solutes that dissolve in the same solvent.
- The method, which is used for the separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points, is called distillation.
- Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. Crystallisation technique is better than simple evaporation technique
- What is an element?
- An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Elements can be normally divided into metals, non-metals and metalloids.
- What is a compound?
- A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.
Chapter 3: Atoms and Molecules
Chapter 4: Structure of the Atom
Chapter 5: The Fundamental Unit of Life
Chapter 6: Tissues
Chapter 7: Diversity in Living Organisms
Chapter 8: Motion
Chapter 9: Force and Laws of Motion
Chapter 10: Gravitation
Chapter 11: Work and Energy
Chapter 12: Sound
Chapter 13: Why do We Fall Ill
Chapter 14: Natural Resources