CoSci P5 Heat and Particles

Describe qualitively the pressure of a gas in terms of the motion of its molecules and their colliding with the walls creating forces.

A higher temperature means the gas molecules move faster and have more collisions. As the gas molecules will collide with the surface of their container more and at a higher speed, the total force they apply on the container per unit area will increase. Pressure is the force applied on a surface per unit area, so the gas pressure increases.

Explain heat transfer in solids in terms of molecular motion.

According to the kinetic theory, all materials are made up of tiny, moving particles. In solids, these particles tend to vibrate around a fixed spot. When you apply heat energy to these particles, they tend to vibrate more vigorously. This heat energy can be transferred from one end of the material to the other, because the particles with more energy collide with the adjacent particles, causing them to vibrate harder and so forth. In other words, the particles with more thermal energy pass on their energy to the particles surrounding them, causing the heat to spread from the hotter regions to the colder regions of the material. thus conduction works the best in metals.

Relate convection in fluids to density changes.

As a liquid or gas gains heat energy, its particles move faster, causing the space between them to increase. Therefore, the density of the liquid or gas also decreases. density=mass/volume Well, that means that the less dense parts of the liquid or gas (as in, the parts with more heat energy) will have a lower mass per unit volume, making them lighter, and causing them to rise to the top while the colder parts sink. If the source of the heat energy is at the bottom of the container, like when you boil water on a stove, then the colder part that sank to the bottom will soon become the hotter part, and that will rise, and so on. The phenomenom of the rising and sinking of heated and cooled liquids create a cycle, a convection current, that will repeat itself until heat is distributed evenly throughout the whole body of liquid.

Describe melting and boiling in terms of energy input without a change in temperature

As you heat a solid, its temperature rises until it reaches its melting point. As more energy is supplied, the solid melts and the temperature doesn't change. It is now a liquid. When you supply heat energy to a liquid, its temperature will rise until it reaches its boiling point. As the liquid vaporises, its temperature doesn't change. It is now gas. Heating a gas will increase its temperature. Since the energy involved in a phase changes is used to break bonds, there is no increase in the kinetic energies of the particles, and therefore no rise in temperature.

Describe the effect of surface colour and texture on the emission, absorption and reflection of radiation

Black dull surface Good absorber Good emitter Poor reflector (black absorbs all the wavelengths of light) Light Shiny surface Poor absorber Poor emitter Good reflector (white reflects all the wavelengths of light)

Distinguish between boiling and evaporation

Boiling - Occurs at fixed temperature - Relatively fast process - Takes place throughout the liquid - bubbles are formed - Temperature remains constant - External thermal energy source required Evaporation - Occurs at any temperature - Relatively slow process - Takes place on surface of liquid only - No bubbles - Temperature may change (evaporative cooling effect) - Does not require external thermal energy source (heat from surroundings is enough

pressure temperature and volume

Boyle's law states that pressure and volume are inversely proportionate when the gas is at a constant temperature. Lowering the volume will increase in a higher pressure, and vice versa. This is because if a gas has a smaller volume, there is less space for the particles to move; they hit the sides of the container more frequently, resulting in higher pressure. A higher temperature means the gas molecules move faster and have more collisions. As the gas molecules will collide with the surface of their container more and at a higher speed, the total force they apply on the container will increase. Pressure is the force applied on a surface per unit area, so the gas pressure increases. Note that this is true when the volume is constant - if the volume increases, then the molecules have to travel further to hit the container, reducing the number of collisions. This could nullify the effect of the temperature on pressure. The opposite is true when the temperature falls at a constant volume - molecules move slower, so fewer collisions and the collisions are less forceful, so the pressure falls.

Brownian motion

Brownian motion refers to the phenomenon of particles in fluids (i.e gases and liquids) moving randomly. They do this because they are bombarded by the other moving particles in the fluid.

Describe condensation and solidification.

Condensation is when a gas changes to a liquid. As a gas loses heat energy, they also lose kinetic energy, causing them to move slower. Their intermolecular forces of attraction become more significant, pulling the molecules together and allowing them to form weak bonds until they form a liquid. e.g. your mirror steams up after a shower. Solidification, or freezing, is when a liquid changes to a solid. As liquids lose energy, their molecules slow down and form more intermolecular bonds with one another. They become locked in place, and thus, they form a solid.

Recognise convection as the main method of heat transfer in liquids and gases.

Convection occurs when particles with more heat energy move and take the place of particles with less heat energy. Because this process requires the movement of particles, it can only occur in liquids or gases. It is more efficient than conduction (heat moves through the substance faster) and requires less energy than radiation (radiation is discussed in the next topic - Unit P6.3), so most of the heat is transferred by convection in liquids and gases.

Relate evaporation to the consequent cooling.

During evaporation, the most energetic molecules escape into the atmosphere, carrying away their energy with them. This means there is less total energy left in the liquid. As temperature is a measure of the average kinetic energy of the liquid, and there is less kinetic energy, the temperature of the liquid also falls, and thus, the liquid cools.

Describe the changes of state in terms of melting, boiling, evaporation, freezing and condensation, sublimation and deposition

Freezing: the substance changes from a liquid to a solid. Melting: the substance changes back from the solid to the liquid. Condensation: the substance changes from a gas to a liquid. Vaporization: the substance changes from a liquid to a gas. Sublimation: the substance changes directly from a solid to a gas without going through the liquid phase. Deposition: the substance changes directly from a gas to a solid without going through the liquid phase.

Identify infra-red radiation as the part of the electromagnetic spectrum often involved in heat transfer by radiation.

Infra-red light is the part of the EM spectrum that is most involved in heat transfer. It is not visible to the naked eye

How does brownian motion act as evidence of the kinetic model of matter

Kinetic model of matter states that particles are in constant random motion Fluid particles (air particles) are in constant random motion. They tend to collide with other particles in the fluid (smoke particles) as a result, these particles in the fluid move in random motion. Therefore supporting the kinetic model of matter.

Latent heat of fusion/vaporization

Latent heat of vaporisation is the energy supplied when a substance changes its physical state from liquid to gas. It is called the latent heat of vaporisation because it is the energy supplied when the liquid vaporises. Latent heat of fusion is the energy supplied when a substance changes from solid to liquid. It is called the latent heat of fusion because a lot of solids are fused together by melting them and combining them, e.g. in the creation of alloys. The temperature does not change when melting or boiling, because this latent heat is used to break the bonds between the molecules. The latent heat of vaporisation is usually greater than the latent heat of fusion because normally, more bonds have to be broken when boiling than when melting.

Explain evaporation in terms of the escape of more energetic molecules for the surface of a liquid

Liquids absorb heat from their surroundings and the sun. This increases their kinetic energy. At the surface of the liquid, a few molecules eventually gain enough kinetic energy to overcome the intermolecular forces surrounding it and escape as a gas. This process is evaporation. Note that evaporation is a superficial process - it only occurs on the surfaces of liquids, unlike vaporisation, which can occur anywhere within the liquid (that's why bubbles of gas form when we boil water).

Describe experiments to demonstrate the properties of good and bad conductors of heat.

One experiment is the wax method: Take rods of different materials with the same dimensions (same length and cross-sectional area), and attach a drawing pin to the end of each using the same mass of wax. Using a container with four rubber-lined holes in the side. Insert the rods through the holes, pushing them in the same amount. Fill the container with boiling water and start the stopwatch. Time how long it takes the drawing pin to fall off each rod and compare the times. The rod that allowed the pin to fall off fastest is the best conductor. Another experiment can show us how good a conductor of thermal energy water is this:

Recognise radiation as the method of heat transfer that does not require a medium to travel through

Radiation is the emission of energy. A lot of radiation travels as waves - examples include visible light, UV (Ultraviolet) light, X-rays, gamma radiation, radio waves, etc. Radiation can travel through a vacuum - that's how we, on Earth, manage to get the Sun's warmth and light despite being separated by the large vacuum of space. In other words, radiation doesn't require a medium. In the same way that light is radiated, heat can also be radiated. This makes radiation the only form of heat transfer that does not need a medium - both conduction and convection use particles to transfer heat energy from one place to the other.

Energy changes

SOLID-(melts energy increases)-LIQUID-(evaporates/boils energy increases)-GAS-(condenses energy decreases)-LIQUID-(solidifies/freezes energy decreases)-SOLID

Describe qualitatively the molecular structure of solids, liquids, and gases in terms of the arrangement, separation, and motion of the molecules.

Solids have very strong intermolecular forces of attraction. This causes their molecules to be tightly bound (solids have the smallest distance between molecules). They are also arranged regularly, or in other words, arranged in a lattice, as this minimises the distance between particles. The strong forces of attraction prevent the molecules from moving, so they vibrate around a fixed point. The strong intermolecular force also means that the solid maintains a fixed volume and shape (all molecules are held tightly in place). Solids cannot be compressed because their molecules are already packed close together. Liquids have a moderately strong intermolecular force of attraction. This means that the molecules are held close together, but not so tightly that they can't move. This allows liquids to rotate and flow over one another. Due to the movement of molecules in a liquid (the molecules in a liquid are constantly in motion, even if the liquid as a whole is stationary), the arrangement of molecules in the liquid is irregular. Because liquid molecules can move, their shape is not fixed - they tend to assume the shape of whatever container they are in. The intermolecular force of attraction in liquids are not as strong as the forces in solids, so the molecules are usually further apart. This means that liquids can be compressed slightly (and thus, their volume can change), but this change is negligible. Gas molecules have so much kinetic energy that they are free to move in any direction, and the molecules are very far apart. The kinetic energy of molecules overcome any forces of attraction, making those forces negligible. The movement of gas molecules also means that they tend to fill out their container, taking its shape. As gas molecules are very far apart, they can be compressed, forcing the molecules closer together. This expansion of gases to fill out its container and the ability to compress gases means that the volume of gases is not fixed.

State the meaning of melting point and boiling point and recall the melting and boiling points of water.

The boiling point is the temperature at which a material changes from a liquid to a gas (boils) while the melting point is the temperature at which a material changes from a solid to a liquid (melts). Keep in mind that a material's melting point is the same as its freezing point. Water: 0 degrees melting, 100 degrees boiling

Demonstrate understanding of how temperature, surface area and air flow over a surface influence evaporation.

The higher the temperature, the greater the rate of evaporation. As the temperature increases, more energy is supplied to the liquid molecules, allowing more surface molecules to gain enough energy to escape the liquid at a time. As the surface area increases, so does the rate of evaporation. This is because more of the liquid is exposed to the atmosphere, so there are more surface molecules that can absorb the surrounding energy and eventually escape. Increased air flow also increases evaporation. When wind blows across the surface of the liquid, the airborne liquid particles are swept away. This decreases the humidity in that region, allowing more molecules to dissipate into the air.

Describe qualitatively the pressure of a gas and the temperature of a gas, liquid or solid in terms of the motion of the particles

The higher the temperature, the more kinetic energy the gas molecules have (heat energy is converted to kinetic energy), and so the faster they move. There will also be more collisions between molecules, making their motion more erratic. The molecules also move further apart, expanding the gas. The lower the temperature, the less kinetic energy the gas molecules have (kinetic energy of the gas is lost as heat to the surroundings), and so they move slower. This may mean the intermolecular forces of attraction become stronger, bringing the molecules closer together. When a solid is heated, its atoms vibrate faster about their fixed points. The relative increase in the size of solids when heated is therefore small. Metal railway tracks have small gaps so that when the sun heats them, the tracks expand into these gaps and don't buckle. Liquids expand for the same reason, but because the bonds between separate molecules are usually less tight they expand more than solids. This is the principle behind liquid-in-glass thermometers. An increase in temperature results in the expansion of the liquid which means it rises up the glass.

explain the changes of state in terms of particle theory and the energy changes involved

The kinetic theory of matter can be used to explain how solids, liquids and gases are interchangeable as a result of increase or decrease in heat energy. When an object is heated the motion of the particles increases as the particles become more energetic. If it is cooled the motion of the particles decreases as they lose energy.

Interpret and describe experiments to investigate the properties of good or bad emitters and good or bad absorbers of infra-red radiation

The transfer of infrared radiation from a hot object to cooler surroundings can be investigated using a piece of apparatus called Leslie's cube.This is a metal cube with four side prepared in different ways: black, white, shiny, or dull. It can be filled with hot water or heated on an electrical hot plate so that all four sides are at the same temperature. Method 1. Measure the temperature a fixed distance from each side of a Leslie's cube using thermometers (or using a thermocouple, an electrical device that produces a potential difference depending on the temperature). 2. Heat the Leslie's cube with boiling water or with a hot plate. 3. Continue to measure and record the temperatures every 30 seconds for five minutes, then plot a graph of temperature against time for each side. 4. Compare the four graphs obtained. Remember that the four thermometers may vary in accuracy, so take this into account when analysing the results.

Describe experiments to illustrate convection in liquids and gases.

There are four things you need: Blue food colour (to represent the cold water), red food colour (representing the warmer water), water and a large transparent container. Add blue food colour to some water, and freeze it in an ice cube tray to create blue ice cubes. Fill the large transparent container with water. Add the blue ice cubes to one side, and add a decent amount of red food colour (2 - 3 teaspoons should do the trick) to the other side. You'll notice that the blue water almost immediately sinks to the bottom of the tub, and the red water mostly stays at the top. You'll also notice that the red water moves towards the blue water and the blue water spreads to the red. What you're seeing is convection.

Thermometers

These are 0°C and 100°C respectively. Thermometers measure temperature. The liquid (usually mercury or coloured alcohol) expands when heated which means that it rises up the glass to show a higher temperature. A thermocouple is used to measure very high temperatures or those that change quickly.

applications

We often use hot water to warm up the lid of a jar. This expands the lid (metals expand more than glass), making it easier to remove . Liquid in thermometers expand and contract as the temperature changes. The volume of the liquid at a given temperature is how we read the temperature off of a thermometer. Overhead cables have to be slack so that on cold days when they contract, they won't snap or detach. Expansion joints - these are found on most large bridges. They look like two metal combs, their teeth interlocking, and have small gaps between each other. When heat causes the bridge to expand, the two sides of the expansion joint move towards each other. As the temperature cools, they gradually retract. This gives the bridge room for expansion and contraction, preventing the cracking/ deformation of the bridge. The expansion joints have interlocking 'teeth' because this minimizes the bump that motorcyclists feel as they ride over it.

thermal expandion of solid liquid gases at constant pressure

When matter is heated, its particles gain energy, which is exerted as kinetic energy. In solids, the particles vibrate harder and faster, creating more space between the particles, causing them to expand. This is most visible in metals. This process is thermal expansion. In liquids, the particles move around faster, weakening the intermolecular forces of attractions, and are thus held less closely together. The liquid expands. In gases, particles move faster as they are heated. If they are heated under constant pressure, the gas particles collide harder with the container surfaces, forcing them out, and allowing the gas to expand. This can be seen when warming the gas in a gas syringe.

Explain in terms of motion and arrangement of molecules the relative order of magnitude of the expansion of solids, liquids and gases.

gases expand the most, followed by liquids, and solids expand the least. This is because gases have the weakest intermolecular forces of attraction, allowing their molecules to move the furthest apart, and solids have the strongest intermolecular forces, limiting the range of motion of the particles.