Chapter 5 — Heat

What is heat?
Heat is energy that moves from a hotter object to a colder one when they are in contact or when rays travel between them. You feel heat when you put your hand near a warm stove or in sunlight. Heat transfer is always from higher temperature to lower temperature until both come to the same temperature.

What is temperature?
Temperature tells how hot or cold a body is. It is connected to how fast the particles of the substance move on average: higher temperature means faster motion. Temperature is measured by thermometers and is what we read as degrees.

How heat and temperature differ
Heat is energy in transit; temperature is a measure. A large bucket of warm water may contain more heat energy than a small cup at the same temperature because heat depends on mass as well as temperature. Temperature alone cannot tell how much heat energy a body holds.

Observations and examples
When a hot cup cools on a table, heat leaves the cup and moves to the air and the table; the cup’s temperature drops. Touching ice makes your hand feel cold because heat flows from your hand into the ice. If you place two objects of different temperatures together, heat flows until they reach the same temperature (thermal equilibrium).

Everyday importance
Understanding heat and temperature helps explain cooking, weather, how engines work and why houses need insulation. It also builds the foundation for learning how heat moves and how materials respond to heating or cooling in later classes.

Why different scales?
Different countries and fields use different temperature scales. The Celsius scale (°C) is common in India and most of the world for daily use. Scientists often use Kelvin (K) because it begins at absolute zero. Some countries, including parts of the USA, use Fahrenheit (°F) for weather and household measures.

Celsius scale details
On the Celsius scale water freezes at 0 °C and boils at 100 °C under standard atmospheric pressure. The scale divides the interval into 100 equal parts, each one degree Celsius. This makes it easy for many everyday and laboratory measurements.

Fahrenheit scale details
Fahrenheit sets freezing point of water at 32 °F and boiling point at 212 °F under standard pressure. While not used widely in India, students may see Fahrenheit in international media or older instruments.

Kelvin scale details
Kelvin is the SI unit for temperature; it starts at absolute zero (0 K), the point where particles have minimum possible motion. Water freezes at about 273 K and boils at about 373 K. Kelvin is helpful in scientific calculations because it avoids negative values in many formulas.

How to think about values
Common reference points help: a comfortable room is about 20–25 °C, a hot day might be 35 °C, and human body temperature is roughly 37 °C. Learn to recognise these numbers across scales so you can convert or understand readings from different thermometers.

Practical note
Thermometers and devices usually show one or more scales. Always note which scale is in use when you record temperatures, and be careful with instruments that use different scales for different purposes (medical vs weather vs laboratory).

Types of thermometers
There are several kinds of thermometers used in classrooms and homes: liquid-in-glass thermometers (mercury or coloured alcohol), clinical thermometers for body temperature, digital thermometers and infrared thermometers for surface readings. Each type reads temperature in a particular way but all indicate how hot or cold something is.

How liquid-in-glass thermometers work
These thermometers have a bulb filled with liquid connected to a narrow capillary tube. When temperature rises, the liquid expands and moves up the tube; when temperature falls, it contracts and the level drops. The tube has a scale marked in degrees, and the position of the top of the liquid column gives the reading.

Clinical thermometers
Clinical thermometers are made to measure body temperature and often have a kink in the capillary so the reading stays until the thermometer is shaken down. When using one, keep the bulb in proper contact (for example, under the tongue) and wait the recommended time for an accurate reading.

Digital and infrared thermometers
Digital thermometers give quick readings and are safer than mercury types. Infrared thermometers measure surface temperature without touching the object and are useful for hot or moving targets. Always follow manufacturer instructions for placement and timing to get correct values.

Correct use and reading
Place the bulb or sensor where you want to measure, wait until the reading stabilises, and read at eye level to avoid parallax errors. For liquid thermometers, ensure the bulb does not touch the container’s bottom, and for clinical use clean and disinfect as needed.

Safety and care
Handle mercury thermometers carefully; if one breaks tell an adult immediately because mercury is dangerous. Use protective holders, store thermometers at room temperature, and prefer digital types for children. Keep thermometers away from direct sunlight and extreme conditions when not measuring.

Observation and meaning
When solids are heated they usually increase in length. This change is small but measurable. For example, a metal rod will become a little longer when warmed and will become shorter again on cooling. This phenomenon is called thermal expansion. It is important in engineering and everyday construction because controlled expansion must be allowed for.

Why solids expand
Heating increases the average vibration of atoms or molecules in a solid. Though they remain in fixed positions relative to each other, these increased vibrations make the average distance between neighbouring particles grow slightly. The accumulation of these small increases along the length gives a noticeable extension in long objects.

Linear expansion rule
For small temperature changes the increase in length is proportional to the original length and to the rise in temperature. That is why a longer rod shows a larger absolute change for the same temperature rise than a short rod of the same material. Different materials expand by different amounts because of their atomic structures.

Practical consequences
Railways, bridges and pipelines are designed with gaps or flexible joints to accommodate expansion on hot days. Overhead electric wires sag more in warm weather. Fitting metal lids onto glass jars requires care; heating or cooling can change fit. Common devices like bimetallic strips exploit unequal expansion of two metals.

Measuring expansion
With careful marking and a scale, small changes in length can be measured in a lab. A thermometer may be mounted on a rod to measure expansion indirectly. Engineers use the linear expansion concept to calculate allowances in design so structures remain safe across temperature ranges.

Liquids and expansion
Liquids expand more on heating than solids do. You can see this in a liquid-in-glass thermometer where the liquid column rises noticeably for a small temperature rise. Different liquids expand by different amounts: alcohol-based liquids expand more than mercury, which is why alcohol thermometers are more sensitive for small temperature changes.

Why liquids expand
Increasing temperature increases the average motion of the particles in a liquid; they move apart slightly and occupy more space. Unlike solids, particles in a liquid are free to move past each other, so the expansion is larger and usually uniform in all directions. A vessel containing heated liquid may overflow if not allowed room to expand.

Gases and strong expansion
Gases expand much more than liquids or solids when heated, since gas particles are far apart and move freely. In a flexible container like a balloon, heating the gas increases its volume and makes the balloon inflate further. In a closed rigid container, heating increases pressure because the gas particles move faster and collide more with the walls.

Special case: water
Water shows unusual behaviour around 4 °C: it reaches maximum density at about 4 °C and expands on cooling below this temperature. This is why ice (frozen water) is less dense than liquid water and floats. This behaviour is crucial for life, because ice floating insulates water below and allows aquatic life to survive winters.

Applications and safety
Understanding liquid and gas expansion helps in designing engines, hot water systems and storage containers. Always leave space at the top of a container for liquid expansion when heating to avoid spills. Pressure vessels require safety valves to release excess pressure if gas is heated.

What are changes of state?
When matter changes from solid to liquid, liquid to gas or vice versa, we call these changes of state. Heat supplied or removed causes particles to rearrange or move faster, producing melting, freezing, boiling or condensation. Knowing these helps explain everyday events like melting ice, boiling water and drying clothes.

Melting and melting point
Melting is the change from solid to liquid. Pure substances melt at definite temperatures under fixed pressure: this is the melting point. For example, pure ice melts at 0 °C under normal atmospheric pressure. During melting, temperature stays nearly constant until the whole solid becomes liquid because heat goes into changing the internal arrangement rather than raising temperature.

Boiling and boiling point
Boiling is when a liquid turns into vapour throughout the liquid, not just at the surface. Boiling occurs at the boiling point when the vapour pressure of the liquid equals the external pressure. For water at normal pressure this is 100 °C. Heating past the boiling point will usually increase the amount of vapour or the intensity of boiling but not the temperature until pressure changes.

Evaporation
Evaporation is the change from liquid to vapour at temperatures below the boiling point and takes place at the surface. Faster molecules escape first, lowering the average kinetic energy and cooling the remaining liquid — this is why sweat cools the body. Evaporation increases with surface area, temperature and air movement.

Everyday examples and significance
Melting explains ice cream softened on a hot day, boiling is used to cook and sterilise, and evaporation dries clothes and cools us. Understanding these processes is useful for household tasks, safety (e.g., handling boiling liquids) and later studies of latent heat and phase diagrams in higher classes.

Definition and mechanism
Conduction is the transfer of heat through a material without any movement of the material as a whole. It happens when faster vibrating particles (or free electrons in metals) pass energy to neighbouring slower particles. The energy moves from the hot end to the cold end until temperatures equilibrate.

Why metals conduct well
Metals are good conductors because they have free electrons that move easily and carry energy quickly across the metal. In non-metals, conduction is slower because heat travels by vibrations passed between atoms or molecules. Materials with tightly bound electrons and rigid structures usually conduct less.

Factors affecting conduction
Thermal conductivity depends on the material’s nature, thickness, cross-sectional area and the temperature difference across it. A thin metal plate transfers heat faster than a thick one of the same metal. A greater temperature difference increases the rate of heat flow.

Everyday examples
Cooking pans have metal bases to conduct heat from the stove to the food quickly. A metal spoon left in hot curry transfers heat to the handle; that is why some utensils have wooden or plastic handles as insulators. Heat sinks in electronic devices use metal fins to conduct and then release heat into the air.

Classroom demonstration
Place a metal rod with wax blobs along its length and heat one end: as heat travels, wax near the hot end melts first and later at positions farther from the flame. This shows heat conduction along the rod and the time it takes to reach different parts.

Convection explained
Convection is heat transfer in fluids (liquids and gases) by the movement of the fluid itself. When part of a fluid heats up, it expands slightly, becomes less dense and rises; cooler, denser fluid sinks to take its place. This circulation pattern — convection currents — moves heat through the fluid and is common in everyday life and nature.

Examples of convection
Boiling water shows convection: hot water from the bottom rises while cooler surface water sinks, setting up loops. Rooms heated by radiators warm as warm air rises and cooler air flows to the heater, creating circulation. Sea breezes result from land heating faster than sea and setting up convection of air masses.

Radiation explained
Radiation is transfer of energy by electromagnetic waves. It needs no medium and can travel through a vacuum; this is how the Sun’s energy reaches Earth. All objects emit thermal radiation; hotter bodies emit more energy and at shorter wavelengths. Dark, matte surfaces emit and absorb radiation more effectively than shiny surfaces.

Comparing the two
Convection requires a fluid and bulk motion, while radiation works through empty space and does not move matter. Many situations involve both: a stove heats a pot by radiation from the flame and conduction through the pot; inside the pot convection distributes heat in the liquid.

Practical uses
Understanding convection and radiation helps in designing heating and cooling systems, weather prediction, and protecting from heat (using reflective clothing or paints to reduce radiative heating). It also helps explain natural phenomena such as wind and ocean currents.

What are conductors and insulators?
Conductors are materials that allow heat to pass through them easily; examples include most metals like copper and aluminium. Insulators are materials that resist heat flow and keep heat from moving quickly; examples include wood, plastic, wool, glass wool and trapped air. Choosing the right material matters when we want to transfer heat or prevent it.

Why choice matters
When cooking, we want quick transfer of heat from flame to food, so we use metal pans. But to avoid burning our hands, the handle is made of wood or plastic which is an insulator. In buildings, insulators in walls and roofs slow heat loss in winter and keep interiors cooler in summer, reducing energy use.

How insulators work
Insulators often trap air in small pockets; air is a poor conductor, so the trapped air slows heat transfer. Reflective surfaces reduce heat loss by radiation by reflecting infrared waves back to the source. Multi-layer combinations (vacuum, reflective coating, foam) reduce conduction, convection and radiation as used in a thermos flask.

Examples of practical uses
Cookware uses metal bases for good conduction; electric iron handles are insulated to protect the user. Thermos flasks use vacuum and reflective layers to keep content hot or cold. Building materials like insulating foam, double-glazed windows and thick curtains reduce heat loss and save fuel.

Selecting materials
Pick conductors when you need fast heat transfer and insulators when you need to preserve temperature or protect people from heat. Consider cost, durability and safety for the application. Testing small samples in class can show which materials conduct heat faster by heating one side and feeling the other.

What is specific heat in simple terms?
Specific heat is a property that tells how much heat energy is needed to raise the temperature of a unit mass of a substance by one degree. For Class 7 we learn it qualitatively: some materials warm up quickly for little heat (low specific heat), while others need much more heat to show the same temperature change (high specific heat).

Why materials differ
The internal structure and bonding of a substance determine how heat energy is distributed among its particles. Water, for instance, stores heat well because energy goes into moving many possible modes of vibration and rotation, so its temperature rises slowly for a given input of heat. Metals have lower specific heat, so they get hot quickly with little energy input.

Everyday examples
Because water has a high specific heat, coastal areas have milder temperature swings and large water bodies moderate climate. When cooking, the metal pan heats up quickly while food or water heats more slowly. This is why heavy-bottomed pans and lids help conserve heat and cook food more evenly: they increase the thermal mass that holds heat.

Simple classroom experiment
Heat equal masses of two different materials with the same flame and measure temperature rise over time. The material showing a smaller temperature rise for the same heat input has the higher specific heat. Record time, starting and final temperatures and compare results to draw conclusions.

Importance and links
Knowing about specific heat helps when discussing climate, heating systems, cooking and thermal storage. Later classes will give the formula and numerical practice, but this qualitative understanding already explains many common observations about heating and cooling behaviour.

What is a bimetallic strip?
A bimetallic strip is made by firmly joining two thin strips of different metals, such as brass and steel, which expand by different amounts when heated. Because one metal expands more than the other for the same temperature rise, the joined strip bends towards the metal with lower expansion. This predictable bending is used to make simple temperature-sensing devices.

How bending produces action
The bending of the strip can be used to move pointers on a dial, operate switches or make and break electrical contacts. In many thermostats the strip bends enough at a set temperature to open or close a contact, controlling a heater, cooler or alarm. On cooling, the strip straightens and the contact returns to its original state, allowing automatic on-off control based on temperature.

Applications
Bimetallic strips are used in household thermostats, electric irons, geysers and thermostatic switches in ovens and refrigerators. They are simple, reliable, and do not require external power for sensing temperature. Their mechanical action makes them robust for many everyday appliances.

Classroom demonstration
A classroom strip made of two different metals can be heated gently to show bending. Attach one end and heat near the free end; as temperature rises, the free end will move. Connect it to a simple circuit and a bell to show how bending can make and break contact to operate the bell at a certain temperature.

Advantages and limitations
Bimetallic devices are inexpensive and durable but less precise than electronic sensors. They are excellent for simple on-off control and for educational demonstrations showing the effects of unequal expansion.

Thermos flasks
A thermos flask keeps liquids hot or cold by reducing all three modes of heat transfer. It has double walls with a vacuum between them that prevents conduction and convection, and a shiny inner surface that reflects thermal radiation. A tight stopper reduces heat loss through air movement. This combination makes thermos flasks useful for storing hot beverages or cold drinks for long periods.

Radiators and space heaters
Radiators transfer heat to room air by conduction through the radiator material and then by convection as air circulates over the warm surfaces. Many radiators have fins to increase surface area so they can warm more air. Space heaters often mix conduction and radiation: they heat surrounding air and also send infrared radiation that warms objects directly.

Expansion joints and design
Long structures like bridges, highways and railway tracks are fitted with expansion joints or small gaps to allow thermal expansion in hot weather. Without these joints, materials could bend, crack or buckle as temperature changes. Engineers calculate expected expansion and include joints, sliding supports or flexible connections to keep structures safe.

Other everyday devices
Cooking vessels, pressure cookers, electric irons and thermostats all use heat principles. Pressure cookers cook faster because steam temperature can exceed 100 °C under pressure. Heat sinks and fans are used in electronics to remove heat by conduction and forced convection to keep components safe.

Design principles
Good thermal design chooses materials and forms that control conduction, convection and radiation as needed. For insulation you want low conductivity and trapped air; for heating you want high conductivity and large surface area. Understanding these choices explains why devices look and work as they do.

Conserving heat at home
Small actions save fuel and electricity. Use lids on pots to cook faster and retain heat, insulate hot water pipes to avoid loss, close doors and curtains to keep warmth inside rooms, and choose thick curtains or double-glazed windows where possible. These measures reduce the need for continuous heating and lower energy bills.

Heat safety rules
Handle hot objects with care: use oven mitts, tongs or potholders. Keep flammable materials away from open flames, and never leave stoves or heaters unattended. Be cautious with hot liquids to avoid scalding children. If a thermometer containing mercury breaks, avoid touching mercury and inform an adult immediately because mercury is toxic.

Practical workplace and classroom safety
During experiments use heatproof mats and stands, keep a fire extinguisher nearby, and wear protective clothing or goggles if splashing is possible. Follow teacher instructions for safe distances from burners and use clamps for holding heated apparatus. Ensure ventilation when heating substances that may release fumes.

Everyday tips linked to heat concepts
Use reflective paint or shades to reduce radiative heating, trap air as insulation in clothing and flasks, and pick materials suitable for tasks: metal for pans, insulating handles for safety. Thicker materials or larger thermal masses change temperature more slowly and can stabilise indoor climates.

Why it matters
Conserving heat reduces environmental impact and saves money. Safety reduces accidents and injuries. Simple knowledge about heat transfer and careful habits make homes and schools safer and more efficient, and builds good habits for life.

Why experiments help
Simple experiments let students see heat concepts directly. Doing hands-on activities builds observation skills, helps link theory to real events and improves careful recording of data. Many heat experiments are safe and use everyday materials so they are ideal for classroom demonstrations and home practise under guidance.

Experiment ideas and procedures
1) Conduction demonstration: place small identical wax blobs at intervals on a clean metal rod and heat one end; observe which blobs melt first to see heat travel along the rod. 2) Convection currents: warm water in a beaker with a small drop of dye near the bottom; heating will create rising coloured streams showing convection paths. 3) Comparing conductors and insulators: place samples of different materials between a hot plate and a cold surface and measure temperature at the cold side after fixed time; the faster the rise, the better the conductor. 4) Specific heat (qualitative): heat equal masses of two substances with the same flame and measure temperature rise—larger rise means lower specific heat.

Recording observations
Write down initial conditions (mass, starting temperature, room conditions), exact steps, times, and final readings. Use simple tables to organise results and draw labelled diagrams to show set-ups. Repeat experiments to check reliability and note sources of error (unequal heating, drafts, contact issues).

Interpreting results
Link observations to concepts: melting of wax shows conduction, rising dye shows convection, differing warm-up rates show conductors vs insulators or differences in specific heat. Discuss why results occurred and how real devices apply the same ideas.

Safety and supervision
Always use teacher supervision when using flames or hot plates. Use tongs, heatproof mats and goggles if needed, and cool hot objects before handling. Ensure safe disposal and careful handling of broken glass or hazardous materials.