It is called a vacuum flask, and also a thermos, an insulated bottle, a vacuum insulated bottle or a thermal flask — all the same object. "Thermos" is originally a brand name that became the everyday word, in the way Xerox did for photocopying. The technical term is vacuum flask, which is what the customs classification uses: HSN 9617 covers vacuum flasks and other vacuum vessels. In India you will hear thermos, flask and insulated bottle used interchangeably.
Key Facts
- The technical name is a vacuum flask; a bottle version is usually called a vacuum insulated bottle.
- "Thermos" began as a brand name and became a generic word for the object in everyday use.
- Customs and GST classification uses HSN 9617 — vacuum flasks and other vacuum vessels.
- The insulation works by a vacuum sealed between two walls, which stops heat moving by conduction and convection.
- A single-wall steel bottle is not a flask of any kind and does not hold temperature.
- In Indian English, "flask" most often means a hot-drinks vessel, and "insulated bottle" the cold-water one, though the construction is identical.
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Short answer: a vacuum flask. Everything else on the shelf is the same thing under a different name.
The names, and where each comes from
- Vacuum flask — the technical and correct term, describing how it works rather than what it is for. The construction was devised in the 1890s for laboratory use, to hold liquefied gases, long before anyone put tea in one.
- Thermos — originally a trademark, applied to the first commercial version. It became the everyday word for the object, in the way people say Xerox for photocopy or Dalda for vanaspati. Still a brand in many markets, but in common Indian speech it just means the object.
- Insulated bottle / vacuum insulated bottle — the modern retail term, usually for the tall carry-around form aimed at water rather than tea.
- Thermal flask, thermo flask, hot and cold bottle — all the same product, in the phrasing of different sellers.
- Tumbler — a wide, short version, usually with a straw or sip lid, meant for a desk rather than a bag.
Nothing meaningful separates them at the product level. If you want the formal answer for paperwork rather than conversation, the customs and GST classification is HSN 9617 — vacuum flasks and other vacuum vessels, which is the category the whole family sits in.
How it actually keeps water hot
Two steel walls with a vacuum sealed between them. Heat moves through material by conduction and through air by convection, and a vacuum has almost neither, so there is very little for heat to travel through in either direction. That is why the same construction keeps tea hot and water cold — it is not doing two things, it is doing one thing, which is refusing to let heat cross.
The consequence people miss: a single-wall steel bottle is not a flask. It looks similar, costs less, and does nothing at all for temperature. If a listing does not say double-wall or vacuum insulated, assume it is single wall.
How to tell a real one
Fill it with ice water, close it, and wait ten minutes. A genuine vacuum vessel stays completely dry and room-temperature on the outside. If the outside is beading with condensation or feels cold, heat is crossing the wall and the vacuum is either absent or has failed.
In India the standard that tests this is BIS IS 17526:2021, which covers stainless steel vacuum insulated bottles and is the certification worth looking for on the label.
Hot and cold in the same vessel
Yes, and it is the same mechanism, which is why "hot and cold bottle" is not a special product category. Two practical notes: pre-warming with hot water for a minute before filling makes a real difference to how long tea stays hot, and pre-chilling does the same for cold. And retention figures on any packaging assume a full, closed vessel — open it repeatedly or fill it two-thirds and expect noticeably less.
If you want the fuller explanation of what "insulated" is actually claiming, see what insulated really means, or browse the insulated range.
Frequently Asked Questions
What is a bottle that keeps water hot called?
A vacuum flask. It is also called a thermos, an insulated bottle, a vacuum insulated bottle or a thermal flask — all the same object. The technical term is vacuum flask, which is what the HSN 9617 customs classification uses.
Is thermos a brand or a product?
Both. It began as a trademark for the first commercial vacuum flasks and became the everyday word for the object, in the way Xerox did for photocopying. It remains a brand in many markets while functioning as a generic term in speech.
What is the difference between a flask and an insulated bottle?
Nothing at the product level. "Flask" tends to be used for hot-drinks vessels and "insulated bottle" for the tall carry-around water form, but the construction — two steel walls with a vacuum between them — is identical.
How does a vacuum flask keep water hot?
A vacuum sealed between two walls removes almost all the material and air that heat would otherwise travel through, so very little heat crosses in either direction. That is also why the same vessel keeps cold drinks cold.
Do all steel bottles keep water hot?
No. Only double-wall vacuum insulated ones do. A single-wall steel bottle looks similar and does nothing for temperature. If a listing does not state double-wall or vacuum insulated, assume it is single wall.
Related guides
Care that decides whether it lasts five years or one
- Rinse daily with hot water; do not leave water standing for days.
- Wash weekly with dish soap and a bottle brush — plus a straw brush if it has a straw, which is the step people skip and then wonder about the smell.
- Deep clean monthly — two tablespoons of baking soda in warm water, 30 minutes, then a white vinegar rinse for odour.
- Replace gaskets and straws every 6-12 months. A ₹100 seal saves a ₹1,900 bottle.
- Never freeze an insulated steel bottle — water expands about 9% and can crack the seal weld, permanently destroying the insulation.
- Add a silicone base boot. Most vacuum failures start with a vertical drop onto the base.
What actually goes wrong over time
Bottles rarely fail all at once. Three things degrade, in rough order of how often they end a bottle's life:
- The gasket. Silicone hardens, stops sealing and holds odour within 6-12 months of daily use. This is the cheapest fix and the most neglected — most bottles that "smell no matter what" have a two-year-old seal.
- The vacuum. Usually from a vertical drop onto the base cracking the seal weld. Silent, invisible, and it halves the bottle's usefulness.
- The finish. Wet paint chips within a year; powder coat survives 3-5. Cosmetic, but it is what makes a bottle feel old.
None of the three is the steel body, which is why buying a bottle on body material alone misses what actually determines lifespan.
The four checks worth making before you buy
- Steel grade stated explicitly — 304, 18/8 or SUS 304. "SS" or "food-grade steel" without a number usually means grade 201, which pits in water contact.
- Double-wall vacuum construction if you want any temperature retention. Single-wall insulates nothing and sweats in humidity.
- BIS IS 17526:2021 certification with a CM/L number — the only retention claim in India that has actually been tested.
- Replaceable silicone gasket. Gaskets perish within 6-12 months of daily use; a sealed lid assembly means the bottle dies when the seal does.
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