Blow Hot Blow Cold Class 5 NCERT EVS Chapter Guide

Chapter Overview: One Breath, Two Effects

This chapter begins with a simple puzzle from everyday life: when we blow on our hands in winter, they feel warmer, but when we blow on hot food, it cools faster. The answer depends on what we compare the blown air with. Air coming from our lungs is usually warmer than cold winter air, so it can warm our hands a little. But hot tea or soup is much hotter than our breath, so our breath is cooler than the food. Also, blowing moves the air near the hot surface away and brings fresh air, helping heat escape faster. EVS asks us to notice such small actions and think like young scientists.

Important Terms in Blow Hot Blow Cold

Exhaled air

The air we breathe out. It is usually warm because it has been inside our body, and it contains water vapour.

Water vapour

Water in the form of gas. When warm breath touches a cool mirror or glass, tiny drops can form and make it look cloudy.

Evaporation

The change of liquid water into vapour. Moving air can make evaporation faster, which can create a cooling effect.

Air movement

Air travelling from one place to another. When we blow, we push air towards an object and disturb the air around it.

Try These Safe Observation Activities

1. Feel air with an open mouth — Hold your palm close to your mouth and say haa slowly. The air feels warm because it has just come from inside your body. Do not touch your mouth to the hand; keep a small distance.
2. Feel air with a narrow mouth — Now make your lips small and blow as if cooling tea. The air may feel cooler on your skin because it moves faster and spreads over the skin, helping moisture evaporate from the surface.
3. Breathe on a mirror — Take a clean mirror and breathe gently on it. You will see a cloudy patch because water vapour in your breath touches the cooler mirror and changes into tiny water droplets.
4. Observe hot food carefully — With an adult nearby, watch steam from hot food. If someone blows gently from a distance, the steam moves away and the food cools faster. Never put your face too close to very hot food or steam.

Worked Examples: Explaining Daily Life Situations

• Example 1: Why do we blow on cold hands in winter?
  Step 1: In winter, the air around our hands is cold.
  Step 2: Air from our mouth has been warmed inside the body.
  Step 3: When this warmer breath touches the cold hands, the hands feel a little warmer.
  Final answer: We blow on cold hands because our breath is warmer than the cold outside air.

• Example 2: Why do we blow on hot tea?
  Step 1: Hot tea is much hotter than our breath.
  Step 2: Blowing pushes away the hot air and steam just above the tea.
  Step 3: Fresh air reaches the surface and heat escapes faster.
  Final answer: We blow on hot tea because moving air helps the tea lose heat and cool down.

• Example 3: Why does a mirror become misty when we breathe on it?
  Step 1: Our breath contains water vapour.
  Step 2: The mirror is cooler than our breath.
  Step 3: Water vapour changes into tiny water drops on the mirror.
  Final answer: The mirror becomes misty because moisture from our breath condenses on the cool surface.

Exam Tip: Do Not Give Only One Fixed Answer

A common mistake is writing that blowing always makes things hot or always makes things cold. In EVS, the correct answer depends on the situation. Compare temperatures: breath is warmer than cold hands, but cooler than hot soup. Also mention moving air when explaining cooling. In NCERT-style answers, use observation words such as warm, cool, moist, steam, mirror, and air movement.

Common Mistakes to Avoid

• Do not say our breath is cold in all cases. Breath from the body is generally warm and moist.
• Do not say blowing alone creates heat. It may only transfer warmth from breath to a colder object.
• Do not touch hot food or steam while doing observations. EVS activities must be safe.
• Do not ignore the role of moving air. Blowing removes warm air near hot food and helps cooling.
• Do not confuse smoke, steam, and mist. Steam or mist is related to water vapour and tiny water droplets, not dust or smoke.

Practice Questions with Solutions

• Q: Why do people blow on their hands during a cold morning?
  A: Step 1: On a cold morning, the hands are colder than the air coming from our mouth.
  Step 2: Our breath has been inside the body, so it feels warm and moist.
  Step 3: When it touches the hands, it gives a little warmth.
  Final answer: People blow on their hands in the cold because their breath is warmer than their hands.

• Q: Riya blows on a spoonful of hot soup before eating it. Explain why.
  A: Step 1: Hot soup has a higher temperature than the air from Riya's mouth.
  Step 2: Blowing moves away hot air and steam from the soup surface.
  Step 3: Fresh moving air helps heat escape faster.
  Final answer: Riya blows on the soup to cool it so that it is safer and more comfortable to eat.

• Q: What will happen if you breathe on a cool glass window? Why?
  A: Step 1: The air we breathe out contains water vapour.
  Step 2: The glass window is cooler than our breath.
  Step 3: Water vapour changes into tiny droplets on the glass.
  Final answer: The window becomes misty or cloudy because moisture from our breath condenses on the cool glass.

• Q: Aman says, Blowing air is always cold. Is he correct? Give a reason.
  A: Step 1: Think of blowing on cold hands; the breath feels warm there.
  Step 2: Think of blowing on hot tea; the same breath helps cooling because tea is hotter than breath.
  Step 3: The effect depends on what the breath touches and on air movement.
  Final answer: Aman is not correct. Blowing can feel warm or help cooling depending on the situation.

Frequently Asked Questions