How to Distill Water: Methods and Equipment

How to distill water: boil it, capture the vapour, and condense it back to liquid on a cold surface, leaving salts, metals and non-volatile residue behind in the boiling vessel. Every method — a pot with ice on the lid, a lab still, a rotary evaporator — is that same three-step loop. What changes is control and throughput.

What actually happens when you distill water?

Distillation separates by volatility. You put heat into a liquid, the molecules that leave the surface most readily go first, and you cool that vapour somewhere else so it condenses into a clean receiving vessel. Whatever cannot evaporate — dissolved salts, heavy metals, scale, sediment, most of what makes tap or well water taste like something — stays in the pot.

That much every tutorial gets right. Here is the part most of them skip: distillation does not remove everything. It removes non-volatiles. Anything in the water that boils near or below water itself rides along with the steam and lands in your distillate, sometimes concentrated rather than removed. If your feed carries light solvents or volatile organics, a single boil-and-catch pass is not a purification step, it is a transfer step. Knowing which of the two you are doing decides whether a kitchen pot is enough or whether you need a condenser, a vacuum line and a temperature you can hold.

The three moves never change: evaporate, transport the vapour, condense it. The equipment question is only ever how tightly you control each move, and how many litres per hour you need to push through it.

How do you distill water at home, and where does that method stop?

The classic setup takes five minutes to build. Water in a wide pot. A heat-proof bowl standing in the middle, above the water line. Invert the lid so it slopes toward the centre, pile ice on top of it, heat the pot. Steam rises, hits the cold underside of the lid, runs downhill to the low point and drips into the bowl. That is a still. A copper coil running from a kettle spout through a bucket of cold water is the same machine with the condenser moved outside.

It works. If you need a litre a day for a steam iron, a battery top-up or a humidifier, stop reading here. The pot is the right answer and nothing in our catalogue will serve you better.

It stops working the moment any of these becomes true:

• You need the same result twice. An open pot has no set point. Heat input drifts with the burner, the ambient air and how much ice is left.
• You need to know when you are done. No temperature readout, no vacuum reading, no clear view of the boil.
• Throughput matters. Your condensing surface is whatever the lid gives you, and it warms up as the ice goes.
• The residue is worth more than the water. If you are recovering a solvent or concentrating an extract rather than pouring the leftovers away, an uncontrolled boil at full atmospheric pressure cooks it.

Consumer distillation stills and survival guides live entirely inside those four limits, which is why the search results for this question stop where they do. Below is the tier they leave out.

How do you distill water in a lab without babysitting a pot?

You hand every variable to a machine that holds it for you. In practice that means a rotary evaporator, which spins the boiling flask inside a heated bath while a vacuum pump pulls vapour across into a condenser. Same evaporate-transport-condense loop, four knobs instead of a burner dial.

Rotation is the part people underestimate. A pot boils off a flat puddle at the bottom. A rotating flask drags a thin film of liquid up its inner wall on every turn, so the evaporating surface becomes a large part of the glass. On the RE units we build, that runs at 0–120 rpm off a 40 W motor. Small motor, because the job is spreading a film, not stirring against resistance.

The bath is the second knob: room temperature to 200 °C, held to ±1 °C. For water that range is generous, and the reason it goes that high is that the same machine is expected to strip high-boiling solvents the week after. The third knob is vacuum, down to −0.095 MPa, and it earns its own section below. The fourth is the cooling source feeding the condenser, which is not part of the evaporator and has to be specified separately.

All the glass is GG3.3 borosilicate. That matters for the same reason it matters anywhere in a process line: you need to watch the boil, and the flask has to survive sitting between a bath rated to 200 °C and a chilled condenser. The rotary evaporator range we manufacture lists the model-by-model breakdown.

Which rotary evaporator size matches your batch?

This is where the top of the search results goes quiet, and where a real parameter sheet earns its keep. Our rotary evaporator ladder runs from a 1 L bench unit to a 5 L unit, and the differences are not cosmetic:

Read that as three decisions, not twelve rows.

Flask volume sets your batch ceiling. The RE-101, RE-201D and RE-301 bench units cover 1 L to 3 L charges on a 1000 W bath, while the RE-501 takes a 5 L evaporating flask on a 280 × 160 mm bath with 1500 W of heating. Skip the RE-501 if your charge never exceeds 3 L. The bigger bath and the extra heating exist to keep a 5 L flask boiling, not to make a 1 L flask boil faster, and you still lift it yourself: bath lift is manual across the whole ladder.

Joint size is a hard fork, not a detail. The RE-101 and RE-201D take a 24# ground joint. The RE-301 moves to a 50# flange port. Those are different fittings. If you already own glassware, or you expect to stock spares across two units, settle this before anything else — a 3 L unit does not inherit your 1 L flasks.

The collecting flask is 1 L on all three bench models. Your evaporating flask can be three times that. Charge an RE-301 to its full 3 L and walk away, and the receiver fills first. Distilling more than the receiver holds means a break to empty it, or a continuous-feed arrangement. Plan for it rather than discovering it.

What the table cannot tell you is that heating power alone does not predict how fast a batch comes off. Rotation speed, vacuum level, condenser area and the temperature of your cooling source all sit in the same equation. That is why we ask about the cooling source before we name a model.

Why does vacuum do more of the work than heat?

Because a liquid boils when its vapour pressure matches the pressure above it, and pulling that pressure down to −0.095 MPa means the liquid reaches that condition at a far lower temperature than it would in an open pot. You are not making the water hotter. You are making it easier to leave.

Three consequences follow, and together they are why the lab tier exists at all:

• Thermal load drops. Anything dissolved in the water sees a milder bath. For an extract, a concentrate or a heat-sensitive product, that is the entire point.
• The bath range stops being about water. Room temperature to 200 °C looks excessive until you remember the machine also has to strip solvents that will not move under a gentle vacuum alone.
• The pump and the chiller are not accessories. A rotary evaporator on its own is a heated, rotating flask. Vacuum comes from the pump you pair with it; condensing capacity comes from the cooling source you feed the condenser. Undersize either one and the vapour you generate does not come back as liquid, it goes through your pump.

Getting that trio right is its own exercise. We wrote up how to match a vacuum pump and chiller to a given flask size separately, because it is the single most common thing a first-time buyer gets wrong.

What if the water is carrying something heat-sensitive?

Then you are no longer asking how to distill water. You are asking how to remove water without damaging what is dissolved in it, and past a certain sensitivity even a vacuum-assisted rotary evaporator gives the product too much time on a hot wall.

That is the job for short-path molecular distillation equipment, where the evaporator and condenser sit close enough that vapour crosses to the cold surface almost immediately under high vacuum. The build is an evaporator, a condenser, a receiving system and vacuum support, sized to the process rather than shipped as a fixed model. It is the tier for heat-sensitive or high-boiling-point materials that need gentle separation. It is not the tier for making clean water, which a rotary evaporator, or even a pot, handles perfectly well.

If your feed sits in that category, the molecular distillation selection guide walks through feed material, vacuum stability, heating temperature, feed rate, condenser support and receiving setup — the six things we need before quoting a build.

What do you have to specify before anyone can quote you?

Buyers usually open with “I need a rotary evaporator, what does it cost”. We cannot answer that, and nobody else can honestly either, until seven things are on the table:

1. Flask volume — your realistic charge per batch, not your best day.
2. Condenser type — driven by what you are condensing and how much of it.
3. Vacuum pump — what you already have, or whether we specify it.
4. Cooling source — chiller, tap water, or nothing yet. This one moves the answer more than any other.
5. Bath temperature — where you actually intend to run inside the room-temp-to-200 °C envelope.
6. Lift mode — the models above are manual lift; if you need otherwise, say so at the start.
7. Solvent characteristics — corrosivity, flammability, boiling behaviour, foaming tendency.

Answer those seven and the model picks itself in about a minute. Our equipment selection guide runs the same checklist across the rest of the process range if you are specifying more than one machine.

FAQ

Is distilled water the same as purified water?

No. Distillation is one purification method among several, and it targets non-volatiles specifically. Salts, metals and sediment stay in the boiling flask; anything volatile enough to boil alongside the water crosses into the distillate with it. “Purified” describes a result you need. Distillation describes one way of getting part of the way there.

Can I use a rotary evaporator just to distill water?

You can, and people do, but check what you are paying for. A rotary evaporator earns its cost through vacuum, a controlled bath and solvent recovery. If you only need clean water in small volume and nothing dissolved in it matters, the machine is overspecified. If you need the residue, the recovered distillate, or repeatability, it is the right tool.

What size rotary evaporator do I need for a 2 L batch?

The RE-301, with its 3 L evaporating flask, is the unit that covers a 2 L charge on our bench ladder — the RE-101 and RE-201D top out at 1 L. Note the 50# flange port on the RE-301 against the 24# ground joint on the smaller two, and note that the collecting flask is 1 L on all three, so a 2 L charge fills the receiver before the flask runs dry.

Does a rotary evaporator come with the vacuum pump and chiller?

The evaporator, the vacuum source and the cooling source are three separate specifications. Tell us which of them you already have and we configure around it. Buying an evaporator without confirming the other two is the fastest route to a machine that will not hold −0.095 MPa, or will not condense what it evaporates.

Why does the bath go to 200 °C if water boils well below that?

Because the same unit has to handle solvents that do not move as easily as water. The room-temperature-to-200 °C range with ±1 °C accuracy is there for the hard jobs. For water you sit near the bottom of it, especially once vacuum is applied.

Picking the setup you actually need

Work down, not up. If you need small volumes of clean water and nothing else, a pot with an inverted iced lid does the job for the price of the ice, and no equipment purchase justifies itself. If you need repeatability, a controlled bath, vacuum, and the ability to keep either the distillate or the residue, start at the RE-101 or RE-201D for 1 L work and move to the RE-301 only when your charge genuinely passes 1 L — remembering the joint size changes with it. Go to the RE-501 when 3 L stops being enough and you have somewhere to lift a 5 L flask by hand. Move to short-path molecular distillation only when the material itself, not the volume, is what limits you.

Send us your batch size, your feed, your cooling source and where you intend to run the bath, and we will tell you which tier you are in, including when the answer is that you do not need us yet.