Bicarb, Castile Soap and Hydrogen Peroxide: What Each Is Actually For

There are 4 ingredients doing most of the real work in a fragrance-free house, and plenty of recipes online that combine them in ways that chemically cancel each other out. Here is what each one does, and what it is quietly bad at.

This sits under our wider guide to choosing genuinely low-tox cleaning products in Australia. That article covers labels and marketing. This one covers what the 4 things in your cupboard physically do.

Bicarb: a soft abrasive with a side job

Sodium bicarbonate sits at roughly 2.5 on the mineral hardness scale. It is softer than glass, porcelain, stainless steel and ceramic tile, and harder than most of the soil sitting on top of them, so it grinds off the mess without scoring the surface underneath.

Made into a paste, it lifts baked-on grease from an oven floor, cooked spills off a stovetop, and the grey film off a bath. It is slow, which is what you want when the alternative is a caustic chemical eating the finish.

The odour job is real but narrower than advertised. Bicarb is mildly alkaline, so it buffers acidic odour compounds, which covers a lot of what makes a fridge, a bin or a pair of shoes unpleasant. It does much less against alkaline odours, which is why it underperforms on ammonia-type smells. Left in an open container it works by contact, so it only reaches what touches it.

There are 2 things bicarb cannot do. It does not disinfect at any concentration you would use at home. And because it is alkaline, it will never dissolve mineral scale or soap scum, both of which need an acid. On a shower screen the abrasion shifts some film mechanically, and on an acrylic screen you are dulling the surface while you do it.

The bicarb-and-vinegar volcano deserves a plain answer. The fizz is carbon dioxide, and what you are left with in the bowl is a weak sodium acetate solution that has neutralised both ingredients. Down a slow drain the physical churn does loosen some build-up, though boiling water and a drain brush do most of the work.

Castile soap: the only real cleaner in the group

Castile is true soap, made by saponifying vegetable oils. Each molecule has a water-loving end and an oil-loving end, so it surrounds grease, lifts it off the surface and holds it in suspension for the rinse to carry away. Nothing else in this article does that. Bicarb scrapes, acids dissolve minerals, peroxide oxidises. The soap is what actually removes oily soil.

It is concentrated, and most people use far too much. Surplus soap has nothing left to emulsify, so it dries as a film that attracts the next lot of soil. A small squeeze in a bucket is enough.

It does not disinfect, and it was never meant to. Soap works by removal, which handles a great deal on a household surface, and that is a different mechanism from killing organisms. Worth understanding properly if you have replaced a spray bottle that made antibacterial claims. We covered that distinction in whether natural cleaners actually disinfect.

Why the vinegar-and-castile recipe cannot work

This is the recipe I get asked about most, and it is chemically self-defeating. Soap is the salt of a fatty acid. Add any acid, and it hands a hydrogen ion back, converting the soap into free fatty acid. Free fatty acid is not water soluble, so it drops out of solution as white curds.

The spray you have made has spent its acid, destroyed its surfactant, and now deposits a fine greasy film on whatever you spray. The bench feels tacky afterwards and people reach for more product. If you want both, use them separately: soap first, rinse properly, acid second.

One last limitation. In hard water, soap reacts with dissolved calcium and magnesium to form scum, the exact deposit you then need an acid to remove. If your shower goes cloudy faster since switching to castile, use less soap and rinse properly.

Hydrogen peroxide: real chemistry, usually stored badly

Household hydrogen peroxide is 3 percent, and it has genuine oxidising power. It breaks down into water and oxygen, so there is no residue to rinse and nothing left behind on a surface a child will touch.

In practice, 3 conditions decide whether it does anything at all.

• It must stay visibly wet on the surface for several minutes. A spray that evaporates in 30 seconds has done nothing.
• The surface has to be clean first. Organic soil consumes peroxide rapidly, so on a dirty surface it is spent before it reaches anything.
• It has to still be active. This is where most bottles fail.

Hydrogen peroxide decomposes on exposure to light, warmth and trace metals, which is why it is sold in an opaque bottle. Decant it into a clear spray bottle, leave it on the vanity in a Brisbane bathroom, and within weeks you are misting your shower with something close to plain water. Keep it in the original bottle in a dark cupboard, decant only into an opaque sprayer, and replace it rather than nursing a 3 year old bottle.

Be honest about the word disinfectant, too. Peroxide has measurable antimicrobial activity, and disinfectant is a regulated claim tied to specific concentrations, contact times and test methods that a supermarket bottle used casually on a benchtop will not be meeting.

And remember it is a mild bleaching agent. It lifts colour. Repeated use on coloured grout, coloured fabric or a timber finish will lighten it, gradually enough that you notice only once the damage is spread across the whole area.

Acid: what handles anything mineral

Mineral build-up on tapware, the white haze on a shower screen, the ring in the toilet, kettle scale, the crust around the base of a mixer. All of it is mineral, and all of it needs an acid.

Citric acid powder is what I mix for this. It dissolves in warm water and you can make it to whatever strength the job needs: a couple of teaspoons in 500 ml for light haze, considerably stronger and left to dwell for a well-established shower screen. It stores as a dry bag rather than litres of liquid, and it does not put a sharp smell through the house, which is exactly what a fragrance-free home is built to avoid.

Vinegar does work on scale. It is weaker at typical strength and it announces itself in every room. There is more on where it earns its place in our piece on whether vinegar is actually a good cleaner.

What an acid cannot do is cut grease. It has no surfactant action at all, so on an oily surface it just sits there. Degrease first, then descale, rinsing between the 2.

What each one will damage

Everything here is capable of ruining something. Match the ingredient to the surface before you match it to the mess.

• Bicarb: acrylic and polycarbonate shower screens, gloss laminate, painted and lacquered surfaces, polished stone (it dulls the polish rather than staining it), and aluminium, which alkalis discolour.
• Castile soap: leaves a film on glass and mirrors, dulls waxed and oiled timber, and builds scum in hard water. Never mix it into an acid solution.
• Hydrogen peroxide: coloured grout, coloured and delicate fabrics, some timber and stone finishes. It keeps oxidising for as long as it sits there.
• Acid: marble, limestone, travertine and any carbonate stone. The etch is a physical change to the surface, not a stain, and it does not clean off. Also avoid prolonged contact with aluminium and with coated or brushed tapware finishes.
• Abrasive pads: fine on glass shower screens and some stainless, and permanently destructive on acrylic, chrome plating, painted surfaces and anodised finishes. Test somewhere invisible, always.

The part the ingredients do not decide

The right 4 bottles do not produce a clean house on their own. Most of what separates a genuinely clean bathroom from a wiped one is mechanical: a professional HEPA vacuum that holds what it collects instead of redistributing it, the correct brush for the grout profile, the right abrasive where a surface can take it, and enough time on the surface for any of it to matter.

A commodity clean is priced on speed, and harsh chemistry is what makes up for a short visit. A lower price usually means fewer hours in your home. Take the harsh chemistry away and you have to pay for the time instead, which is how our natural house cleaning is structured: $85 per hour, with technique doing the work the chemicals used to. Grout is the clearest example, and we walked through the method in how to clean grout.

The short version

Soap for anything greasy. Bicarb for gentle abrasion or an acidic odour. An acid for anything mineral. Hydrogen peroxide, stored dark and used on a clean wet surface, for oxidising action. Never soap and acid in the same bottle. Never peroxide and vinegar in the same bottle. Never bleach and anything.

You do not need more products than that. Most people already own 3 of the 4 and have been combining them into something weaker than either ingredient on its own.